Electrical Engineering

Learn Digital Logic and Microprocessor Design with VHDL

2010-02-16T17:06:00.001-08:00

Digital Logic and Microprocessor Design with VHDL (soon with Verilog)- Enoch O. Hwang, La Sierra University, Riverside, CA. This book will teach students how to design digital logic circuits, specifically combinational and sequential circuits. Students will learn how to put these two types of circuits together to form dedicated and general-purpose microprocessors. This book is unique in that it combines the use of logic principles and the building of individual components to create data paths and control units, and finally the building of real dedicated custom microprocessors and general-purpose microprocessors. After understanding the material in the book, students will be able to design simple microprocessors and implement them in real hardware.

Cover
Preface
contents
1 Designing Microprocessors
2 Digital Circuits
3 Combinational Circuits
4 Standard Combinational Components
5 Implementation Technologies
6 Latches and Flip-Flops
7 Sequential Circuits
8 Standard Sequential Components
9 Datapaths
10 Control Units
11 Dedicated Microprocessors
12 General-Purpose Microprocessors Appendix    (Quartus version)
        Max+Plus    Quartus
A Schematic Entry Tutorial 1Q
B VHDL Entry Tutorial 2Q
C UP2 Programming Tutorial 3Q
D VHDL Summary
    Verilog Summary

Free Electrical Book

2009-12-28T03:03:00.000-08:00

Electrical based ebooks are in great demand these days. However if you search hard enough, there’s a lot of freely available online resources out there which you can take advantage of. Here are a few (do leave your comments if you know of others):-
* Note: Some of the PDF’s listed down here might be too big in size to be viewed directly from your browser. Unless you have a very fast connection or just want to take a peek or view it briefly, I’d suggest right clicking and saving it straight to your computer for each of the ebooks under the PDF category.

Electrical Facilities Safety (PDF)
Electronics Tutorials (HTML)
Op Amp Circuit Collection (PDF)
Digital Logic basics (HTML)
555 timer circuits (HTML)
Handbook of operational amplifier applications (PDF)
Electromagnetic Fields and Energy (free college textbook online) (HTML)
Classical Electromagnetism by Richard Fitzpatrick (HTML)
Directory of Ebooks by Pacontrol.com – 54 ebooks (PDF)
Electrical Science (PDF)
- Electrical Science Volume 1
- Electrical Science Volume 2
- Electrical Science Volume 3
- Electrical Science Volume 4
Lazar’s power electronics guide (HTML & PDF)
Electric Motor Controls Tutorials (PDF)
gerbv – A Free/Open Source Gerber Viewer (Software)
PCB – interactive printed circuit board editor for the X11 window system (Software)
Programmable Controllers Theory and Implementation (PDF)
Automating Manufacturing Systems with PLCs (PDF) – 11MB
More PLC stuffs – Basics, Glossary, Laws (PDF)
Process Control Fundamentals (PDF)
Ebooks from Texas Instruments (Source: SMPS.US)
- Introduction and Basic Magnetics (PDF)
- Magnetic Core Characteristics (PDF)
- Windings data (PDF)
- Power supply transformer design (PDF)
- Inductor and Flyback Transformer Design (PDF)
- For the remaining 7 ebooks (click here and scroll down under “Unitrode seminar magnetics handbook”
Lessons In Electric Circuits (Posted before, but worth re-mentioning) – HTML & PDF
- Volume I – DC
- Volume II – AC
- Volume III – Semiconductors
- Volume IV – Digital
- Volume V – Reference
- Volume VI – Experiments

PIC Microcontrollers - Programming in C

2009-12-17T19:03:00.000-08:00

PIC Microcontrollers - Programming in C | What are microcontrollers, anyway? Electronics built in one single chip capable of controlling a small submarine, a crane or an elevator… It’s up to you to decide what you want them to do and dump a program containing appropriate instructions into the chip.

Title: PIC Microcontrollers - Programming in C
Author: Milan Verle
Paperback: 336 pages
Publisher: mikroElektronika; 1st edition (2009)
Language: English
ISBN-13: 978-86-84417-17-8
Paperback Color: Two Color
Covers Color: Full Color
Buy print book including CD:
Price: $24.00 USD

Numerical Analysis For Differential Equation Solution

2009-12-05T13:25:00.000-08:00

1. Introduction
Differential equations play an important role in many problem of engineering and science. This is so because of the involvement of differential equation in the mathematical models. It naturally leads to the determination of solution numerically. The use of the numerical methods have become vital in the absence of explicit solutions. Normally numerical methods have two major roles:
(a) the method should be implementable on a computer;
(b) the method should deal with the analysis of error estimates.
For the present we deal with some of the numerical methods to solve initial value problems (IVP's) (on finite intervals). We also mention that no effort is made to study boundary value problems. Let us consider an initial value problem.
Full Reference...

Papers About Theory of planned behavior

2009-11-14T15:39:00.001-08:00

TPB posits that individual behavior is driven by behavioral intentions where behavioural intentions are a function of an individual's attitude toward the behaviour, the subjective norms surrounding the performance of the behavior, and the individual's perception of the ease with which the behavior can be performed (behavioral control). Attitude toward the behavior is defined as the individual's positive or negative feelings about performing a behaviour. It is determined through an assessment of one's beliefs regarding the consequences arising from a behavior and an evaluation of the desirability of these consequences. Formally, overall attitude can be assessed as the sum of the individual consequence x desirability assessments for all expected consequences of the behavior. Subjective norm is defined as an individual's perception of whether people important to the individual think the behavior should be performed. The contribution of the opinion of any given referent is weighted by the motivation that an individual has to comply with the wishes of that referent. Hence, overall subjective norm can be expressed as the sum of the individual perception x motivation assessments for all relevant referents. Behavioral control is defined as one's perception of the difficulty of performing a behavior. TPB views the control that people have over their behavior as lying on a continuum from behaviors that are easily performed to those requiring considerable effort, resources, etc. Although Ajzen has suggested that the link between behavior and behavioral control outlined in the model should be between behavior and actual behavioural control rather than perceived behavioural control, the difficulty of assessing actual control has led to the use of perceived control as a proxy.

abstract paper share

Source: Eagly, A. H., & Chaiken, S. (1993). The psychology of attitudes. Publishers.

Electrical Pdf Ebook

2009-07-31T03:18:00.001-07:00

Topical worksheets

Together, these worksheets constitute a body of questions far too large to be comprehensively covered in any single electronics course. That is not the point, though. This collection is purposely oversized so you may pick and choose enough questions to meet your specific teaching needs. Download those worksheets containing questions you wish to use, and then only assign those questions to your students for research and discussion. The rest may be ignored, or your students may find them useful as study aids outside of class.

Although I strive to make each of these worksheets self-contained, some topics will (necessarily) assume coverage of prior topics. For instance, it would be impossible for a beginning student to understand a worksheet on series-parallel resistor circuits without first having understood Ohm's Law.

General:

Index of question files: [Plain-text]
How to turn any breadboarded circuit into a valid troubleshooting assessment: [PDF]
Student surveys (for regular inclusion in worksheets/exams): [PDF]
Questions relating to project management: [PDF]
Test page for adjustment of copy machine: [Encapsulated PostScript] [PDF]

Printed Circuit Boards:

Gerber files for 2x3 inch breadboard PCB (.zip archive): [ZIP]

General explanation for troubleshooting PCBs

Schematic diagram for simple 10-resistor series troubleshooting circuit: [PDF]
Gerber files for 4x4 inch troubleshooting PCB of a simple 10-resistor series circuit (.zip archive): [ZIP]

Schematic diagram for loaded voltage divider troubleshooting circuit: [PDF]
Gerber files for 4x4 inch troubleshooting PCB of a loaded voltage divider circuit (.zip archive): [ZIP]

Schematic diagram for quarter-active Wheatstone bridge troubleshooting circuit: [PDF]
Gerber files for 4x4 inch troubleshooting PCB of a Wheatstone bridge circuit (.zip archive): [ZIP]

Basic electricity:

Atomic structure: [PDF]
Static electricity: [PDF]
Voltage, Current, and Resistance: [PDF]
Conductors and insulators: [PDF]
Elementary circuits: [PDF]
Electrical connections: [PDF]
Soldering: [PDF]
Sources of electricity: [PDF]
Physical effects of electricity: [PDF]
Resistors: [PDF]
Switches: [PDF]
Basic voltmeter use: [PDF]
Basic ammeter use: [PDF]
Basic circuit troubleshooting: [PDF]
Ohm's Law: [PDF]
Energy, work, and power: [PDF]
Electric shock: [PDF]
Arc flash and arc blast: [PDF]
Safety grounding: [PDF]
Lock-out / Tag-out: [PDF]
Wire types and sizes: [PDF]
Design Project: Telegraph system [PDF]
Magnetism: [PDF]
Basic electromagnetism and electromagnetic induction: [PDF]
Basic relays: [PDF]
Series DC circuits: [PDF]
Parallel DC circuits: [PDF]
Basic ohmmeter use: [PDF]
Specific resistance: [PDF]
Temperature coefficient of resistance: [PDF]
Batteries: [PDF]
Overcurrent protection: [PDF]
Basic troubleshooting strategies: [PDF]
Performance assessments for basic electricity: [PDF]

DC electric circuits:

Voltage divider circuits: [PDF]
Current divider circuits: [PDF]
Kirchhoff's Laws: [PDF]
Potentiometers: [PDF]
Series-parallel DC circuits: [PDF]
Voltmeter design: [PDF]
Ammeter design: [PDF]
Design Project: Voltmeter [PDF]
DC bridge circuits: [PDF]
DC metrology: [PDF]
Magnetic units of measurement: [PDF]
Intermediate electromagnetism and electromagnetic induction: [PDF]
Capacitance: [PDF]
Capacitors: [PDF]
Inductance: [PDF]
Inductors: [PDF]
Time constant circuits: [PDF]
Time constant calculations: [PDF]
DC transducers (INCOMPLETE): [PDF]
DC generator theory: [PDF]
DC motor theory: [PDF]
Design Project: DC motor [PDF]
DC motor control circuits: [PDF]
Performance assessments for DC: [PDF]

AC electric circuits:

AC waveforms: [PDF]
Basic oscilloscope operation: [PDF]
Peak, average, and RMS measurements: [PDF]
Design Project: Four-channel audio mixer [PDF]
AC phase: [PDF]
Inductive reactance: [PDF]
Capacitive reactance: [PDF]
Impedance: [PDF]
Trigonometry for AC circuits: [PDF]
Phasor mathematics (INCOMPLETE): [PDF]
Series and parallel AC circuits: [PDF]
Resonance: [PDF]
Series-parallel combination AC circuits: [PDF]
Mixed-frequency signals: [PDF]
Decibel measurements: [PDF]
Passive filter circuits: [PDF]
Design Project: Audio tone control [PDF]
Passive integrator and differentiator circuits: [PDF]
Oscilloscope trigger controls: [PDF]
Mutual inductance: [PDF]
Step-up, step-down, and isolation transformers: [PDF]
Autotransformers: [PDF]
Impedance matching with transformers: [PDF]
Advanced electromagnetism and electromagnetic induction: [PDF]
Electrical noise and interference (INCOMPLETE): [PDF]
Design Project: Sensitive audio detector [PDF]
AC power: [PDF]
Characteristic impedance: [PDF]
AC transducers (PENDING): [PDF]
AC metrology: [PDF]
Polyphase power systems: [PDF]
Delta and Wye 3-phase circuits: [PDF]
AC generator theory: [PDF]
AC motor theory: [PDF]
AC motor control circuits: [PDF]
Microphones (INCOMPLETE): [PDF]
Fundamentals of radio communication (INCOMPLETE): [PDF]
Performance assessments for AC: [PDF]

Network analysis techniques:

Component modeling: [PDF]
Superposition theorem: [PDF]
Thevenin's, Norton's, and Maximum Power Transfer theorems: [PDF]
Millman's theorem: [PDF]
Simultaneous equations for circuit analysis: [PDF]
DC branch current analysis (INCOMPLETE): [PDF]
DC mesh current analysis (INCOMPLETE): [PDF]
AC network analysis: [PDF]
Performance assessments for network analysis (INCOMPLETE): [PDF]

Discrete semiconductor devices and circuits:

Electrical conduction in semiconductors: [PDF]
PN junctions: [PDF]
Electron versus Conventional flow: [PDF]
Rectifying diodes: [PDF]
Rectifier circuits: [PDF]
Basic AC-DC power supplies: [PDF]
Design Project: AC-DC power supply [PDF]
Design Project: Dual-output AC-DC power supply [PDF]
Design Project: Simple component curve-tracer circuit [PDF]
Clipper and clamper circuits: [PDF]
Miscellaneous diode applications: [PDF]
Zener diodes: [PDF]
Special diodes (INCOMPLETE): [PDF]
Elementary amplifier theory: [PDF]
Bipolar junction transistor theory: [PDF]
Bipolar junction transistors as switches: [PDF]
Bipolar junction transistors in active mode: [PDF]
Bipolar transistor biasing circuits: [PDF]
Regulated power sources: [PDF]
Design Project: DC voltage regulator [PDF]
Class A BJT amplifiers: [PDF]
Class B BJT amplifiers: [PDF]
Class C BJT amplifiers (INCOMPLETE): [PDF]
Design Project: Audio power amplifier [PDF]
BJT amplifier troubleshooting: [PDF]
Junction field effect transistors: [PDF]
JFET amplifiers: [PDF]
Insulated gate field effect transistors: [PDF]
Insulated gate bipolar transistors (INCOMPLETE): [PDF]
IGFET amplifiers (INCOMPLETE): [PDF]
Conventional transistor overview and special transistors (INCOMPLETE): [PDF]
Active loads in amplifier circuits (INCOMPLETE): [PDF]
Optoelectronic devices: [PDF]
Differential transistor amplifiers: [PDF]
Multi-stage transistor amplifiers: [PDF]
Oscillator circuits: [PDF]
Design Project: Radio transmitter [PDF]
Thyristors: [PDF]
Thyristor application circuits: [PDF]
Signal modulation (INCOMPLETE): [PDF]
Power conversion circuits: [PDF]
Fiber optics (PENDING): [PDF]
Performance assessments for semiconductors: [PDF]

