21

CHAPTER

Bipolar Transistors

THE WORD TRANSISTOR IS A CONTRACTION OF “CURRENT-TRANSFERRING RESISTOR.” A BIPOLAR TRANsistor has two P-N junctions. Two configurations exist for bipolar transistors: a P type layer between two N type layers (called NPN), or an N type layer between two P type layers (called PNP).

NPN versus PNP

Figure 21-1A is a simplified drawing of an NPN bipolar transistor, and Fig. 21-1B shows the symbol that engineers use for it in schematic diagrams. The P type, or center, layer constitutes the base. One of the N type semiconductor layers forms the emitter, and the other N type layer forms the collector. We label the base, the emitter, and the collector B, E, and C, respectively.

21-1   At A, a simplified structural drawing of an NPN transistor. At B, the schematic symbol. We identify the electrodes as E = emitter, B = base, and C = collector.

A PNP bipolar transistor has two P type layers, one on either side of a thin, N type layer as shown in Fig. 21-2A. The schematic symbol appears at Fig. 21-2B. The N type layer constitutes the base. One of the P type layers forms the emitter, and the other P type layer forms the collector. As with the NPN device, we label these electrodes B, E, and C.

21-2   At A, a simplified structural drawing of a PNP transistor. At B, the schematic symbol. We identify the electrodes as E = emitter, B = base, and C = collector.

We can tell from a schematic diagram whether the circuit designer means for a transistor to be NPN type or PNP type. Once we realize that the arrow always goes with the emitter, we can identify the three electrodes without having to label them. In an NPN transistor, the arrow at the emitter points outward. In a PNP transistor, the arrow at the emitter points inward.

Generally, PNP and NPN transistors can perform the same electronic functions. However, they require different voltage polarities, and the currents flow in different directions. In many situations, we can replace an NPN device with a PNP device or vice versa, reverse the power-supply polarity, and expect the circuit to work with the replacement device if it has the appropriate specifications.

Biasing

For a while, let’s imagine a bipolar transistor as two diodes connected in reverse series (that is, in series but in opposite directions). We can’t normally connect two diodes together this way and get a working transistor, but the analogy works for modeling (technically describing) the behavior of bipolar transistors. Figure 21-3A shows a dual-diode NPN transistor model. The base is at the connection point between the two anodes. The cathode of one diode forms the device’s emitter, while the cathode of the other diode forms the collector. Figure 21-3B shows the equivalent “real-world” NPN transistor circuit.

21-3   At A, the dual-diode model of a simple NPN circuit. At B, the actual transistor circuit.

NPN Biasing

In an NPN transistor, we normally bias the device so that the collector voltage is positive with respect to the emitter. We illustrate this scheme by indicating the battery’s polarity in Figs. 21-3A and 21-3B. Typical DC voltages for a transistor’s power supply range between about 3 V and 50 V, meaning that the collector electrode and the emitter electrode differ in potential by 3 V to 50 V.

In these diagrams, we label the base point “control” because the flow of current through the transistor depends on what happens at this electrode. Any change in the voltage that we apply to the base—either in the form of DC or AC—profoundly affects what happens inside the transistor, and also what happens in other components that we connect to it.

Zero Bias for NPN

Suppose that we connect an NPN transistor so that the base and the emitter are at the same voltage. We call this condition zero bias because the potential difference between the two electrodes equals 0 V. In this situation, the emitter-base current, often called simply the base current and denoted I_B, equals zero. The emitter-base (E-B) junction, which is a P-N junction, operates below its forward breakover voltage, preventing current from flowing between the emitter and the base. Zero bias also prevents any current from flowing between the emitter and the collector unless we inject an AC signal at the base to change things. Such a signal must, at least momentarily, attain a positive voltage equal to or greater than the forward breakover voltage of the E-B junction. When no current flows between the emitter and the collector in a bipolar transistor under no-signal conditions, we say that the device is operating in a state of cutoff.

Reverse Bias for NPN

Imagine that we connect an extra battery between the base and the emitter in the circuit of Fig. 21-3B, with the polarity set such as to force the base voltage E_B to become negative with respect to the emitter. The presence of the new battery causes the E-B junction to operate in a state of reverse bias. No current flows through the E-B junction in this situation (as long as the new battery voltage is not so great that avalanche breakdown occurs at the E-B junction). In that sense, the transistor behaves in the same way when reverse-biased as it does when zero-biased. If we inject an AC signal at the base with the intent to cause a flow of current during part of the cycle, the signal must attain, at least momentarily, a positive voltage high enough to overcome the sum of the reverse bias voltage (produced by the battery) and the forward breakover voltage of the E-B junction. We’ll have a harder time getting a reverse-biased transistor to conduct than we’ll have getting a zero-biased transistor to conduct.

