Wednesday, December 10, 2014

Transistor configurations , transistor specifications and precautions

TRANSISTOR CONFIGURATIONS

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.

image
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 (IC) and increases collector voltage (making VC 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 VC 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:

    image
    (D is the Greek letter delta, it is used to indicate a small change)
    For example, if the input current (IB) in a common emitter changes from 75 mA to 100 mA and the output current (IC) changes from 1.5 mA to 2.6 mA, the current gain (b) will be 44.

    image
    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 hfe used in place of b. The terms hfe and b are equivalent and may be used interchangeably. This is because "hfe" 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:

    image
    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 RL (not shown), thus lowering the collector voltage VC. 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 IB 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:

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

    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, IE = IB + IC) Therefore, ALPHA is ALWAYS LESS THAN ONE FOR A COMMON-BASE CONFIGURATION.

    Another term for "a" is hfb. These terms (and hfb) are equivalent and may be used interchangeably. The meaning for the term hfb is derived in the same manner as the term hfe 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 hfe. To find alpha (a) when given beta (b), use the following formula to convert b to a for use with the common-base configuration:

    image
    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 asimage


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

    image
    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:image
    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:image
    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:
    image

    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

    imageimage

    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.

    image
    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)?


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    TRANSISTOR SPECIFICATIONS


    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:

    image

    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?


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    PRECAUTIONS


    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.

    image

    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.
    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.

    image
    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

    image
    Figure 2-19B. - Testing a transistor's leakage with an ohmmeter.
    BASE-TO-COLLECTOR TESTimage

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

    BASE-TO-EMITTER TEST

    image
    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

    image

    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?

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