Analog integrated circuits:

IC fabrication and packaging (INCOMPLETE): [PDF]
Printed circuit board layout and manufacture (INCOMPLETE): [PDF]
Design Project: LED stroboscope [PDF]
Design Project: Signal generator [PDF]
Basic operational amplifiers: [PDF]
Open-loop opamp circuits: [PDF]
Design Project: Pulse-width modulation (PWM) signal generator [PDF]
Negative feedback opamp circuits: [PDF]
Positive feedback opamp circuits: [PDF]
Inverting and noninverting opamp voltage amplifier circuits: [PDF]
Design Project: Intercom system [PDF]
Design Project: Audio media-based signal generator [PDF]
Design Project: Sensitive microphone amplifier [PDF]
Summer and subtractor opamp circuits: [PDF]
Voltage/current converter opamp circuits (INCOMPLETE): [PDF]
Linear computational circuitry: [PDF]
Servo motor systems (PENDING): [PDF]
Precise diode circuits: [PDF]
AC negative feedback opamp circuits: [PDF]
Opamp oscillator circuits: [PDF]
Active filters: [PDF]
Logarithms for analog circuits: [PDF]
Nonlinear opamp circuits: [PDF]
Phase-locked loops (INCOMPLETE): [PDF]
Performance assessments for analog integrated circuits: [PDF]

Digital circuits:

Design Project: Logic probe [PDF]
Digital logic signals: [PDF]
Basic logic gates: [PDF]
Numeration systems: [PDF]
Binary arithmetic: [PDF]
Digital codes: [PDF]
TTL logic gates: [PDF]
CMOS logic gates: [PDF]
Basic logic gate troubleshooting: [PDF]
Electromechanical relay logic: [PDF]
Time-delay electromechanical relays: [PDF]
Protective relay circuits (INCOMPLETE): [PDF]
Boolean algebra: [PDF]
Sum-of-Products and Product-of-sums expressions: [PDF]
Karnaugh mapping: [PDF]
Binary math circuits: [PDF]
Encoders and decoders: [PDF]
Multiplexers and demultiplexers: [PDF]
Digital display circuits: [PDF]
Programmable logic technology: [PDF]
Latch circuits: [PDF]
Timer circuits: [PDF]
Flip-flop circuits: [PDF]
Design Project: Light-pulse switch [PDF]
Design Project: Power inverter [PDF]
Counters: [PDF]
Design Project: Event counter [PDF]
Shift registers: [PDF]
Design Project: Arbitrary waveform generator [PDF]
Digital-to-Analog conversion: [PDF]
Analog-to-Digital conversion: [PDF]
Switched capacitor circuitry: [PDF]
Digital communication (INCOMPLETE): [PDF]
Memory devices: [PDF]
Finite state machines (PENDING): [PDF]
Microprocessor function (PENDING): [PDF]
Microprocessor programming (INCOMPLETE): [PDF]
Microcontroller principles (INCOMPLETE): [PDF]
Programmable logic controllers (INCOMPLETE): [PDF]
High-reliability circuits (INCOMPLETE): [PDF]
Stepper motors (INCOMPLETE): [PDF]
Design Project: Stepper motor driver [PDF]
Performance assessments for digital: [PDF]

Mathematics for electronics:

Scientific notation and metric prefixes: [PDF]
Basic algebra and graphing for electric circuits: [PDF]
Algebraic equation manipulation for electric circuits: [PDF]
Algebraic substitution for electric circuits: [PDF]
Logarithms for analog circuits: [PDF]
Simultaneous equations for circuit analysis: [PDF]
Trigonometry for AC circuits: [PDF]
Phasor mathematics (INCOMPLETE): [PDF]
Calculus for electric circuits: [PDF]

Circuit animations:

Simple switch circuit animation: [fast GIF] [slow GIF] [PDF]
Soldering a wire to a lug: [fast GIF] [slow GIF] [PDF]
Electromagnetic induction: [fast GIF] [slow GIF] [PDF]
Thevenin's theorem demonstrated: [fast GIF] [slow GIF] [PDF]
Lissajous figures on an oscilloscope (0 degrees phase shift): [fast GIF] [slow GIF] [PDF]
Lissajous figures on an oscilloscope (90 degrees phase shift): [fast GIF] [slow GIF] [PDF]
Lissajous figures on an oscilloscope (180 degrees phase shift): [fast GIF] [slow GIF] [PDF]
Lissajous figures on an oscilloscope (2:1 frequency ratio): [fast GIF] [slow GIF] [PDF]
Three-phase motor, rotating magnetic field: [fast GIF] [slow GIF] [PDF]
Transmission line with open end: [fast GIF] [slow GIF] [PDF]
Transmission line with shorted end: [fast GIF] [slow GIF] [PDF]
Transmission line with terminated end: [fast GIF] [slow GIF] [PDF]
Semiconductor diode junction (forward biased): [fast GIF] [slow GIF] [PDF]
Bridge rectifier circuit with ideal diodes: [fast GIF] [slow GIF] [PDF]
Bridge rectifier circuit with real diodes: [fast GIF] [slow GIF] [PDF]
BJT characteristic curve sketching: [fast GIF] [slow GIF] [PDF]
Push-pull transistor amplifier with crossover distortion: [fast GIF] [slow GIF] [PDF]
Pulse-width modulation comparator circuit: [fast GIF] [slow GIF] [PDF]
Telephony multiplexer system: [fast GIF] [slow GIF] [PDF]
Johnson ring counter circuit (with timing diagram): [fast GIF] [slow GIF] [PDF]
ROM memory addressing: [fast GIF] [slow GIF] [PDF]

Math animations:

Differentiation and integration: [fast GIF] [slow GIF] [PDF]

Bellingham Technical College students ("Core" Electronics course worksheets):

DC Electric Circuits

ELTR100 (DC 1), Section 1: [PDF]	ELTR100 (DC 1), Section 2: [PDF]	ELTR100 (DC 1), Section 3: [PDF]

ELTR105 (DC 2), Section 1: [PDF]	ELTR105 (DC 2), Section 2: [PDF]	ELTR105 (DC 2), Section 3: [PDF]

AC Electric Circuits

ELTR110 (AC 1), Section 1: [PDF]	ELTR110 (AC 1), Section 2: [PDF]	ELTR110 (AC 1), Section 3: [PDF]

ELTR115 (AC 2), Section 1: [PDF]	ELTR115 (AC 2), Section 2: [PDF]	ELTR115 (AC 2), Section 3: [PDF]

Discrete Semiconductor Circuits

ELTR120 (Semi 1), Section 1: [PDF]	ELTR120 (Semi 1), Section 2: [PDF]	ELTR120 (Semi 1), Section 3: [PDF]

ELTR125 (Semi 2), Section 1: [PDF]	ELTR125 (Semi 2), Section 2: [PDF]	ELTR125 (Semi 2), Section 3: [PDF]

Operational Amplifier Circuits

ELTR130 (Opamps 1), Section 1: [PDF]	ELTR130 (Opamps 1), Section 2: [PDF]

ELTR135 (Opamps 2), Section 1: [PDF]	ELTR135 (Opamps 2), Section 2: [PDF]

Digital Circuits

ELTR140 (Digital 1), Section 1: [PDF]	ELTR140 (Digital 1), Section 2: [PDF]	ELTR140 (Digital 1), Section 3: [PDF]

ELTR145 (Digital 2), Section 1: [PDF]	ELTR145 (Digital 2), Section 2: [PDF]	ELTR145 (Digital 2), Section 3: [PDF]

Engineering Analysis

2009-07-28T04:34:00.001-07:00

A book used for teaching upper level and graduate courses in engineering analysis. It is currently being used to teach EGR 600 - Analysis. This book emphasizes programming - this is not found in any other autoamtion books available. At present, half of the book is written chapters, while the other half is in note form.

Chapters

The complete book in PDF

Chapters

Numbers and units [PDF]
Algebra [PDF]
Trigonometry [PDF]
Vectors [PDF]
Matrices [PDF]
Graphing [PDF]
Programming [PDF]
Permutations [PDF]
Statistics [PDF]
Reliability [PDF]
Calculus [PDF]
Differential Equations [PDF]
ECE Review [PDF]
ME Review [PDF]
PDM Review [PDF]
MO Review [PDF]
Boolean [PDF]
Number Systems [PDF]
Transforms [PDF]
Geometry [PDF]
Financial [PDF]
Optimization [PDF]
Projects [PDF]
Graphs [PDF]
Trees [PDF]

Downloads

Source code directories

Links

Numerical Recipes in C

Dynamic System Modeling and Control

2009-07-28T04:32:00.000-07:00

This book introduces the basic concepts of system modeling with differential equations.

Status

The draft version includes updates made during the fall of 2004, including many corrections and clarifications.

Chapters (FROM THE DRAFT VERSION 2.6)

Introduction (PDF)
Translation (PDF) (PPT)
Solving Differential Equations (PDF) (PPT)
Numerical Methods (PDF) (PPT)
Rotation (PDF) (PPT)
Transfer Functions (PDF) (PPT)
Circuits (PDF) (PPT)
Feedback Control Systems and Block Diagram (PDF)s (PPT)
Phasor Analysis (PDF) (PPT)
Bode Plots (PDF) (PPT)
Root Locus Plots (PDF) (PPT)
Non-linear systems (PDF) (PPT)
Analog IO (PDF) (PPT)
Sensors (PDF) (PPT)
Actuators (PDF) (PPT)
Motion Control Systems (PDF) (PPT)
Laplace Techniques (PDF) (PPT)
Linear Systems (PDF) (PPT)
Convolution (PDF)
State Analysis (PDF)
State Based Controls (PDF)
System Identification(PDF)
Magnetic Systems (PDF) (PPT)
Fluids (PDF) (PPT)
Thermal (PDF) (PPT)
Optimization(PDF)
Finite Element Analysis(PDF)
Fuzzy Logic(PDF)
Neural Networks(PDF)
Embedded Systems(PDF)
Lab Guide (PDF)
Writing (PDF)
Math Review Guide (PDF)
C Programming Review (PDF)
Units (PDF)
Materials (PDF)

Downloads

Version 2.6 (CURRENT DRAFT VERSION) PDF (5.5MB) January 5, 2005
Version 2.4 PDF (5.2MB) January 26, 2004
Version 2.3 PDF (4.4MB) August 25, 2003
Version 2.2 PDF (4.4MB) July 22, 2002
Version 2.1 PDF (4.3MB) March 31, 2002 (HTML)
Version 2.0 PDF (4.3MB) January 10, 2002
Version 1.0 PDF (3.5MB) August 10, 2001

NONLINEAR SYSTEM REPRESENTATION

2009-07-28T04:31:00.000-07:00

Tommy W. S. Chow, City University of Hong Kong
Hong-Zhou Tan, University of Manitoba
Yong Fang, Shanghai University
J. Webster (ed.), Wiley Encyclopedia of Electrical and Electronics Engineering Online
Copyright © 2001 by John Wiley & Sons, Inc. All rights reserved.
Article Online Posting Date: December 20, 2001

Content :

Top Of Article
Input-Output System Representation
Nonlinear Differential Algebraic Representation
Volterra Representation
State-Space Representation
Bilinear Representation
Narma Representation
Fuzzy-Logic Nonlinear Representation
Nonlinear Representation Using Neural Networks
Model-Free Representation
Features Of Nonlinear Representations
Example
Concluding Remarks
Acknowledgment

Free Download eBook : NONLINEAR SYSTEM REPRESENTATION

Tuesday,Jul Automated Manufacturing Systems - PLCs

2009-07-28T04:30:00.000-07:00

This is a manuscript for a PLC based control system book that is currently being used for teaching an undergraduate controls course EGR 450 - Manufacturing Controls. The course and book focus on the Allen Bradley family of controllers, thus allowing a deeper topic coverage than is normal in PLC books.

Status

Versions 5.0 and newer of the book are updated to focus on the Allen Bradley ControlLogix controllers. The older book focuses on the PLC-5 and Micrologix platforms and can be found on a copy of the previous page.

A Free Book

Please note that the book is available under the GFDL (GNU Free Document License). This permits copying, distribution, modification, etc. without my permission, within the terms of the license.

Interesting facts

Between May 6 and August 22, 2003 the book was downloaded 88973 times from the main website. This does not include downloads from other websites that have copies of the book. It also does not include copies downloaded over the previous years. On August 21, 2003 the book was downloaded 563 times. In the same time period individual chapters were downloaded in PDF and RTF formats 88086 times. Most interestingly is that the PLC book webpage was only accessed 14001 times. This suggests that people have created links, or are using external links to access the book directly.

Downloads

Version 5.2 (newest) available at the book web site September 30, 2008
Version 5.1 PDF (6MB) April 21, 2008
Version 5.0 PDF (6MB) May 5, 2007
Version 4.9 PDF (6MB) January 12, 2007
Version 4.7 PDF (5.4MB) April 14, 2005
Version 4.6 PDF (5.3MB) December 15, 2004
Version 4.5 PDF (5MB) May 5, 2004
Version 4.2 PDF (5MB) April 3, 2003
Version 4.1 PDF (5.0MB) July 22, 2002
Version 4.0 PDF (4.9MB) March 31, 2002 (HTML)
Version 3.1 PDF (4.7MB) November 27, 2001
Version 3 PDF (4.7MB) July 12, 2001
Version 2 PDF (4.5MB)

Resources

Labguide PDF - updated April 21, 2008
Chapters and Powerpoint slides

Other Stuff

Links
- More online books books
- EGR 450 - A PLC controls course
- An open source PLC for Linux
- An online PLC tutorial
- A general information and discussion site with ladder logic to download
- A publisher with another PLC book on-line
- A general information PLC site
- Tools for analysis of PLC based control systems Tek Supply
- A site dedicated to communications Data Transfer HQ
- PLC maintenance and safety - Business Industrial Network

Field Effect Transistors

2009-06-03T20:46:00.000-07:00

Although it has brought about a revolution in the design of electronic equipment, the bipolar (PNP/NPN) transistor still has one very undesirable characteristic. The low input impedance associated with its base-emitter junction causes problems in matching impedances between interstage amplifiers.