Forward Bias for NPN

Now suppose that we make E_B positive with respect to the emitter, starting at small voltages and gradually increasing. This situation gives us a state of forward bias at the E-B junction. If the forward bias remains below the forward breakover voltage, no current flows, either in the E-B junction or from the emitter to the collector. But when the base voltage E_B reaches and then exceeds the breakover point, the E-B junction conducts, and current starts to flow through the E-B junction.

The base-collector (B-C) junction of a bipolar transistor is normally reverse-biased. It will remain reverse-biased as long as E_B stays smaller than the supply voltage between the emitter and the collector. In practical transistor circuits, engineers commonly set E_B at a small fraction of the supply voltage. For example, if the battery between the emitter and the collector in Figs. 21-3A and B provides 12 V, the small battery between the emitter and the base might consist of a single 1.5-V cell. Despite the reverse bias of the B-C junction, a significant emitter-collector current, called collector current and denoted I_C, flows through the transistor once the E-B junction conducts.

In a real transistor circuit, such as the one shown in Fig. 21-3B, the meter reading will jump when we apply DC base bias to reach and then exceed the forward breakover voltage of the E-B junction. If we continue to increase the forward bias at the E-B junction, even a small rise in E_B, attended by a rise in the base current I_B, will cause a large increase in the collector current I_C. Figure 21-4 portrays the situation as a graph. Once current starts to flow in the collector, increasing E_B a tiny bit will cause the I_C to go up a lot! However, if E_B continues to rise, we’ll eventually arrive at a voltage where the curve for I_C versus E_B levels off. Then we say that the transistor has reached a state of saturation. It’s “running wide open,” conducting as much as it possibly can, given a fixed potential difference between the collector and the emitter.

21-4   Relative collector current (I_C) as a function of base voltage (E_B) for a hypothetical NPN silicon transistor.

PNP Biasing

We can describe the situation inside a PNP transistor, as we vary the voltage of the small battery or cell between the emitter and the base, as a “mirror image” of the case for an NPN device. The diodes are reversed, the arrow points inward rather than outward in the transistor symbol, and all the polarities are reversed. The dual-diode PNP model, along with the “real-world” transistor circuit, appear in Fig. 21-5. We can repeat the foregoing discussion (for the NPN case) almost verbatim, except that we must replace every occurrence of the word “positive” with the word “negative.” Qualitatively, the same things happen in the PNP device as in the NPN case.

21-5   At A, the dual-diode model of a simple PNP circuit. At B, the actual transistor circuit.

Amplification

Because a small change in I_B causes a large variation in I_C when we set the DC bias voltages properly, a transistor can operate as a current amplifier. Engineers use several expressions to describe the current-amplification characteristics of bipolar transistors.

Collector Current versus Base Current

Figure 21-6 is a graph of the way the collector current I_C changes in a typical bipolar transistor as the base current I_B changes. We can see some points along this I_C versus I_B curve at which a transistor won’t provide any current amplification. For example, if we operate the transistor in saturation (shown by the extreme upper right-hand part of the curve), the I_C versus I_B curve runs horizontally. In this zone, a small change in I_B causes little or no change in I_C. But if we bias the transistor near the middle of the “ramped-up” straight-line part of the curve in Fig. 21-6, the transistor will work as a current amplifier.

21-6   Three different transistor bias points. We observe the most current amplification when we bias the device near the middle of the straight-line portion of the curve.

Whenever we want a bipolar transistor to amplify a signal, we must bias the device so that a small change in the current between the emitter and the base will result in a large change in the current between the emitter and the collector. The ideal voltages for E_B (the base bias) and E_C (the power-supply voltage) depend on the internal construction of the transistor, and also on the chemical composition of the semiconductor materials that make up its N type and P type sections.

Static Current Characteristics

We can describe the current-carrying characteristics of a bipolar transistor in simplistic terms as the static forward current transfer ratio. This parameter comes in two “flavors,” one that describes the collector current versus the emitter current when we place the base at electrical ground (symbolized H_FB), and the other that describes the collector current versus the base current when we place the emitter at electrical ground (symbolized H_FE).

