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?

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

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?

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:

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?