For years, scientists searched for a solution that would combine the high input impedance of the vacuum tube with the many other advantages of the transistor. The result of this research is the FIELD-EFFECT TRANSISTOR (FET). In contrast to the bipolar transistor, which uses bias current between base and emitter to control conductivity, the FET uses voltage to control an electrostatic field within the transistor. Because the FET is voltage-controlled, much like a vacuum tube, it is sometimes called the "solid-state vacuum tube."

The elements of one type of FET, the junction type (JFET), are compared with the bipolar transistor and the vacuum tube in figure 3-44. As the figure shows, the JFET is a three-element device comparable to the other two. The "gate" element of the JFET corresponds very closely in operation to the base of the transistor and the grid of the vacuum tube. The "source" and "drain" elements of the JFET correspond to the emitter and collector of the transistor and to the cathode and plate of the vacuum tube.

Figure 3-44. - Comparison of JFET, transistor, and vacuum tube symbols.

The construction of a JFET is shown in figure 3-45. A solid bar, made either of N-type or P-type material, forms the main body of the device. Diffused into each side of this bar are two deposits of material of the opposite type from the bar material, which form the "gate." The portion of the bar between the deposits of gate material is of a smaller cross section than the rest of the bar and forms a "channel" connecting the source and the drain. Figure 3-45 shows a bar of N-type material and a gate of P-type material. Because the material in the channel is N-type, the device is called an N-channel JFET.

Figure 3-45. - JFET structure.

In a P-channel JFET, the channel is made of P-type material and the gate of N-type material. In figure 3-46, schematic symbols for the two types of JFET are compared with those of the NPN and PNP bipolar transistors. Like the bipolar transistor types, the two types of JFET differ only in the configuration of bias voltages required and in the direction of the arrow within the symbol. Just as it does in transistor symbols, the arrow in a JFET symbol always points towards the N-type material. Thus the symbol of the N-channel JFET shows the arrow pointing toward the drain/source channel, whereas the P-channel symbol shows the arrow pointing away from the drain/source channel toward the gate.

Figure 3-46. - Symbols and bias voltages for transistors and JFET.

The key to FET operation is the effective cross-sectional area of the channel, which can be controlled by variations in the voltage applied to the gate. This is demonstrated in the figures which follow.

Figure 3-47 shows how the JFET operates in a zero gate bias condition. Five volts are applied across the JFET so that current flows through the bar from source to drain, as indicated by the arrow. The gate terminal is tied to ground. This is a zero gate bias condition. In this condition, a typical bar represents a resistance of about 500 ohms. A milliammeter, connected in series with the drain lead and dc power, indicates the amount of current flow. With a drain supply (V_DD) of 5 volts, the milliammeter gives a drain current (I_D) reading of 10 milliamperes. The voltage and current subscript letters (V_DD, I_D) used for an FET correspond to the elements of the FET just as they do for the elements of transistors.

Figure 3-47. - JFET operation with zero gate bias.

In figure 3-48, a small reverse-bias voltage is applied to the gate of the JFET. A gate-source voltage (V_GG) of negative 1 volt applied to the P-type gate material causes the junction between the P- and N-type material to become reverse biased. Just as it did in the varactor diode, a reverse-bias condition causes a "depletion region" to form around the PN junction of the JFET. Because this region has a reduced number of current carriers, the effect of reverse biasing is to reduce the effective cross-sectional area of the "channel." This reduction in area increases the source-to-drain resistance of the device and decreases current flow.

Figure 3-48. - JFET with reverse bias.

The application of a large enough negative voltage to the gate will cause the depletion region to become so large that conduction of current through the bar stops altogether. The voltage required to reduce drain current (I_D) to zero is called "pinch-off" voltage and is comparable to "cut-off" voltage in a vacuum tube. In figure 3-48, the negative 1 volt applied, although not large enough to completely stop conduction, has caused the drain current to decrease markedly (from 10 milliamperes under zero gate bias conditions to 5 milliamperes). Calculation shows that the 1-volt gate bias has also increased the resistance of the JFET (from 500 ohms to 1 kilohm). In other words, a 1-volt change in gate voltage has doubled the resistance of the device and cut current flow in half.

These measurements, however, show only that a JFET operates in a manner similar to a bipolar transistor, even though the two are constructed differently. As stated before, the main advantage of an FET is that its input impedance is significantly higher than that of a bipolar transistor. The higher input impedance of the JFET under reverse gate bias conditions can be seen by connecting a microammeter in series with the gate-source voltage (V_GG), as shown in figure 3-49.

Figure 3-49. - JFET input impedance.

With a V_GG of 1 volt, the microammeter reads .5 microamps. Applying Ohm's law (1V ¸ .5mA) illustrates that this very small amount of current flow results in a very high input impedance (about 2 megohms). By contrast, a bipolar transistor in similar circumstances would require higher current flow (e.g., .1 to -1 mA), resulting in a much lower input impedance (about 1000 ohms or less). The higher input impedance of the JFET is possible because of the way reverse-bias gate voltage affects the cross-sectional area of the channel.

The preceding example of JFET operation uses an N-channel JFET. However, a P-channel JFET operates on identical principles. The differences between the two types are shown in figure 3-50.

Figure 3-50. - JFET symbols and bias voltages.

Because the materials used to make the bar and the gate are reversed, source voltage potentials must also be reversed. The P-channel JFET therefore requires a positive gate voltage to be reverse biased, and current flows through it from drain to source.

Figure 3-51 shows a basic common-source amplifier circuit containing an N-channel JFET. The characteristics of this circuit include high input impedance and a high voltage gain. The function of the circuit components in this figure is very similar to those in a triode vacuum tube common-cathode amplifier circuit. C1 and C3 are the input and output coupling capacitors. R1 is the gate return resistor and functions much like the grid return resistor in a vacuum tube circuit. It prevents unwanted charge buildup on the gate by providing a discharge path for C1. R2 and C2 provide source self-bias for the JFET, which operates like cathode self-bias. R3 is the drain load resistor, which acts like the plate or collector load resistor.

Figure 3-51. - JFET common source amplifier.

The phase shift of 180 degrees between input and output signals is the same as that of common-cathode vacuum tube circuits (and common-emitter transistor circuits). The reason for the phase shift can be seen easily by observing the operation of the N-channel JFET. On the positive alternation of the input signal, the amount of reverse bias on the P-type gate material is reduced, thus increasing the effective cross-sectional area of the channel and decreasing source-to-drain resistance. When resistance decreases, current flow through the JFET increases. This increase causes the voltage drop across R3 to increase, which in turn causes the drain voltage to decrease. On the negative alternation of the cycle, the amount of reverse bias on the gate of the JFET is increased and the action of the circuit is reversed. The result is an output signal, which is an amplified 180-degree-out-of-phase version of the input signal.

A second type of field-effect transistor has been introduced in recent years that has some advantages over the JFET. This device is the metal oxide semiconductor field effect transistor (MOSFET). The MOSFET has an even higher input impedance than the JFET (10 to 100 million megohms). Therefore, the MOSFET is even less of a load on preceding circuits. The extremely high input impedance, combined with a high gain factor, makes the MOSFET a highly efficient input device for RF/IF amplifiers and mixers and for many types of test equipment.

The MOSFET is normally constructed so that it operates in one of two basic modes: the depletion mode or the enhancement mode. The depletion mode MOSFET has a heavily doped channel and uses reverse bias on the gate to cause a depletion of current carriers in the channel. The JFET also operates in this manner. The enhancement mode MOSFET has a lightly doped channel and uses forward bias to enhance the current carriers in the channel. A MOSFET can be constructed that will operate in either mode depending upon what type of bias is applied, thus allowing a greater range of input signals.

In addition to the two basic modes of operation, the MOSFET, like the JFET, is of either the P-channel type or the N-channel type. Each type has four elements: gate, source, drain, and substrate. The schematic symbols for the four basic variations of the MOSFET are shown in views A, B, C, and D of figure 3-52.

Figure 3-52A. - MOSFET symbols.

Figure 3-52B. - MOSFET symbols.

Figure 3-52C. - MOSFET symbols.

Figure 3-52D. - MOSFET symbols.

The construction of an N-channel MOSFET is shown in figure 3-53. Heavily doped N-type regions (indicated by the N+) are diffused into a P-type substrate or base. A channel of regular N-type material is diffused between the heavily doped N-type regions. A metal oxide insulating layer is then formed over the channel, and a metal gate layer is deposited over the insulating layer. There is no electrical connection between the gate and the rest of the device. This construction method results in the extremely high input impedance of the MOSFET. Another common name for the device, derived from the construction method, is the insulated gate field effect transistor (IGFET).

Figure 3-53. - MOSFET structure.

The operation of the MOSFET, or IGFET, is basically the same as the operation of the JFET. The current flow between the source and drain can be controlled by using either of two methods or by using a combination of the two methods. In one method the drain voltage controls the current when the gate potential is at zero volts. A voltage is applied to the gate in the second method. An electric field is formed by the gate voltage that affects the current flow in the channel by either depleting or enhancing the number of current carriers available. As previously stated, a reverse bias applied to the gate depletes the carriers, and a forward bias enhances the carriers. The polarity of the voltages required to forward or reverse bias a MOSFET depends upon whether it is of the P-channel type or the N-channel type. The effects of reverse-bias voltage on a MOSFET designed to operate in the depletion mode are illustrated in views A, B, and C of figure 3-54. The amount of reverse bias applied has a direct effect on the width of the current channel and, thus, the amount of drain current (I_D).

Figure 3-54A. - Effects of bias on N-channel depletion MOSFET.

Figure 3-54B. - Effects of bias on N-channel depletion MOSFET.

Figure 3-54C. - Effects of bias on N-channel depletion MOSFET.

Figure 3-55 (view A, view B, and view C) illustrates the effect of forward bias on an enhancement mode N-channel MOSFET. In this case, a positive voltage applied to the gate increases the width of the current channel and the amount of drain current (I_D)

Figure 3-55A. - Effects of bias on N-channel enhancement MOSFET.

Figure 3-55B. - Effects of bias on N-channel enhancement MOSFET.

Figure 3-55C. - Effects of bias on N-channel enhancement MOSFET.

Another type of MOSFET is the induced-channel type MOSFET. Unlike the MOSFETs discussed so far, the induced-channel type has no actual channel between the source and the drain. The induced channel MOSFET is constructed by making the channel of the same type material as the substrate, or the opposite of the source and the drain material. As shown in figure 3-56, the source and the drain are of P-type material, and the channel and the substrate are of N-type material.

Figure 3-56. - Induced channel MOSFET construction.

The induced-channel MOSFET is caused to conduct from source to drain by the electric field that is created when a voltage is applied to the gate. For example, assume that a negative voltage is applied to the MOSFET in figure 3-56. The effect of the negative voltage modifies the conditions in the substrate material. As the gate builds a negative charge, free electrons are repelled, forming a depletion region. Once a certain level of depletion has occurred (determined by the composition of the substrate material), any additional gate bias attracts positive holes to the surface of the substrate. When enough holes have accumulated at the surface channel area, the channel changes from an N-type material to a P-type material, since it now has more positive carriers than negative carriers. At this point the channel is considered to be to inverted, and the two P-type regions at the source and the drain are now connected by a P-type inversion layer or channel. As with the MOSFET, the gate signal determines the amount of current flow through the channel as long as the source and drain voltages remain constant. When the gate voltage is at zero, essentially no current flows since a gate voltage is required to form a channel.

The MOSFETs discussed up to this point have been single-gate MOSFETs. Another type of MOSFET, the dual-gate type, is shown in figure 3-57. As the figure shows, the gates in a dual-gate MOSFET can be compared to the grids in a multi-grid vacuum tube. Because the substrate has been connected directly to the source terminal, the dual-gate MOSFET still has only four leads: one each for source and drain, and two for the gates. Either gate can control conduction independently, making this type of MOSFET a truly versatile device.

Figure 3-57. - Dual-gate MOSFET.

One problem with both the single- and dual-gate MOSFETs is that the oxide layer between gate and channel can be destroyed very easily by ordinary static electricity. Replacement MOSFETs come packaged with their leads shorted together by a special wire loop or spring to avoid accidental damage. The rule to remember with these shorting springs is that they must not be removed until after the MOSFET has been soldered or plugged into a circuit. One such spring is shown in figure 3-58.

Figure 3-58. - MOSFET shorting spring.

Q.30 What is one of the primary advantages of the FET when compared to the bipolar transistor?
Q.31 The FET and the vacuum tube have what in common?
Q.32 The base of a transistor serves a purpose similar to what element of the FET?
Q.33 What are the two types of JFET?
Q.34 The source and drain of an N-channel JFET are made of what type of material?
Q.35 What is the key to FET operation?
Q.36 What is the normal current path in an N-channel JFET?
Q.37 Applying a reverse bias to the gate of an FET has what effect?
Q.38 The input and output signals of a JFET amplifier have what phase relationship?
Q.39 When compared to the JFET, what is the input impedance of the MOSFET?
Q.40 What are the four elements of the MOSFET?
Q.41 The substrate of an N-channel MOSFET is made of what material?
Q.42 In a MOSFET, which element is insulated from the channel material?
Q.43 What type of MOSFET can be independently controlled by two separate signals?
Q.44 What is the purpose of the spring or wire around the leads of a new MOSFET?