The quantity H_FB equals the ratio of the collector current to the emitter current at a given instant in time with the base grounded:

H_FB = I_C/I_E

For example, if an emitter current I_E of 100 mA results in a collector current I_C of 90 mA, then we can calculate

H_FB = 90/100 = 0.90

If I_E = 100 mA and I_C = 95 mA, then

H_FB = 95/100 = 0.95

The quantity H_FE equals the ratio of the collector current to the base current at a given instant in time with the emitter grounded:

H_FE = I_C/I_B

For example, if a base current I_B of 10 mA results in a collector current I_C of 90 mA, then we can calculate

H_FE = 90/10 = 9.0

If I_B = 5.0 mA and I_C = 95 mA, then

H_FE = 95/5.0 = 19

Alpha

We can describe the current variations in a bipolar transistor by dividing the difference in I_C by the difference in I_E that occurs when we apply a small signal to the emitter of a transistor with the base connected to electrical ground (or placed at the same potential difference as electrical ground). We call this ratio the alpha, symbolized as the lowercase Greek letter alpha (α). Let’s abbreviate the words “the difference in” by writing d. Then mathematically, we can define

α = dI_C/dI_E

We call this quantity the dynamic current gain of the transistor for the grounded-base situation. The alpha for any transistor is always less than 1, because whenever we apply a signal to the input, the base “bleeds off” at least a little current from the emitter before it shows up at the collector.

Beta

We get an excellent definition of current amplification for “real-world signals” when we divide the difference in I_C by the difference in I_B as we apply a small signal to the base of a transistor with the emitter at electrical ground. Then we get the dynamic current gain for the grounded-emitter case. We call this ratio the beta, symbolized as the lowercase Greek letter beta (β). Once again, let’s abbreviate the words “the difference in” as d. Then we have

β = dI_C/dI_B

The beta for any transistor can exceed 1—and often does, greatly!—so this expression for “current gain” lives up to its name. However, under some conditions, we might observe a beta of less than 1. This condition can occur if we improperly bias a transistor, if we choose the wrong type of transistor for a particular application, or if we attempt to operate the transistor at a signal frequency that’s far higher than the maximum frequency for which it is designed.

How Alpha and Beta Relate

Whenever base current flows in a bipolar transistor, we can calculate the beta in terms of the alpha with the formula

β = α/(1 − α)

and we can calculate the alpha in terms of the beta using the formula

α = β/(1 + β)

With a little bit of algebra, we can derive these formulas from the fact that, at any instant in time, the collector current equals the emitter current minus the base current; that is,

I_C = I_E − I_B

“Real-World” Amplification

Let’s look at Fig. 21-6 again. It’s a graph of the collector current as a function of the base current (I_C versus I_B) for a hypothetical transistor. We can infer both H_FE and β from this graph. We can find H_FE at any particular point on the curve when we divide I_C by I_B at that point. Geometrically, the value of β at any given point on the curve equals the slope (“rise over run”) of a tangent line at that point. On a two-dimensional coordinate grid, the tangent to a curve at a point constitutes the straight line that intersects the curve at that point without crossing the curve.

In Fig. 21-6, the tangent to the curve at point B appears as a dashed, straight line; the tangents to the curve at points A and C lie precisely along the curve (and, therefore, don’t show up visually). As the slope of the line tangent to the curve increases, the value of β increases. Point A provides the highest value of β for this particular transistor, as long as we don’t let the input signal get too strong. Points in the immediate vicinity of A provide good β values as well.

For small-signal amplification, point A in Fig. 21-6 represents a good bias level. Engineers would say that it’s a favorable operating point. The β figure at point B is smaller (the curve slopes less steeply upward as we move toward the right) than the β figure at point A, so point B represents a less favorable operating point for small-signal amplification than point A does. At point C, we can surmise that β = 0 because the slope of the curve equals zero in that vicinity (it doesn’t “rise” at all as we “run” toward the right). The transistor won’t amplify weak signals when we bias it at point C or beyond.

Overdrive

Even when we bias a transistor so that it can produce the greatest possible current amplification (at or near point A in Fig. 21-6), we can encounter problems if we inject an AC input signal that’s too strong. If the input-signal amplitude gets large enough, the transistor’s operating point might move to or beyond point B, off the coordinate grid to the left, or both, during part of the signal cycle. In that case, the effective value of β will decrease. Figure 21-7 shows why this effect occurs. Points X and Y represent the instantaneous current extremes during the signal cycle in this particular case. Note that the slope of the line connecting points X and Y is less than the slope of the straight-line part of the curve at and near point A.

21-7   Excessive input reduces amplification.