TRANSISTOR PRECAUTIONS

2009-05-29T10:11:00.001-07:00

Transistors, although generally more rugged mechanically than electron tubes, are susceptible to damage by electrical overloads, heat, humidity, and radiation. Damage of this nature often occurs during transistor servicing by applying the incorrect polarity voltage to the collector circuit or excessive voltage to the input circuit. Careless soldering techniques that overheat the transistor have also been known to cause considerable damage. One of the most frequent causes of damage to a transistor is the electrostatic discharge from the human body when the device is handled. You may avoid such damage before starting repairs by discharging the static electricity from your body to the chassis containing the transistor. You can do this by simply touching the chassis. Thus, the electricity will be transferred from your body to the chassis before you handle the transistor.

To prevent transistor damage and avoid electrical shock, you should observe the following precautions when you are working with transistorized equipment:

Test equipment and soldering irons should be checked to make certain there is no leakage current from the power source. If leakage current is detected, isolation transformers should be used.

Always connect a ground between test equipment and circuit before attempting to inject or monitor a signal.

Ensure test voltages do not exceed maximum allowable voltage for circuit components and transistors. Also, never connect test equipment outputs directly to a transistor circuit.

Ohmmeter ranges that require a current of more than one milliampere in the test circuit should not be used for testing transistors.

Battery eliminators should not be used to furnish power for transistor equipment because they have poor voltage regulation and, possibly, high-ripple voltage.

The heat applied to a transistor, when soldered connections are required, should be kept to a minimum by using a low-wattage soldering iron and heat shunts, such as long-nose pliers, on the transistor leads.

When it becomes necessary to replace transistors, never pry transistors to loosen them from printed circuit boards.

All circuits should be checked for defects before replacing a transistor.

The power must be removed from the equipment before replacing a transistor.

Using conventional test probes on equipment with closely spaced parts often causes accidental shorts between adjacent terminals. These shorts rarely cause damage to an electron tube but may ruin a transistor.

To prevent these shorts, the probes can be covered with insulation, except for a very short length of the tips.

LEAD IDENTIFICATION

Transistor lead identification plays an important part in transistor maintenance; because, before a transistor can be tested or replaced, its leads or terminals must be identified. Since there is no standard method of identifying transistor leads, it is quite possible to mistake one lead for another. Therefore, when you are replacing a transistor, you should pay close attention to how the transistor is mounted, particularly to those transistors that are soldered in, so that you do not make a mistake when you are installing the new transistor. When you are testing or replacing a transistor, if you have any doubts about which lead is which, consult the equipment manual or a transistor manual that shows the specifications for the transistor being used.

There are, however, some typical lead identification schemes that will be very helpful in transistor troubleshooting. These schemes are shown in figure 2-17. In the case of the oval-shaped transistor shown in view A, the collector lead is identified by a wide space between it and the base lead. The lead farthest from the collector, in line, is the emitter lead. When the leads are evenly spaced and in line, as shown in view B, a colored dot, usually red, indicates the collector. If the transistor is round, as in view C, a red line indicates the collector, and the emitter lead is the shortest lead. In view D the leads are in a triangular arrangement that is offset from the center of the transistor. The lead opposite the blank quadrant in this scheme is the base lead. When viewed from the bottom, the collector is the first lead clockwise from the base. The leads in view E are arranged in the same manner as those is view D except that a tap is used to identify the leads. When viewed from the bottom in a clockwise direction, the first lead following the tab is the emitter, followed by the base and collector.

Figure 2-17. - Transistor lead identification.

In a conventional power transistor as shown in views F and G, the collector lead is usually connected to the mounting base. For further identification, the base lead in view F is covered with green sleeving. While the leads in view G are identified by viewing the transistor from the bottom in a clockwise direction (with mounting holes occupying 3 o'clock and 9 o'clock positions), the emitter lead will be either at the 5 o'clock or 11 o'clock position. The other lead is the base lead.

TRANSISTOR TESTING

There are several different ways of testing transistors. They can be tested while in the circuit, by the substitution method mentioned, or with a transistor tester or ohmmeter.

Transistor testers are nothing more than the solid-state equivalent of electron-tube testers (although they do not operate on the same principle). With most transistor testers, it is possible to test the transistor in or out of the circuit.

There are four basic tests required for transistors in practical troubleshooting: gain, leakage, breakdown, and switching time. For maintenance and repair, however, a check of two or three parameters is usually sufficient to determine whether a transistor needs to be replaced.

Since it is impractical to cover all the different types of transistor testers and since each tester comes with its own operator's manual, we will move on to something you will use more frequently for testing transistors-the ohmmeter.

Testing Transistors with an Ohmmeter

Two tests that can be done with an ohmmeter are gain, and junction resistance. Tests of a transistor's junction resistance will reveal leakage, shorts, and opens.

TRANSISTOR GAIN TEST. - A basic transistor gain test can be made using an ohmmeter and a simple test circuit. The test circuit can be made with just a couple of resistors and a switch, as shown in figure 2-18. The principle behind the test lies in the fact that little or no current will flow in a transistor between emitter and collector until the emitter-base junction is forward biased. The only precaution you should observe is with the ohmmeter. Any internal battery may be used in the meter provided that it does not exceed the maximum collector-emitter breakdown voltage.

Figure 2-18. - Testing a transistor's gain with an ohmmeter.

With the switch in figure 2-18 in the open position as shown, no voltage is applied to the PNP transistor's base, and the emitter-base junction is not forward biased. Therefore, the ohmmeter should read a high resistance, as indicated on the meter. When the switch is closed, the emitter-base circuit is forward biased by the voltage across R1 and R2. Current now flows in the emitter-collector circuit, which causes a lower resistance reading on the ohmmeter. A 10-to-1 resistance ratio in this test between meter readings indicates a normal gain for an audio-frequency transistor.

To test an NPN transistor using this circuit, simply reverse the ohmmeter leads and carry out the procedure described earlier.

TRANSISTOR JUNCTION RESISTANCE TEST. - An ohmmeter can be used to test a transistor for leakage (an undesirable flow of current) by measuring the base-emitter, base-collector, and collector-emitter forward and reverse resistances.

For simplicity, consider the transistor under test in each view of figure 2-19 (view A, view Band view C) as two diodes connected back to back. Therefore, each diode will have a low forward resistance and a high reverse resistance. By measuring these resistances with an ohmmeter as shown in the figure, you can determine if the transistor is leaking current through its junctions. When making these measurements, avoid using the R1 scale on the meter or a meter with a high internal battery voltage. Either of these conditions can damage a low-power transistor.

Figure 2-19A. - Testing a transistor's leakage with an ohmmeter.

COLLECTOR-TO-EMITTER TEST

Figure 2-19B. - Testing a transistor's leakage with an ohmmeter.

BASE-TO-COLLECTOR TEST

Figure 2-19C. - Testing a transistor's leakage with an ohmmeter.

BASE-TO-EMITTER TEST

Now consider the possible transistor problems that could exist if the indicated readings in figure 2-19 are not obtained. A list of these problems is provided in table 2-2.

Table 2-2. - Possible Transistor Problems from Ohmmeter Readings

RESISTANCE READINGS

PROBLEMS

FORWARD	REVERSE	The transistor is:
LOW (NOT SHORTED)	LOW (NOT SHORTED)	LEAKING
LOW (SHORTED)	LOW (SHORTED)	SHORTED
HIGH	HIGH	OPEN
**Except collector-to-emitter test.

By now, you should recognize that the transistor used in figure 2-19 (view A, view B and view C) is a PNP transistor. If you wish to test an NPN transistor for leakage, the procedure is identical to that used for testing the PNP except the readings obtained are reversed.

When testing transistors (PNP or NPN), you should remember that the actual resistance values depend on the ohmmeter scale and the battery voltage. Typical forward and reverse resistances are insignificant. The best indicator for showing whether a transistor is good or bad is the ratio of forward-to-reverse resistance. If the transistor you are testing shows a ratio of at least 30 to 1, it is probably good. Many transistors show ratios of 100 to 1 or greater.

Q.38 What safety precaution must be taken before replacing a transistor?
Q.39 How is the collector lead identified on an oval-shaped transistor?
Q.40 What are two transistor tests that can be done with an ohmmeter?
Q.41 When you are testing the gain of an audio-frequency transistor with an ohmmeter, what is indicated by a 10-to-1 resistance ratio?
Q.42 When you are using an ohmmeter to test a transistor for leakage, what is indicated by a low, but not shorted, reverse resistance reading?

TRANSISTOR SPECIFICATIONS

2009-05-29T10:09:00.000-07:00

Transistors are available in a large variety of shapes and sizes, each with its own unique characteristics. The characteristics for each of these transistors are usually presented on SPECIFICATION SHEETS or they may be included in transistor manuals. Although many properties of a transistor could be specified on these sheets, manufacturers list only some of them. The specifications listed vary with different manufacturers, the type of transistor, and the application of the transistor. The specifications usually cover the following items.

A general description of the transistor that includes the following information:

The kind of transistor. This covers the material used, such as germanium or silicon; the type of transistor(NPN or PNP); and the construction of the transistor(whether alloy-junction, grown, or diffused junction, etc.).

Some of the common applications for the transistor, such as audio amplifier, oscillator, rf amplifier, etc.

General sales features, such as size and packaging(mechanical data).

The "Absolute Maximum Ratings" of the transistor are the direct voltage and current values that if exceeded in operation may result in transistor failure. Maximum ratings usually include collector-to-base voltage, emitter-to-base voltage, collector current, emitter current, and collector power dissipation. The typical operating values of the transistor. These values are presented only as a guide. The values vary widely, are dependent upon operating voltages, and also upon which element is common in the circuit. The values listed may include collector-emitter voltage, collector current, input resistance, load resistance, current-transfer ratio(another name for alpha or beta), and collector cutoff current, which is leakage current from collector to base when no emitter current is applied. Transistor characteristic curves may also be included in this section. A transistor characteristic curve is a graph plotting the relationship between currents and voltages in a circuit. More than one curve on a graph is called a "family of curves." Additional information for engineering-design purposes.

So far, many letter symbols, abbreviations, and terms have been introduced, some frequently used and others only rarely used. For a complete list of all semiconductor letter symbols and terms, refer to EIMB series 000-0140, Section III.

TRANSISTOR IDENTIFICATION

Transistors can be identified by a Joint Army-Navy (JAN) designation printed directly on the case of the transistor. The marking scheme explained earlier for diodes is also used for transistor identification. The first number indicates the number of junctions. The letter "N" following the first number tells us that the component is a semiconductor. And, the 2- or 3-digit number following the N is the manufacturer's identification number. If the last number is followed by a letter, it indicates a later, improved version of the device. For example, a semiconductor designated as type 2N130A signifies a three-element transistor of semiconductor material that is an improved version of type 130:

2	N	130
A NUMBER OF JUNCTIONS (TRANSISTOR)	SEMI-CONDUCTOR IDENTIFICATION	NUMBER FIRST MODIFICATION

You may also find other markings on transistors that do not relate to the JAN marking system. These markings are manufacturers' identifications and may not conform to a standardized system. If in doubt, always replace a transistor with one having identical markings. To ensure that an identical replacement or a correct substitute is used, consult an equipment or transistor manual for specifications on the transistor.

TRANSISTOR MAINTENANCE

Transistors are very rugged and are expected to be relatively trouble free. Encapsulation and conformal coating techniques now in use promise extremely long life expectancies. In theory, a transistor should last indefinitely. However, if transistors are subjected to current overloads, the junctions will be damaged or even destroyed. In addition, the application of excessively high operating voltages can damage or destroy the junctions through arc-over or excessive reverse currents. One of the greatest dangers to the transistor is heat, which will cause excessive current flow and eventual destruction of the transistor.

To determine if a transistor is good or bad, you can check it with an ohmmeter or a transistor tester. In many cases, you can substitute a transistor known to be good for one that is questionable and thus determine the condition of a suspected transistor. This method of testing is highly accurate and sometimes the quickest, but it should be used only after you make certain that there are no circuit defects that might damage the replacement transistor. If more than one defective transistor is present in the equipment where the trouble has been localized, this testing method becomes cumbersome, as several transistors may have to be replaced before the trouble is corrected. To determine which stages failed and which transistors are not defective, all the removed transistors must be tested. This test can be made by using a standard Navy ohmmeter, transistor tester, or by observing whether the equipment operates correctly as each of the removed transistors is reinserted into the equipment. A word of caution-indiscriminate substitution of transistors in critical circuits should be avoided.

When transistors are soldered into equipment, substitution is not practicable; it is generally desirable to test these transistors in their circuits.

Q.34 List three items of information normally included in the general description section of a specification sheet for a transistor.
Q.35 What does the number "2" (before the letter "N") indicate in the JAN marking scheme?
Q.36 What is the greatest danger to a transistor?
Q.37 What method for checking transistors is cumbersome when more than one transistor is bad in a circuit?

TRANSISTOR CONFIGURATIONS

2009-05-29T10:08:00.000-07:00

A transistor may be connected in any one of three basic configurations (fig. 2-16): common emitter (CE), common base (CB), and common collector (CC). The term common is used to denote the element that is common to both input and output circuits. Because the common element is often grounded, these configurations are frequently referred to as grounded emitter, grounded base, and grounded collector.

Figure 2-16. - Transistor configurations.

Each configuration, as you will see later, has particular characteristics that make it suitable for specific applications. An easy way to identify a specific transistor configuration is to follow three simple steps:

Identify the element (emitter, base, or collector) to which the input signal is applied.

Identify the element (emitter, base, or collector) from which the output signal is taken.

The remaining element is the common element, and gives the configuration its name.

Therefore, by applying these three simple steps to the circuit in figure 2-12, we can conclude that this circuit is more than just a basic transistor amplifier. It is a common-emitter amplifier.

Common Emitter

The common-emitter configuration (CE) shown in figure 2-16 view A is the arrangement most frequently used in practical amplifier circuits, since it provides good voltage, current, and power gain. The common emitter also has a somewhat low input resistance (500 ohms-1500 ohms), because the input is applied to the forward-biased junction, and a moderately high output resistance (30 kilohms-50 kilohms or more), because the output is taken off the reverse-biased junction. Since the input signal is applied to the base-emitter circuit and the output is taken from the collector-emitter circuit, the emitter is the element common to both input and output.

Since you have already covered what you now know to be a common-emitter amplifier (fig. 2-12), let's take a few minutes and review its operation, using the PNP common-emitter configuration shown in figure 2-16 view A.

When a transistor is connected in a common-emitter configuration, the input signal is injected between the base and emitter, which is a low resistance, low-current circuit. As the input signal swings positive, it also causes the base to swing positive with respect to the emitter. This action decreases forward bias which reduces collector current (I_C) and increases collector voltage (making V_C more negative). During the negative alternation of the input signal, the base is driven more negative with respect to the emitter. This increases forward bias and allows more current carriers to be released from the emitter, which results in an increase in collector current and a decrease in collector voltage (making V_C less negative or swing in a positive direction). The collector current that flows through the high resistance reverse-biased junction also flows through a high resistance load (not shown), resulting in a high level of amplification.

Since the input signal to the common emitter goes positive when the output goes negative, the two signals (input and output) are 180 degrees out of phase. The common-emitter circuit is the only configuration that provides a phase reversal.

The common-emitter is the most popular of the three transistor configurations because it has the best combination of current and voltage gain. The term GAIN is used to describe the amplification capabilities of the amplifier. It is basically a ratio of output versus input. Each transistor configuration gives a different value of gain even though the same transistor is used. The transistor configuration used is a matter of design consideration. However, as a technician you will become interested in this output versus input ratio (gain) to determine whether or not the transistor is working properly in the circuit.

The current gain in the common-emitter circuit is called BETA (b). Beta is the relationship of collector current (output current) to base current (input current). To calculate beta, use the following formula:

(D is the Greek letter delta, it is used to indicate a small change)

For example, if the input current (I_B) in a common emitter changes from 75 mA to 100 mA and the output current (I_C) changes from 1.5 mA to 2.6 mA, the current gain (b) will be 44.

This simply means that a change in base current produces a change in collector current which is 44 times as large.

You may also see the term h_fe used in place of b. The terms h_fe and b are equivalent and may be used interchangeably. This is because "h_fe" means: h = hybrid (meaning mixture)

f = forward current transfer ratio
e = common emitter configuration

The resistance gain of the common emitter can be found in a method similar to the one used for finding beta:

Once the resistance gain is known, the voltage gain is easy to calculate since it is equal to the current gain (b) multiplied by the resistance gain (E = bR). And, the power gain is equal to the voltage gain multiplied by the current gain b (P = bE).

Common Base

The common-base configuration (CB) shown in figure 2-16, view B is mainly used for impedance matching, since it has a low input resistance (30 ohms-160 ohms) and a high output resistance (250 kilohms-550 kilohms). However, two factors limit its usefulness in some circuit applications: (1) its low input resistance and (2) its current gain of less than 1. Since the CB configuration will give voltage amplification, there are some additional applications, which require both a low-input resistance and voltage amplification, that could use a circuit configuration of this type; for example, some microphone amplifiers.

In the common-base configuration, the input signal is applied to the emitter, the output is taken from the collector, and the base is the element common to both input and output. Since the input is applied to the emitter, it causes the emitter-base junction to react in the same manner as it did in the common-emitter circuit. For example, an input that aids the bias will increase transistor current, and one that opposes the bias will decrease transistor current.

Unlike the common-emitter circuit, the input and output signals in the common-base circuit are in phase. To illustrate this point, assume the input to the PNP version of the common-base circuit in figure 2-16 view B is positive. The signal adds to the forward bias, since it is applied to the emitter, causing the collector current to increase. This increase in Ic results in a greater voltage drop across the load resistor R_L (not shown), thus lowering the collector voltage V_C. The collector voltage, in becoming less negative, is swinging in a positive direction, and is therefore in phase with the incoming positive signal.

The current gain in the common-base circuit is calculated in a method similar to that of the common emitter except that the input current is I_E not I_B and the term ALPHA (a) is used in place of beta for gain. Alpha is the relationship of collector current (output current) to emitter current (input current). Alpha is calculated using the formula:

For example, if the input current (I_E) in a common base changes from 1 mA to 3 mA and the output current (I_C) changes from 1 mA to 2.8 mA, the current gain (a) will be 0.90 or:

This is a current gain of less than 1.

Since part of the emitter current flows into the base and does not appear as collector current, collector current will always be less than the emitter current that causes it. (Remember, I_E= I_B + I_C) Therefore, ALPHA is ALWAYS LESS THAN ONE FOR A COMMON-BASE CONFIGURATION.

Another term for "a" is h_fb. These terms (and h_fb) are equivalent and may be used interchangeably. The meaning for the term h_fb is derived in the same manner as the term h_fe mentioned earlier, except that the last letter "e" has been replaced with "b" to stand for common- base configuration.

Many transistor manuals and data sheets only list transistor current gain characteristics in terms of b or h_fe. To find alpha (a) when given beta (b), use the following formula to convert b to a for use with the common-base configuration:

To calculate the other gains (voltage and power) in the common-base configuration when the current gain (a) is known, follow the procedures described earlier under the common-emitter section.

Common Collector

The common-collector configuration (CC) shown in figure 2-16 view C is used mostly for impedance matching. It is also used as a current driver, because of its substantial current gain. It is particularly useful in switching circuitry, since it has the ability to pass signals in either direction (bilateral operation).

In the common-collector circuit, the input signal is applied to the base, the output is taken from the emitter, and the collector is the element common to both input and output. The common collector is equivalent to our old friend the electron-tube cathode follower. Both have high input and low output resistance. The input resistance for the common collector ranges from 2 kilohms to 500 kilohms, and the output resistance varies from 50 ohms to 1500 ohms. The current gain is higher than that in the common emitter, but it has a lower power gain than either the common base or common emitter. Like the common base, the output signal from the common collector is in phase with the input signal. The common collector is also referred to as an emitter-follower because the output developed on the emitter follows the input signal applied to the base.

Transistor action in the common collector is similar to the operation explained for the common base, except that the current gain is not based on the emitter-to-collector current ratio, alpha (a). Instead, it is based on the emitter-to-base current ratio called GAMMA (g), because the output is taken off the emitter. Since a small change in base current controls a large change in emitter current, it is still possible to obtain high current gain in the common collector. However, since the emitter current gain is offset by the low output resistance, the voltage gain is always less than 1 (unity), exactly as in the electron-tube cathode follower

The common-collector current gain, gamma (g), is defined as

and is related to collector-to-base current gain, beta (b), of the common-emitter circuit by the formula:

Since a given transistor may be connected in any of three basic configurations, there is a definite relationship, as pointed out earlier, between alpha (a), beta (b), and gamma (g). These relationships are listed again for your convenience:

Take, for example, a transistor that is listed on a manufacturer's data sheet as having an alpha of 0.90. We wish to use it in a common emitter configuration. This means we must find beta. The calculations are:

Therefore, a change in base current in this transistor will produce a change in collector current that will be 9 times as large.

If we wish to use this same transistor in a common collector, we can find gamma (g) by:

To summarize the properties of the three transistor configurations, a comparison chart is provided in table 2-1 for your convenience.

Table 2-1. - Transistor Configuration Comparison Chart

AMPLIFIER TYPE	COMMON BASE	COMMON EMITTER	COMMON COLLECTOR
INPUT/OUTPUT PHASE RELATIONSHIP	0°	180°	0°
VOLTAGE GAIN	HIGH	MEDIUM	LOW
CURRENT GAIN	LOW(a)	MEDIUM(b)	HIGH(g)
POWER GAIN	LOW	HIGH	MEDIUM
INPUT RESISTANCE	LOW	MEDIUM	HIGH
OUTPUT RESISTANCE	HIGH	MEDIUM	LOW

Now that we have analyzed the basic transistor amplifier in terms of bias, class of operation, and circuit configuration, let's apply what has been covered to figure 2-12. A reproduction of figure 2-12 is shown below for your convenience.

This illustration is not just the basic transistor amplifier shown earlier in figure 2-12 but a class A amplifier configured as a common emitter using fixed bias. From this, you should be able to conclude the following:

Because of its fixed bias, the amplifier is thermally unstable.

Because of its class A operation, the amplifier has low efficiency but good fidelity.

Because it is configured as a common emitter, the amplifier has good voltage, current, and power gain.

In conclusion, the type of bias, class of operation, and circuit configuration are all clues to the function and possible application of the amplifier.

Q.26 What are the three transistor configurations?
Q.27 Which transistor configuration provides a phase reversal between the input and output signals?
Q.28 What is the input current in the common-emitter circuit?
Q.29 What is the current gain in a common-base circuit called?
Q.30 Which transistor configuration has a current gain of less than 1?
Q.31 What is the output current in the common-collector circuit?
Q.32 Which transistor configuration has the highest input resistance?
Q.33 What is the formula for GAMMA (g)?

AMPLIFIER CLASSES OF OPERATION

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In the previous discussions, we assumed that for every portion of the input signal there was an output from the amplifier. This is not always the case with amplifiers. It may be desirable to have the transistor conducting for only a portion of the input signal. The portion of the input for which there is an output determines the class of operation of the amplifier. There are four classes of amplifier operations. They are class A, class AB, class B, and class C.

Class A Amplifier Operation

Class A amplifiers are biased so that variations in input signal polarities occur within the limits of CUTOFF and SATURATION. In a PNP transistor, for example, if the base becomes positive with respect to the emitter, holes will be repelled at the PN junction and no current can flow in the collector circuit. This condition is known as cutoff. Saturation occurs when the base becomes so negative with respect to the emitter that changes in the signal are not reflected in collector-current flow.

Biasing an amplifier in this manner places the dc operating point between cutoff and saturation and allows collector current to flow during the complete cycle (360 degrees) of the input signal, thus providing an output which is a replica of the input. Figure 2-12 is an example of a class A amplifier. Although the output from this amplifier is 180 degrees out of phase with the input, the output current still flows for the complete duration of the input.

The class A operated amplifier is used as an audio- and radio-frequency amplifier in radio, radar, and sound systems, just to mention a few examples.

For a comparison of output signals for the different amplifier classes of operation, refer to figure 2-15 during the following discussion.

Figure 2-15. - A comparison of output signals for the different amplifier classes of operation.

Class AB Amplifier Operation Amplifiers designed for class AB operation are biased so that collector current is zero (cutoff) for a portion of one alternation of the input signal. This is accomplished by making the forward-bias voltage less than the peak value of the input signal. By doing this, the base-emitter junction will be reverse biased during one alternation for the amount of time that the input signal voltage opposes and exceeds the value of forward-bias voltage. Therefore, collector current will flow for more than 180 degrees but less than 360 degrees of the input signal, as shown in figure 2-15 view B. As compared to the class A amplifier, the dc operating point for the class AB amplifier is closer to cutoff.

The class AB operated amplifier is commonly used as a push-pull amplifier to overcome a side effect of class B operation called crossover distortion.

Class B Amplifier Operation

Amplifiers biased so that collector current is cut off during one-half of the input signal are classified class B. The dc operating point for this class of amplifier is set up so that base current is zero with no input signal. When a signal is applied, one half cycle will forward bias the base-emitter junction and I_C will flow. The other half cycle will reverse bias the base-emitter junction and I_C will be cut off. Thus, for class B operation, collector current will flow for approximately 180 degrees (half) of the input signal, as shown in figure 2-15 view C.

The class B operated amplifier is used extensively for audio amplifiers that require high-power outputs. It is also used as the driver- and power-amplifier stages of transmitters.

Class C Amplifier Operation

In class C operation, collector current flows for less than one half cycle of the input signal, as shown in figure 2-15 view D. The class C operation is achieved by reverse biasing the emitter-base junction, which sets the dc operating point below cutoff and allows only the portion of the input signal that overcomes the reverse bias to cause collector current flow.

The class C operated amplifier is used as a radio-frequency amplifier in transmitters.

From the previous discussion, you can conclude that two primary items determine the class of operation of an amplifier - (1) the amount of bias and (2) the amplitude of the input signal. With a given input signal and bias level, you can change the operation of an amplifier from class A to class B just by removing forward bias. Also, a class A amplifier can be changed to class AB by increasing the input signal amplitude. However, if an input signal amplitude is increased to the point that the transistor goes into saturation and cutoff, it is then called an OVERDRIVEN amplifier.

You should be familiar with two terms used in conjunction with amplifiers - FIDELITY and EFFICIENCY. Fidelity is the faithful reproduction of a signal. In other words, if the output of an amplifier is just like the input except in amplitude, the amplifier has a high degree of fidelity. The opposite of fidelity is a term we mentioned earlier - distortion. Therefore, a circuit that has high fidelity has low distortion. In conclusion, a class A amplifier has a high degree of fidelity. A class AB amplifier has less fidelity, and class B and class C amplifiers have low or "poor" fidelity.

The efficiency of an amplifier refers to the ratio of output-signal power compared to the total input power. An amplifier has two input power sources: one from the signal, and one from the power supply. Since every device takes power to operate, an amplifier that operates for 360 degrees of the input signal uses more power than if operated for 180 degrees of the input signal. By using more power, an amplifier has less power available for the output signal; thus the efficiency of the amplifier is low. This is the case with the class A amplifier. It operates for 360 degrees of the input signal and requires a relatively large input from the power supply. Even with no input signal, the class A amplifier still uses power from the power supply. Therefore, the output from the class A amplifier is relatively small compared to the total input power. This results in low efficiency, which is acceptable in class A amplifiers because they are used where efficiency is not as important as fidelity.

Class AB amplifiers are biased so that collector current is cut off for a portion of one alternation of the input, which results in less total input power than the class A amplifier. This leads to better efficiency.

Class B amplifiers are biased with little or no collector current at the dc operating point. With no input signal, there is little wasted power. Therefore, the efficiency of class B amplifiers is higher still.