When our AC input signal is so strong that it drives the transistor to the extreme points X and Y, as shown in Fig. 21-7, a transistor amplifier introduces distortion into the signal, meaning that the output wave does not have the same shape as the input wave. We call this phenomenon nonlinearity. We can sometimes tolerate this condition, but often it’s undesirable. Under most circumstances, we’ll want our amplifier to remain linear (or to exhibit excellent linearity), meaning that the output wave has the same shape as (although probably stronger than) the input wave.

When the input signal to a transistor amplifier exceeds a certain critical maximum, we get a condition called overdrive. An overdriven transistor operates in or near saturation during part of the input signal cycle. Overdrive reduces the overall circuit efficiency, causes excessive collector current to flow, and can overheat the base-collector (B-C) junction. Sometimes overdrive can physically destroy a transistor.

Gain versus Frequency

A bipolar transistor exhibits an amplification factor (gain) that decreases as the signal frequency increases. Some bipolar transistors can amplify effectively at frequencies up to only a few megahertz. Other devices can work into the gigahertz range. The maximum operating frequency for a particular bipolar transistor depends on the capacitances of the P-N junctions inside the device. A low junction capacitance value translates into a high maximum usable frequency.

Expressions of Gain

You’ve learned about current gain expressed as a ratio. You’ll also hear or read about voltage gain or power gain in amplifier circuits. You can express any gain figure as a ratio. For example, if you read that a circuit has a voltage gain of 15, then you know that the output signal voltage equals 15 times the input signal voltage. If someone tells you that the power gain of a circuit is 25, then you know that the output signal power equals 25 times the input signal power.

Alpha Cutoff

Suppose that we operate a bipolar transistor as a current amplifier, and we deliver an input signal to it at 1 kHz. Then we steadily increase the input-signal frequency so that the value of α declines. We define the alpha cutoff frequency of a bipolar transistor, symbolized f_α, as the frequency at which α decreases to 0.707 times its value at 1 kHz. (Don’t confuse this use of the term “cutoff” with the state of “cutoff” that we get when we zero-bias or reverse-bias a transistor under no-signal conditions!) A transistor can have considerable gain at its alpha cutoff frequency. By looking at this specification for a particular transistor, we can get an idea of how rapidly it loses its ability to amplify as the frequency goes up.

Beta Cutoff Frequency

Imagine that we repeat the above-described variable-frequency experiment while watching β instead of α. We discover that β decreases as the frequency increases. We define the beta cutoff frequency (also called the gain bandwidth product) for a bipolar transistor, symbolized f_β or f_T, as the frequency at which β gets down to 1. If we try to make a transistor amplify above its beta cutoff frequency, we’ll fail!

Figure 21-8 shows the alpha cutoff and beta cutoff frequencies for a hypothetical transistor on a graph of gain versus signal frequency. Note that the scales of this graph are not linear and the divisions are unevenly spaced. We call this type of plot a log-log graph because both scales are logarithmic rather than linear. The value on either scale increases in proportion to the base-10 logarithm of the distance from the origin, rather than varying in direct proportion to that distance.

21-8   Alpha cutoff and beta cutoff frequencies for a hypothetical transistor.

Common-Emitter Configuration

A bipolar transistor can be “wired up” in three general ways. We can ground the emitter for the signal, we can ground the base for the signal, or we can ground the collector for the signal. An often-used arrangement is the common-emitter circuit. “Common” means “grounded for the signal.” Figure 21-9 shows the basic configuration.

21-9   Common-emitter configuration for an NPN transistor circuit.

Even if a circuit point remains at ground potential for signals, it can have a significant DC voltage with respect to electrical ground. In the circuit shown, capacitor C₁ appears as a short circuit to the AC signal, so the emitter remains at signal ground. But resistor R₁ causes the emitter to attain and hold a certain positive DC voltage with respect to electrical ground (or a negative voltage, if we replace the NPN transistor with a PNP device). The exact DC voltage at the emitter depends on the resistance of R₁, and on the bias at the base. We set the DC base bias by adjusting the ratio of the values of resistors R₂ and R₃. The DC base bias can range from 0 V, or ground potential, to +12 V, which equals the power-supply voltage. Normally it’s a couple of volts.

Capacitors C₂ and C₃ block DC to or from the external input and output circuits, while letting the AC signal pass. Resistor R₄ keeps the output signal from “shorting out” through the power supply. A signal enters the common-emitter circuit through C₂, so the signal causes the base current I_B to vary. The small fluctuations in I_B cause large changes in the collector current I_C. This current passes through resistor R₄, producing a fluctuating DC voltage across it. The AC component of this voltage passes unhindered through capacitor C₃ to the output.