The efficiency of class C is the highest of the four classes of amplifier operations.

Q.22 What amplifier class of operation allows collector current to flow during the complete cycle of the input?
Q.23 What is the name of the term used to describe the condition in a transistor when the emitter-base junction has zero bias or is reverse biased and there is no collector current?
Q.24 What two primary items determine the class of operation of an amplifier?
Q.25 What amplifier class of operation is the most inefficient but has the least distortion?

BIAS TYPES OF TRANSISTOR

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One of the basic problems with transistor amplifiers is establishing and maintaining the proper values of quiescent current and voltage in the circuit. This is accomplished by selecting the proper circuit-biasing conditions and ensuring these conditions are maintained despite variations in ambient (surrounding) temperature, which cause changes in amplification and even distortion (an unwanted change in a signal). Thus a need arises for a method to properly bias the transistor amplifier and at the same time stabilize its dc operating point (the no signal values of collector voltage and collector current). As mentioned earlier, various biasing methods can be used to accomplish both of these functions. Although there are numerous biasing methods, only three basic types will be considered.

Base-Current Bias (Fixed Bias)

The first biasing method, called BASE CURRENT BIAS or sometimes FIXED BIAS, was used in figure 2-12. As you recall, it consisted basically of a resistor (R_B) connected between the collector supply voltage and the base. Unfortunately, this simple arrangement is quite thermally unstable. If the temperature of the transistor rises for any reason (due to a rise in ambient temperature or due to current flow through it), collector current will increase. This increase in current also causes the dc operating point, sometimes called the quiescent or static point, to move away from its desired position (level). This reaction to temperature is undesirable because it affects amplifier gain (the number of times of amplification) and could result in distortion, as you will see later in this discussion.

Self-Bias

A better method of biasing is obtained by inserting the bias resistor directly between the base and collector, as shown in figure 2-13. By tying the collector to the base in this manner, feedback voltage can be fed from the collector to the base to develop forward bias. This arrangement is called SELF-BIAS. Now, if an increase of temperature causes an increase in collector current, the collector voltage (V_C) will fall because of the increase of voltage produced across the load resistor (R_L). This drop in V_C will be fed back to the base and will result in a decrease in the base current. The decrease in base current will oppose the original increase in collector current and tend to stabilize it. The exact opposite effect is produced when the collector current decreases.

Figure 2-13. - A basic transistor amplifier with self-bias.

Self-bias has two small drawbacks: (1) It is only partially effective and, therefore, is only used where moderate changes in ambient temperature are expected; (2) it reduces amplification since the signal on the collector also affects the base voltage. This is because the collector and base signals for this particular amplifier configuration are 180 degrees out of phase (opposite in polarity) and the part of the collector signal that is fed back to the base cancels some of the input signal. This process of returning a part of the output back to its input is known as DEGENERATION or NEGATIVE FEEDBACK. Sometimes degeneration is desired to prevent amplitude distortion (an output signal that fails to follow the input exactly) and self-bias may be used for this purpose.

Combination Bias

A combination of fixed and self-bias can be used to improve stability and at the same time overcome some of the disadvantages of the other two biasing methods. One of the most widely used combination-bias systems is the voltage-divider type shown in figure 2-14. Fixed bias is provided in this circuit by the voltage-divider network consisting of R1, R2, and the collector supply voltage (V_CC). The dc current flowing through the voltage-divider network biases the base positive with respect to the emitter. Resistor R3, which is connected in series with the emitter, provides the emitter with self-bias. Should I_E increase, the voltage drop across R3 would also increase, reducing V_C. This reaction to an increase in I_E by R3 is another form of degeneration, which results in less output from the amplifier. However, to provide long-term or dc thermal stability, and at the same time, allow minimal ac signal degeneration, the bypass capacitor (C_bp) is placed across R3. If C_bp is large enough, rapid signal variations will not change its charge materially and no degeneration of the signal will occur.

Figure 2-14. - A basic transistor amplifier with combination bias.

In summary, the fixed-bias resistors, R1 and R2, tend to keep the base bias constant while the emitter bias changes with emitter conduction. This action greatly improves thermal stability and at the same time maintains the correct operating point for the transistor.

Q.18 Which biasing method is the most unstable?
Q.19 What type of bias is used where only moderate changes in ambient temperature are expected?
Q.20 When is degeneration tolerable in an amplifier?
Q.21 What is the most widely used combination-bias system?

THE BASIC TRANSISTOR AMPLIFIER

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In the preceding pages we explained the internal workings of the transistor and introduced new terms, such as emitter, base, and collector. Since you should be familiar by now with all of the new terms mentioned earlier and with the internal operation of the transistor, we will move on to the basic transistor amplifier.

To understand the overall operation of the transistor amplifier, you must only consider the current in and out of the transistor and through the various components in the circuit. Therefore, from this point on, only the schematic symbol for the transistor will be used in the illustrations, and rather than thinking about majority and minority carriers, we will now start thinking in terms of emitter, base, and collector current.

Before going into the basic transistor amplifier, there are two terms you should be familiar with: AMPLIFICATION and AMPLIFIER. Amplification is the process of increasing the strength of a SIGNAL. A signal is just a general term used to refer to any particular current, voltage, or power in a circuit. An amplifier is the device that provides amplification (the increase in current, voltage, or power of a signal) without appreciably altering the original signal.

Transistors are frequently used as amplifiers. Some transistor circuits are CURRENT amplifiers, with a small load resistance; other circuits are designed for VOLTAGE amplification and have a high load resistance; others amplify POWER.

Now take a look at the NPN version of the basic transistor amplifier in figure 2-12 and let's see just how it works.

So far in this discussion, a separate battery has been used to provide the necessary forward-bias voltage. Although a separate battery has been used in the past for convenience, it is not practical to use a battery for emitter-base bias. For instance, it would take a battery slightly over .2 volts to properly forward bias a germanium transistor, while a similar silicon transistor would require a voltage slightly over .6 volts. However, common batteries do not have such voltage values. Also, since bias voltages are quite critical and must be held within a few tenths of one volt, it is easier to work with bias currents flowing through resistors of high ohmic values than with batteries.

By inserting one or more resistors in a circuit, different methods of biasing may be achieved and the emitter-base battery eliminated. In addition to eliminating the battery, some of these biasing methods compensate for slight variations in transistor characteristics and changes in transistor conduction resulting from temperature irregularities. Notice in figure 2-12 that the emitter-base battery has been eliminated and the bias resistor R_B has been inserted between the collector and the base. Resistor R_B provides the necessary forward bias for the emitter-base junction. Current flows in the emitter-base bias circuit from ground to the emitter, out the base lead, and through R_B to V_CC. Since the current in the base circuit is very small (a few hundred microamperes) and the forward resistance of the transistor is low, only a few tenths of a volt of positive bias will be felt on the base of the transistor. However, this is enough voltage on the base, along with ground on the emitter and the large positive voltage on the collector, to properly bias the transistor.

Figure 2-12. - The basic transistor amplifier.

With Q1 properly biased, direct current flows continuously, with or without an input signal, throughout the entire circuit. The direct current flowing through the circuit develops more than just base bias; it also develops the collector voltage (V_C) as it flows through Q1 and R_L. Notice the collector voltage on the output graph. Since it is present in the circuit without an input signal, the output signal starts at the V_C level and either increases or decreases. These dc voltages and currents that exist in the circuit before the application of a signal are known as QUIESCENT voltages and currents (the quiescent state of the circuit).

Resistor R_L, the collector load resistor, is placed in the circuit to keep the full effect of the collector supply voltage off the collector. This permits the collector voltage (V_C) to change with an input signal, which in turn allows the transistor to amplify voltage. Without R_L in the circuit, the voltage on the collector would always be equal to V_CC.

The coupling capacitor (C_C) is another new addition to the transistor circuit. It is used to pass the ac input signal and block the dc voltage from the preceding circuit. This prevents dc in the circuitry on the left of the coupling capacitor from affecting the bias on Q1. The coupling capacitor also blocks the bias of Q1 from reaching the input signal source.

The input to the amplifier is a sine wave that varies a few millivolts above and below zero. It is introduced into the circuit by the coupling capacitor and is applied between the base and emitter. As the input signal goes positive, the voltage across the emitter-base junction becomes more positive. This in effect increases forward bias, which causes base current to increase at the same rate as that of the input sine wave. Emitter and collector currents also increase but much more than the base current. With an increase in collector current, more voltage is developed across R_L. Since the voltage across R_L and the voltage across Q1 (collector to emitter) must add up to V_CC, an increase in voltage across R_L results in an equal decrease in voltage across Q1. Therefore, the output voltage from the amplifier, taken at the collector of Q1 with respect to the emitter, is a negative alternation of voltage that is larger than the input, but has the same sine wave characteristics.

During the negative alternation of the input, the input signal opposes the forward bias. This action decreases base current, which results in a decrease in both emitter and collector currents. The decrease in current through R_L decreases its voltage drop and causes the voltage across the transistor to rise along with the output voltage. Therefore, the output for the negative alternation of the input is a positive alternation of voltage that is larger than the input but has the same sine wave characteristics.

By examining both input and output signals for one complete alternation of the input, we can see that the output of the amplifier is an exact reproduction of the input except for the reversal in polarity and the increased amplitude (a few millivolts as compared to a few volts).

The PNP version of this amplifier is shown in the upper part of the figure. The primary difference between the NPN and PNP amplifier is the polarity of the source voltage. With a negative V_CC, the PNP base voltage is slightly negative with respect to ground, which provides the necessary forward bias condition between the emitter and base.

When the PNP input signal goes positive, it opposes the forward bias of the transistor. This action cancels some of the negative voltage across the emitter-base junction, which reduces the current through the transistor. Therefore, the voltage across the load resistor decreases, and the voltage across the transistor increases. Since V_CC is negative, the voltage on the collector (V_C) goes in a negative direction (as shown on the output graph) toward -V_CC (for example, from -5 volts to -7 volts). Thus, the output is a negative alternation of voltage that varies at the same rate as the sine wave input, but it is opposite in polarity and has a much larger amplitude .

During the negative alternation of the input signal, the transistor current increases because the input voltage aids the forward bias. Therefore, the voltage across R_L increases, and consequently, the voltage across the transistor decreases or goes in a positive direction (for example: from -5 volts to -3 volts). This action results in a positive output voltage, which has the same characteristics as the input except that it has been amplified and the polarity is reversed.

In summary, the input signals in the preceding circuits were amplified because the small change in base current caused a large change in collector current. And, by placing resistor R_L in series with the collector, voltage amplification was achieved.

Q.14 What is the name of the device that provides an increase in current, voltage, or power of a signal without appreciably altering the original signal?
Q.15 Besides eliminating the emitter-base battery, what other advantages can different biasing methods offer?
Q.16 In the basic transistor amplifier discussed earlier, what is the relationship between the polarity of the input and output signals?
Q.17 What is the primary difference between the NPN and PNP amplifiers?

PNP Transistor Operation

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The PNP transistor works essentially the same as the NPN transistor. However, since the emitter, base, and collector in the PNP transistor are made of materials that are different from those used in the NPN transistor, different current carriers flow in the PNP unit. The majority current carriers in the PNP transistor are holes. This is in contrast to the NPN transistor where the majority current carriers are electrons. To support this different type of current (hole flow), the bias batteries are reversed for the PNP transistor. A typical bias setup for the PNP transistor is shown in figure 2-8.

Notice that the procedure used earlier to properly bias the NPN transistor also applies here to the PNP transistor. The first letter (P) in the PNP sequence indicates the polarity of the voltage required for the emitter (positive), and the second letter (N) indicates the polarity of the base voltage ( negative). Since the base-collector junction is always reverse biased, then the opposite polarity voltage (negative) must be used for the collector. Thus, the base of the PNP transistor must be negative with respect to the emitter, and the collector must be more negative than the base. Remember, just as in the case of the NPN transistor, this difference in supply voltage is necessary to have current flow (hole flow in the case of the PNP transistor) from the emitter to the collector. Although hole flow is the predominant type of current flow in the PNP transistor, hole flow only takes place within the transistor itself, while electrons flow in the external circuit. However, it is the internal hole flow that leads to electron flow in the external wires connected to the transistor.

Figure 2-8. - A properly biased PNP transistor.

PNP FORWARD-BIASED JUNCTION. - Now let us consider what happens when the emitter-base junction in figure 2-9 is forward biased. With the bias setup shown, the positive terminal of the battery repels the emitter holes toward the base, while the negative terminal drives the base electrons toward the emitter. When an emitter hole and a base electron meet, they combine. For each electron that combines with a hole, another electron leaves the negative terminal of the battery, and enters the base. At the same time, an electron leaves the emitter, creating a new hole, and enters the positive terminal of the battery. This movement of electrons into the base and out of the emitter constitutes base current flow (I_B), and the path these electrons take is referred to as the emitter-base circuit.

Figure 2-9. - The forward-biased junction in a PNP transistor.

PNP REVERSE-BIASED JUNCTION. - In the reverse-biased junction (fig. 2-10), the negative voltage on the collector and the positive voltage on the base block the majority current carriers from crossing the junction.

However, this same negative collector voltage acts as forward bias for the minority current holes in the base, which cross the junction and enter the collector. The minority current electrons in the collector also sense forward bias-the positive base voltage-and move into the base. The holes in the collector are filled by electrons that flow from the negative terminal of the battery. At the same time the electrons leave the negative terminal of the battery, other electrons in the base break their covalent bonds and enter the positive terminal of the battery. Although there is only minority current flow in the reverse-biased junction, it is still very small because of the limited number of minority current carriers.

Figure 2-10. - The reverse-biased junction in a PNP transistor.

PNP JUNCTION INTERACTION. - The interaction between the forward- and reverse-biased junctions in a PNP transistor is very similar to that in an NPN transistor, except that in the PNP transistor, the majority current carriers are holes. In the PNP transistor shown in figure 2-11, the positive voltage on the emitter repels the holes toward the base. Once in the base, the holes combine with base electrons. But again, remember that the base region is made very thin to prevent the recombination of holes with electrons. Therefore, well over 90 percent of the holes that enter the base become attracted to the large negative collector voltage and pass right through the base. However, for each electron and hole that combine in the base region, another electron leaves the negative terminal of the base battery (V_BB) and enters the base as base current (I_B). At the same time an electron leaves the negative terminal of the battery, another electron leaves the emitter as IE (creating a new hole) and enters the positive terminal of V_BB. Meanwhile, in the collector circuit, electrons from the collector battery (V_CC) enter the collector as Ic and combine with the excess holes from the base. For each hole that is neutralized in the collector by an electron, another electron leaves the emitter and starts its way back to the positive terminal of V_CC.

Figure 2-11. - PNP transistor operation.

Although current flow in the external circuit of the PNP transistor is opposite in direction to that of the NPN transistor, the majority carriers always flow from the emitter to the collector. This flow of majority carriers also results in the formation of two individual current loops within each transistor. One loop is the base-current path, and the other loop is the collector-current path. The combination of the current in both of these loops (I_B+ I_C) results in total transistor current (I_E). The most important thing to remember about the two different types of transistors is that the emitter-base voltage of the PNP transistor has the same controlling effect on collector current as that of the NPN transistor. In simple terms, increasing the forward-bias voltage of a transistor reduces the emitter-base junction barrier. This action allows more carriers to reach the collector, causing an increase in current flow from the emitter to the collector and through the external circuit. Conversely, a decrease in the forward-bias voltage reduces collector current.

Q.10 What are the majority current carriers in a PNP transistor?
Q.11 What is the relationship between the polarity of the voltage applied to the PNP transistor and that applied to the NPN transistor?
Q.12 What is the letter designation for base current?
Q.13 Name the two current loops in a transistor.

TRANSISTOR THEORY

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You should recall from an earlier discussion that a forward-biased

PN junction is comparable to a low-resistance circuit element because it passes a high current for a given voltage. In turn, a reverse-biased PN junction is comparable to a high-resistance circuit element. By using the Ohm's law formula for power (P = I²R) and assuming current is held constant, you can conclude that the power developed across a high resistance is greater than that developed across a low resistance. Thus, if a crystal were to contain two PN junctions (one forward-biased and the other reverse-biased), a low-power signal could be injected into the forward-biased junction and produce a high-power signal at the reverse-biased junction. In this manner, a power gain would be obtained across the crystal. This concept, which is merely an extension of the material covered in chapter 1, is the basic theory behind how the transistor amplifies. With this information fresh in your mind, let's proceed directly to the NPN transistor.

NPN Transistor Operation

Just as in the case of the PN junction diode, the N material comprising the two end sections of the NP N transistor contains a number of free electrons, while the center P section contains an excess number of holes. The action at each junction between these sections is the same as that previously described for the diode; that is, depletion regions develop and the junction barrier appears. To use the transistor as an amplifier, each of these junctions must be modified by some external bias voltage. For the transistor to function in this capacity, the first PN junction (emitter-base junction) is biased in the forward, or low-resistance, direction. At the same time the second PN junction (base-collector junction) is biased in the reverse, or high-resistance, direction. A simple way to remember how to properly bias a transistor is to observe the NPN or PNP elements that make up the transistor. The letters of these elements indicate what polarity voltage to use for correct bias. For instance, notice the NPN transistor below:

The emitter, which is the first letter in the NPN sequence, is connected to the negative side of the battery while the base, which is the second letter(NPN), is connected to the positive side. However, since the second PN junction is required to be reverse biased for proper transistor operation, the collector must be connected to an opposite polarity voltage(positive) than that indicated by its letter designation(NPN). The voltage on the collector must also be more positive than the base, as shown below:

We now have a properly biased NPN transistor.

In summary, the base of the NPN transistor must be positive with respect to the emitter, and the collector must be more positive than the base.

NPN FORWARD-BIASED JUNCTION. - An important point to bring out at this time, which was not necessarily mentioned during the explanation of the diode, is the fact that the N material on one side of the forward-biased junction is more heavily doped than the P material. This results in more current being carried across the junction by the majority carrier electrons from the N material than the majority carrier holes from the P material. Therefore, conduction through the forward-biased junction, as shown in figure 2-5, is mainly by majority carrier electrons from the N material (emitter).

Figure 2-5. - The forward-biased junction in an NPN transistor.

With the emitter-to-base junction in the figure biased in the forward direction, electrons leave the negative terminal of the battery and enter the N material (emitter). Since electrons are majority current carriers in the N material, they pass easily through the emitter, cross over the junction, and combine with holes in the P material (base). For each electron that fills a hole in the P material, another electron will leave the P material (creating a new hole) and enter the positive terminal of the battery.

NPN REVERSE-BIASED JUNCTION. - The second PN junction (base-to-collector), or reverse-biased junction as it is called (fig. 2-6), blocks the majority current carriers from crossing the junction. However, there is a very small current, mentioned earlier, that does pass through this junction. This current is called minority current, or reverse current. As you recall, this current was produced by the electron-hole pairs. The minority carriers for the reverse-biased PN junction are the electrons in the P material and the holes in the N material. These minority carriers actually conduct the current for the reverse-biased junction when electrons from the P material enter the N material, and the holes from the N material enter the P material. However, the minority current electrons (as you will see later) play the most important part in the operation of the NPN transistor.

Figure 2-6. - The reverse-biased junction in an NPN transistor.

At this point you may wonder why the second PN junction (base-to-collector) is not forward biased like the first PN junction (emitter-to-base). If both junctions were forward biased, the electrons would have a tendency to flow from each end section of the N P N transistor (emitter and collector) to the center P section (base). In essence, we would have two junction diodes possessing a common base, thus eliminating any amplification and defeating the purpose of the transistor. A word of caution is in order at this time. If you should mistakenly bias the second PN junction in the forward direction, the excessive current could develop enough heat to destroy the junctions, making the transistor useless. Therefore, be sure your bias voltage polarities are correct before making any electrical connections.

NPN JUNCTION INTERACTION. - We are now ready to see what happens when we place the two junctions of the NPN transistor in operation at the same time. For a better understanding of just how the two junctions work together, refer to figure 2-7 during the discussion.

Figure 2-7. - NPN transistor operation.

The bias batteries in this figure have been labeled V_CC for the collector voltage supply, and V_BB for the base voltage supply. Also notice the base supply battery is quite small, as indicated by the number of cells in the battery, usually 1 volt or less. However, the collector supply is generally much higher than the base supply, normally around 6 volts. As you will see later, this difference in supply voltages is necessary to have current flow from the emitter to the collector.

As stated earlier, the current flow in the external circuit is always due to the movement of free electrons. Therefore, electrons flow from the negative terminals of the supply batteries to the N-type emitter. This combined movement of electrons is known as emitter current (I_E). Since electrons are the majority carriers in the N material, they will move through the N material emitter to the emitter-base junction. With this junction forward biased, electrons continue on into the base region. Once the electrons are in the base, which is a P-type material, they become minority carriers. Some of the electrons that move into the base recombine with available holes. For each electron that recombines, another electron moves out through the base lead as base current I_B (creating a new hole for eventual combination) and returns to the base supply battery V

_BB. The electrons that recombine are lost as far as the collector is concerned. Therefore, to make the transistor more efficient, the base region is made very thin and lightly doped. This reduces the opportunity for an electron to recombine with a hole and be lost. Thus, most of the electrons that move into the base region come under the influence of the large collector reverse bias. This bias acts as forward bias for the minority carriers (electrons) in the base and, as such, accelerates them through the base-collector junction and on into the collector region. Since the collector is made of an N-type material, the electrons that reach the collector again become majority current carriers. Once in the collector, the electrons move easily through the N material and return to the positive terminal of the collector supply battery V_CC as collector current (I_C).

To further improve on the efficiency of the transistor, the collector is made physically larger than the base for two reasons: (1) to increase the chance of collecting carriers that diffuse to the side as well as directly across the base region, and (2) to enable the collector to handle more heat without damage.

In summary, total current flow in the NPN transistor is through the emitter lead. Therefore, in terms of percentage, I_E is 100 percent. On the other hand, since the base is very thin and lightly doped, a smaller percentage of the total current (emitter current) will flow in the base circuit than in the collector circuit. Usually no more than 2 to 5 percent of the total current is base current (I_B) while the remaining 95 to 98 percent is collector current (I_C). A very basic relationship exists between these two currents:

I_E = I_B + I_C

In simple terms this means that the emitter current is separated into base and collector current. Since the amount of current leaving the emitter is solely a function of the emitter-base bias, and because the collector receives most of this current, a small change in emitter-base bias will have a far greater effect on the magnitude of collector current than it will have on base current. In conclusion, the relatively small emitter-base bias controls the relatively large emitter-to-collector current.

Q.6 To properly bias an NPN transistor, what polarity voltage is applied to the collector, and what is its relationship to the base voltage?
Q.7 Why is conduction through the forward-biased junction of an NPN transistor primarily in one direction, namely from the emitter to base?
Q.8 In the NPN transistor, what section is made very thin compared with the other two sections?
Q.9 What percentage of current in an NPN transistor reaches the collector?

Storm: Great expectations unmet

2009-05-29T09:46:00.001-07:00

Zatni Arbi , Contributor , Jakarta | Mon, 05/25/2009 2:39 PM | Sci-Tech

One of the problems caused by the Internet is that consumers and IT journalists usually know about the arrival of a new product long before it becomes available on the local market.

"Bloggers, too, are always several steps ahead in their information gathering," my dear friend Budiputra recently told me, and how right he was.

On the Internet we can read about a product being launched in its home country. If the product has great reviews, we may hold our purchase plan and wait until it becomes available. But, on the other hand, if it is overpromoted, it may backfire because our expectations are too great.

When the BlackBerry Curve 8900, aka BlackBerry Javelin, and the Pearl Flip 8220 were launched in Jakarta, we were aware that the touch screen-based BlackBerry Storm was already available in the United States.

The good news is that the Javelin seems to have been well-received by consumers here. It should be, as it is a great product. The not-so-good news is that we had great expectations for the Storm.

The Storm has a lot of potential, of course. First, it is the first touch-based smartphone that incorporates Research in Motion's (RIM) award-winning SurePress technology. To activate a button on the screen, you place your finger on it and then you press the entire screen panel down. It is very much like the multi-touch glass touchpad on the Unibody MacBook, on which we have to press down to register a click. As I mentioned in one of my past reviews, RIM received a prestigious award from GSM Association during the Mobile Congress in Barcelona earlier this year.

Courtesy of BlackBerry Asia Pacific, I had a couple of days to play around with a Storm around one month ago. The SurePress technology surely impressed me, but whether it is the right technology for an e-mail-centric smartphone is a different question.

The BlackBerrys have of course evolved over the years to include dozens of features and applications. Today, if you have a BlackBerry, you can download applications for news and weather, music and video, social networking and sharing, travel, maps and navigation, and even games.

However, when people buy a BlackBerry, they have two possible reasons. First, people want it as a status symbol, and that is the reason they even go as far as to buy a "black market" BlackBerry in places such as Roxy Mas, Jakarta. Second, people need it because they are heavy email users, and they find the BlackBerry to be the best device to support their business activities. These people usually obtain their BlackBerry and the email service from the operators and their appointed distributors - the way it should actually work.

So, emailing has remained the driving force behind all the BlackBerrys. Now, to create email or to respond to email quickly, nothing beats the real QWERTY keypad like the one found on the other BlackBerrys, on the Nokia Communicator Series, on a growing number of Nokia E-Series, on Palm smartphones, etc.

During my brief test, it became clear that the SurePress approach required some learning. It was not as quick to master as a hardware keypad. I guess, no matter how much time I spend learning to use a touch screen to enter my text I will never be able to do it faster than with a real keypad.

Besides, as is the case with any touch screen-based smartphone, we need to look at the screen to ensure that we are aiming our fingertip at the right virtual button.

I never approve of drivers who enter text while driving, but sometimes we have to send email while doing something else that does not involving putting someone's life at risk.

We may have to SMS someone while reading an email message on our netbook, for example.

One question I asked the RIM executive was why there was no Wi-Fi on the Storm. The answer I got was that the Storm already supports HSDPA and that RIM is more interested in offering more choice to customers.

Wi-Fi is available on the Bold and the other new models.

On the flipside, however, the Storm is a sturdily built smartphone. A little bit on the heavy and bulky side, it also features a sharp display with the four BlackBerry signature buttons still there.

Frankly, my experience with the Storm in handling real-world email did not meet my great expectations. Rumor has it that RIM is now working on a really hot BlackBerry Storm 2, even 3. Let us wait and see how it will meet our raising expectations. In the meantime, I think BlackBerry Bold is still the best BlackBerry of all.

Source : www.thejakartapost.com

Messaging for the masses

2009-05-29T09:45:00.001-07:00

Zatni Arbi, Contributor, Jakarta | Mon, 05/18/2009 10:54 AM | Sci-Tech

Email on the go is a boon to mobile professionals and travelers. In the past, before I had my first BlackBerry, I had to spend a lot of money to get Internet access in my hotel room.

On average, 24-hour Internet access costs around US$20 in many cities in the Asia Pacific. And, then again, I used it only for eight hours at the most, as I could use it only in the room.

Then I had the BlackBerry. All the incoming email messages that arrived at my CBN mailbox would be delivered to the device. I was able to read them and respond to them immediately if necessary.

There was no longer any need for the rush to find the Ethernet cable in my hotel room and plug it into my notebook or to go out and find an Internet café to check my email. With push email, I can now be closer to being in two places at one time.

That is the kind of power of email that Nokia has recognized. No wonder they have set up a group that handles messaging for the masses, which is headed by Tom Furlong. Tom’s official position is senior vice president and general manager for consumer messaging, service and software.