The circuit of Fig. 21-9 represents the basis for many amplifier systems at all commonly encountered signal frequencies. The common-emitter configuration can produce more gain than any other arrangement. The output wave appears inverted (in phase opposition) with respect to the input wave. If the input signal constitutes a pure sine wave, then the common-emitter circuit shifts the signal phase by 180°.

Common-Base Configuration

As its name implies, the common-base circuit (Fig. 21-10) has the base at signal ground. The DC bias is the same as that for the common emitter circuit, but we apply the input signal at the emitter instead of at the base. This arrangement gives rise to fluctuations in the voltage across resistor R₁, causing variations in I_B. These small current fluctuations produce large variations in the current through R₄. Therefore, amplification occurs. The output wave follows along in phase with the input wave.

21-10   Common-base configuration for an NPN transistor circuit.

The signal enters the transistor through capacitor C₁. Resistor R₁ keeps the input signal from “shorting out” to ground. Resistors R₂ and R₃ provide the base bias. Capacitor C₂ holds the base at signal ground. Resistor R₄ keeps the output signal from “shorting out” through the power supply. We get the output through capacitor C₃. The common-base circuit exhibits a relatively low input impedance, and provides somewhat less gain than a common-emitter circuit.

A common-base amplifier offers better stability than most common-emitter circuits do. By “better stability,” we mean that the common-base circuit is less likely to break into oscillation (generate a signal of its own) as a result of amplifying some of its own output. The main reason for this “good behavior” is the low input impedance of the common-base circuit, which requires the input signal to offer significant power to drive the system into amplification. The common-base circuit isn’t sensitive enough to get out of control easily!

Sensitive amplifiers, such as optimally-biased common-emitter circuits, can pick up some of their own output as a result of stray capacitance between the input and output wires. This little bit of “signal leakage” can provide enough energy at the input to cause the whole circuit to “chase its own tail.” When positive (in-phase) feedback gives rise to oscillation in an amplifier, we say that the amplifier suffers from parasitic oscillation, or parasitics, which can cause a radio transmitter to put out signals on unauthorized frequencies, or make a radio receiver stop working altogether.

Common-Collector Configuration

A common-collector circuit (Fig. 21-11) operates with the collector at signal ground. We apply the AC input signal at the transistor base, just as we do with the common-emitter circuit. The signal passes through C₂ onto the base. Resistors R₂ and R₃ provide the correct base bias. Resistor R₄ limits the current through the transistor. Capacitor C₃ keeps the collector at signal ground. Fluctuating DC flows through R₁, and a fluctuating voltage, therefore, appears across it. The AC part of this voltage passes through C₁ to the output. Because the output follows the emitter current, some engineers and technicians call this arrangement an emitter-follower circuit.

21-11   Common-collector configuration, also known as an emitter-follower circuit, using an NPN transistor.

The output wave of a common-collector circuit appears exactly in phase with the input wave. The transistor exhibits a relatively high input impedance, while its output impedance remains low. For this reason, the common-collector circuit can take the place of a transformer when we want to match a high-impedance circuit to a low-impedance circuit or load. A well-designed emitter-follower circuit can function over a wider range of frequencies than a typical wirewound transformer can.

Quiz

Refer to the text in this chapter if necessary. A good score is at least 18 correct. Answers are in the back of the book.

1.  In a common-base transistor circuit, the output and input waves differ in phase by

(a)  ¼ of a cycle.

(b)  ⅓ of a cycle.

(c)  ½ of a cycle.

(d)  None of the above

2.  Which of the following circuit configurations do engineers sometimes use in place of conventional wirewound transformers to match a high input impedance to a low output impedance?

(a)  Common emitter

(b)  Common base

(c)  Common collector

(d)  Any of the above

3.  Current will never flow in the B-C junction of a grounded-emitter bipolar transistor when we

(a)  reverse-bias the E-B junction and apply no input signal.

(b)  forward-bias the E-B junction beyond forward breakover and apply no input signal.

(c)  zero-bias the E-B junction and apply a strong input signal.

(d)  forward-bias the E-B junction beyond forward breakover and apply a weak input signal.

4.  Figure 21-12 illustrates a bipolar transistor and several other components in

21-12   Illustration for Quiz Questions 4 through 8.

(a)  a common-emitter configuration.

(b)  an emitter-follower configuration.