The group, which is part of Nokia’s service organization, is responsible for the development of consumer email, instant messaging and, in the future, social network participation on Nokia devices.

Recently I had the opportunity to have a teleconference interview with him. “Why call it consumer messaging?” was my first question.

“Other mobile email services are targeted to the upper segment of the users. At Nokia, we aim at bringing email messaging to millions of users around the world, not only company executives and other corporate users,” he answered.

According to Tom, people in the majority of the developing world are having their first Internet experience on their mobile devices.

So the company’s vision is to enable mobile phone users to set up their first digital ID and then start enjoying the Internet experience without having to go to an Internet café or find a desktop PC to create their email accounts. It should be so easy that they should be able to do it right from their devices.

Needless to say, as my personal experience has shown, push email is the heart of the messaging for the masses. Nokia Messaging is the push email service provided by Nokia to make messaging more effective.

“At the moment, it is available in 16 S60 devices,” said Tom. “By the end of the year, every S60 and a very large portion of S40 devices will have the capability,” he added.

The idea is that a mobile phone user can choose whichever model he or she prefers, and the Finnish company will provide the email and Web browsing capability on it.

To expedite the realization of this “messaging for the masses” vision, Nokia has also acquired two companies, namely, Intellisync and OZ Communications.

Intellisync, which was acquired three years ago, was strong in platform-independent, enterprise solutions for wireless messaging and applications for mobile devices.

Meanwhile, the recently acquired OZ Communications has an excellent consumer messaging suite, which includes email, instant messaging and the ability to attach to social networks.

Thus, in about two years from now, it is very likely that we will no longer be limited to 160 characters per SMS. Email is accessible anywhere there is network coverage. The technology developed by OZ Communications, for example, also reduces the data rates required to deliver the email messages, drives the operator cost down and makes the subscription more affordable to the masses.

There will be no need to locate the nearest public hotspot or pay for the expensive broadband access in the hotel rooms.

With email for the masses, more opportunities will emerge, and it will be even easier to connect to other people around the world.

The question is, will Nokia’s move be followed by others such as BlackBerry?

Source : www.thejakartapost.com

Japan university gives away iPhones to nab truants

2009-05-29T09:42:00.000-07:00

By YURI KAGEYAMA, AP Business Writer - Fri May 29, 2009 10:48AM EDT
TOKYO - A prestigious Japanese university is giving away hundreds of iPhones, in part to use its Global Positioning System to nab students that skip class.

Truants in Japan often fake attendance by getting friends to answer roll-call or hand in signed attendance cards. That's verging on cheating since attendance is a key requirement for graduation here.

Aoyama Gakuin University in Tokyo is giving Apple Inc.'s iPhone 3G to 550 students in its School of Social Informatics, which studies the use of Internet and computer technology in society.

The gadget will work as a tool for studies, but it also comes with GPS, a satellite navigation system that automatically checks on its whereabouts. The university plans to use that as a way check attendance.

Students who skip class could still fake attendance by giving their iPhone to a friend who goes to class. But youngsters aren't likely to lend their mobile phones, which are packed with personal information and e-mail, according to the university.

U.S. universities use the iPhone for various, other purposes. At Stanford University, students have developed iPhone applications in a course. At Duke University, the gadget is used to get around the campus and find information about course listings and other events.

Aoyama Gakuin signed a deal earlier this month with Softbank Corp., the exclusive vendors of the iPhone in Japan.

The number of students using the iPhone is expected to reach 1,000 in the program — the first time the iPhone is being used on such a scale at a Japanese university.

The iPhone will be used to relay course materials, lecture videos and tests. The university hopes students will develop software applications and other lifestyle uses for the cell phone.

Source : tech.yahoo.com

Microsoft revamps search engine, dubbed "Bing"

2009-05-29T09:36:00.002-07:00

By Bill Rigby - Thu May 28, 2009 6:55PM EDT
SEATTLE (Reuters) - Microsoft Corp is revamping its search engine to counter the dominance of Google Inc in the Web search and related advertising business.
The world's largest software company, which is still in talks with Yahoo Inc over a potential partnership, has long been determined to play a major role in the lucrative Web search market after watching upstart Google take a stranglehold.

Microsoft, which has been testing the search engine internally under the name Kumo for several months, plans to introduce the new service, re-christened "Bing," over the next few days, with a full launch next Wednesday. The service will be available at http://www.bing.com.

Advertising Age reported earlier this week that Microsoft was planning a $80 million to $100 million ad campaign to promote Bing. Microsoft declined to comment on the report.

"We'll have what I would call a big budget -- big enough that I had to gulp when I approved the budget," said Microsoft Chief Executive Steve Ballmer, who unveiled Bing at a technology conference in Carlsbad, California, run by the All Things Digital tech blog.

The Redmond, Washington-based firm has lots of ground to make up. Last month Google took 64.2 percent of U.S. Internet searches -- up half a percentage point from the month before -- handling 9.5 billion out of a total of 14.8 billion searches.

Yahoo was a distant second with 20.4 percent of searches and Microsoft third with 8.2 percent, both down slightly from the month before, according to data firm comScore.

Ballmer offered no quick turnaround in those numbers. "My timeframe is lots of years," he said at the conference. "I don't have a specific forecast, but this is lots of years."

The new name, Bing, is short, universal and can be "verbed-up," said Ballmer, a clear reference to the fact that 'to Google' has become the generic verb for searching the Internet for information.

NEW FEATURES

Both Google and Yahoo have recently introduced new features in their search engines to attract users, making Microsoft's task even harder.

Microsoft is calling its new product a "decision engine," promising to make things like buying a digital camera, booking a flight or searching for a restaurant easier by serving up results based on similar previous searches.

A search on a make of car, for example, will bring up clickable categories on the left-hand sidebar, such as 'problems,' 'reviews' and 'dealers,' which Microsoft has calculated are the most likely places a Web user will want to go from the initial search.

Bing also incorporates the increasingly popular Farecast service in its flight booking section -- making use of the company it bought last year -- which predicts whether fares will rise or fall.

Other new features include getting directions to locations with only one mouse click, and the ability to hover over a search result to see more information, without having to open a new link.

Microsoft's shares rose 36 cents to $20.49 on Nasdaq at mid-afternoon. Google edged up 1.1 percent to $409.96 and Yahoo shares rose 1 percent to $15.09.

(Reporting by Bill Rigby; Additional reporting by Alexei Oreskovic; Editing by Derek Caney, Richard Chang)

Engineered circuits can count cellular events

2009-05-29T09:36:00.001-07:00

Anne Trafton, News Office
May 28, 2009

MIT and Boston University engineers have designed cells that can count and "remember" cellular events, using simple circuits in which a series of genes are activated in a specific order.

Such circuits, which mimic those found on computer chips, could be used to count the number of times a cell divides, or to study a sequence of developmental stages. They could also serve as biosensors that count exposures to different toxins.

The team developed two types of cellular counters, both described in the May 29 issue of Science. Though the cellular circuits resemble computer circuits, the researchers are not trying to create tiny living computers.

"I don't think computational circuits in biology will ever match what we can do with a computer," said Timothy Lu, a graduate student in the Harvard-MIT Division of Health Sciences and Technology (HST) and one of two lead authors of the paper.

Performing very elaborate computing inside cells would be extremely difficult because living cells are much harder to control than silicon chips. Instead, the researchers are focusing on designing small circuit components to accomplish specific tasks.

"Our goal is to build simple design tools that perform some aspect of cellular function," said Lu.

Ari Friedland, a graduate student at Boston University, is also a lead author of the Science paper. Other authors are Xiao Wang, postdoctoral associate at BU; David Shi, BU undergraduate; George Church, faculty member at Harvard Medical School and HST; and James Collins, professor of biomedical engineering at BU.

Learning to count

To demonstrate their concept, the team built circuits that count up to three cellular events, but in theory, the counters could go much higher.

The first counter, dubbed the RTC (Riboregulated Transcriptional Cascade) Counter, consists of a series of genes, each of which produces a protein that activates the next gene in the sequence.

With the first stimulus -- for example, an influx of sugar into the cell -- the cell produces the first protein in the sequence, an RNA polymerase (an enzyme that controls transcription of another gene). During the second influx, the first RNA polymerase initiates production of the second protein, a different RNA polymerase.

The number of steps in the sequence is, in theory, limited only by the number of distinct bacterial RNA polymerases. "Our goal is to use a library of these genes to create larger and larger cascades," said Lu.

The counter's timescale is minutes or hours, making it suitable for keeping track of cell divisions. Such a counter would be potentially useful in studies of aging.

The RTC Counter can be "reset" to start counting the same series over again, but it has no way to "remember" what it has counted. The team's second counter, called the DIC (DNA Invertase Cascade) Counter, can encode digital memory, storing a series of "bits" of information.

The process relies on an enzyme known as invertase, which chops out a specific section of double-stranded DNA, flips it over and re-inserts it, altering the sequence in a predictable way.

The DIC Counter consists of a series of DNA sequences. Each sequence includes a gene for a different invertase enzyme. When the first activation occurs, the first invertase gene is transcribed and assembled. It then binds the DNA and flips it over, ending its own transcription and setting up the gene for the second invertase to be transcribed next.

When the second stimulus is received, the cycle repeats: The second invertase is produced, then flips the DNA, setting up the third invertase gene for transcription. The output of the system can be determined when an output gene, such as the gene for green fluorescent protein, is inserted into the cascade and is produced after a certain number of inputs or by sequencing the cell's DNA.

This circuit could in theory go up to 100 steps (the number of different invertases that have been identified). Because it tracks a specific sequence of stimuli, such a counter could be useful for studying the unfolding of events that occur during embryonic development, said Lu.

Other potential applications include programming cells to act as environmental sensors for pollutants such as arsenic. Engineers would also be able to specify the length of time an input needs to be present to be counted, and the length of time that can fall between two inputs so they are counted as two events instead of one.

They could also design the cells to die after a certain number of cell divisions or night-day cycles.

"There's a lot of concern about engineered organisms -- if you put them in the environment, what will happen?" said Collins, who is also a Howard Hughes Medical Institute investigator. These counters "could serve as a programmed expiration date for engineered organisms."

The research was funded by the National Institute of Health Director's Pioneer Award Program, the National Science Foundation FIBR program, and the Howard Hughes Medical Institute.

Long-distance brain waves focus attention, McGovern study finds

2009-05-29T09:30:00.000-07:00

Just as our world buzzes with distractions -- from phone calls to e-mails to tweets -- the neurons in our brain are bombarded with messages. Research has shown that when we pay attention, some of these neurons begin firing in unison, like a chorus rising above the noise. Now, a study in the May 29 issue of Science reveals the likely brain center that serves as the conductor of this neural chorus.

MIT neuroscientists found that neurons in the prefrontal cortex -- the brain's planning center -- fire in unison and send signals to the visual cortex to do the same, generating high-frequency waves that oscillate between these distant brain regions like a vibrating spring. These waves, also known as gamma oscillations, have long been associated with cognitive states such as attention, learning and consciousness.

"We are especially interested in gamma oscillations in the prefrontal cortex because it provides top-down influences over other parts of the brain," explains senior author Robert Desimone, director of the McGovern Institute for Brain Research and the Doris and Don Berkey Professor of Neuroscience at MIT. "We know that the prefrontal cortex is affected in people with schizophrenia, ADHD and many other brain disorders, and that gamma oscillations are also altered in these conditions. Our results suggest that altered neural synchrony in the prefrontal cortex could disrupt communication between this region and other areas of the brain, leading to altered perceptions, thoughts, and emotions."

Neurons in the visual cortex (area V4) encode the visual scene, and neurons in the FEF portion of prefrontal cortex control the focus of attention. When attention (cone and circle) is directed to the red book, neurons in FEF and V4 (represented by red triangles) start firing rhythmically, and the neural activity becomes synchronized across the two areas. View this post on MIT TechTV.

To explain neural synchrony, Desimone uses the analogy of a crowded party with people talking in different rooms. If individuals raise their voices at random, the noise just becomes louder. But if a group of individuals in one room chant together in unison, the next room is more likely to hear the message. And if people in the next room chant in response, the two rooms can communicate.

In the Science study, Desimone looked for patterns of neural synchrony in two "rooms" of the brain associated with attention -- the frontal eye field (FEF) within the prefrontal cortex and the V4 region of the visual cortex. Lead authors Georgia Gregoriou, a postdoctoral associate in the Desimone lab, and Stephen Gotts of the National Institute of Mental Health, trained two macaque monkeys to watch a monitor displaying multiple objects, and to concentrate on one of the objects when cued. They monitored neural activity from the FEF and the V4 regions of the brain when the monkeys were either paying attention to the object or ignoring it.

When the monkeys first paid attention to the appropriate object, neurons in both areas showed strong increases in activity. Then, as if connected by a spring, the oscillations in each area began to synchronize with one another. Desimone's team analyzed the timing of the neural activity and found that the prefrontal cortex became engaged by attention first, followed by the visual cortex -- as if the prefrontal cortex commanded the visual region to snap to attention. The delay between neural activity in these areas during each wave cycle reflected the speed at which signals travel from one region to the other -- indicating that the two brain regions were talking to one another.

Desimone suspects this pattern of oscillation is not just specific to attention, but could also represent a more general mechanism for communication between different parts of the brain. These findings support speculation that gamma synchrony enables far-flung regions of the brain to rapidly communicate with each other -- which has important implications for understanding and treating disorders ranging from schizophrenia to impaired vision and attention. "This helps us think about how to approach studying and treating these disorders by finding ways to restore gamma rhythms in the affected brain regions."

Huihui Zhou, a research scientist in the Desimone lab, contributed to this study. The NIH/National Eye Institute and National Institute of Mental Health supported this research.

Source : http://web.mit.edu

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Storm: Great expectations unmet

Messaging for the masses

Japan university gives away iPhones to nab truants

Microsoft revamps search engine, dubbed "Bing"

Engineered circuits can count cellular events

Learning to count

Long-distance brain waves focus attention, McGovern study finds