(c)  a common-base configuration.

(d)  a common-collector configuration.

5.  What, if any, major errors exist in the circuit of Fig. 21-12?

(a)  Nothing is wrong, assuming that we choose the component values properly.

(b)  We should use an NPN transistor, not a PNP transistor.

(c)  The power-supply polarity at the collector should be positive, not negative.

(d)  We should transpose the input and output terminals.

6.  In the circuit of Fig. 21-12, what purpose does component X serve?

(a)  It keeps the signal from “shorting out” to ground.

(b)  It helps to establish the proper bias at the base.

(c)  It ensures that the circuit won’t break into oscillation.

(d)  It keeps the base at signal ground.

7.  In the circuit of Fig. 21-12, what purpose does component Y serve?

(a)  It keeps the input isolated from the output.

(b)  It keeps the output signal from “shorting out” through the power supply.

(c)  It ensures that the circuit won’t break into oscillation.

(d)  It helps to establish the proper bias at the base.

8.  In the circuit of Fig. 21-12, what purpose does component Z serve?

(a)  It helps the circuit to function as an oscillator by providing feedback.

(b)  It keeps the output signal from “shorting out” through the power supply.

(c)  It helps to establish the proper bias at the base.

(d)  It ensures that the output wave remains in phase opposition with respect to the input wave.

9.  In an emitter-follower circuit, we apply the input signal between the

(a)  collector and ground.

(b)  emitter and collector.

(c)  base and ground.

(d)  base and collector.

10.  In the dual-diode model of a PNP transistor, the base corresponds to

(a)  the point at which the cathodes meet.

(b)  the point at which the cathode of one diode meets the anode of the other.

(c)  the point at which the anodes meet.

(d)  either of the anodes.

11.  Suppose that we encounter a schematic diagram of a complicated circuit that uses bipolar transistors. For some reason, the draftsperson didn’t put the arrows inside the transistor symbols. Can we nevertheless differentiate between NPN and PNP devices? If so, how?

(a)  No, we can’t.

(b)  Yes, we can. For a PNP device, the applied DC collector voltage is always positive with respect to the emitter voltage, while for an NPN device, the applied DC collector voltage is always negative with respect to the emitter voltage.

(c)  Yes, we can. For a PNP device, the applied DC collector voltage is always negative with respect to the emitter voltage, while for an NPN device, the applied DC collector voltage is always positive with respect to the emitter voltage.

(d)  Yes, we can. For a PNP device, the E-B junction is always forward-biased, while for an NPN device, the E-B junction is always reverse-biased.

12.  With no signal input, a properly connected common-emitter NPN bipolar transistor would have the highest value of I_C when

(a)  we forward-bias the E-B junction considerably beyond forward breakover.

(b)  we connect the base directly to the negative power-supply terminal.

(c)  we reverse-bias the E-B junction.

(d)  we connect the base directly to electrical ground.

13.  Suppose that for a certain transistor at a specific constant frequency, we find that the alpha equals 0.9315. What’s the beta?

(a)  We can’t determine it because our figure for the alpha makes no sense. We must have made a mistake when we determined the alpha!

(b)  13.60

(c)  0.4823

(d)  1.075

14.  Suppose that for a certain transistor at a certain frequency, we find that the beta equals 0.5572. What’s the alpha?

(a)  We can’t determine it because our figure for the beta makes no sense. We must have made a mistake when we determined the beta!

(b)  1.258

(c)  0.3578

(d)  1.795

15.  Suppose that for a certain transistor at a certain frequency, we find that the alpha equals exactly 1.00. What’s the beta?

(a)  We can’t define it.

(b)  0.333

(c)  0.500

(d)  1.00

16.  In a common-emitter circuit, we normally take the output from the

(a)  emitter.

(b)  base.

(c)  collector.

(d)  More than one of the above

17.  What major error exists in the circuit of Fig. 21-13?

21-13   Illustration for Quiz Questions 17 through 20.

(a)  The power supply voltage is too high for any bipolar transistor to handle.

(b)  We should use an NPN transistor, not a PNP transistor.

(c)  The power-supply polarity at the collector should be positive, not negative.

(d)  We should transpose the input and output terminals.

18.  In the circuit of Fig. 21-13, what purpose does component X serve?

(a)  It keeps the signal from “shorting out” through the emitter.

(b)  It helps to establish the proper bias at the collector.

(c)  It keeps the signal from “feeding back” into the input device.

(d)  It blocks DC to or from the external input device, while letting the AC signal pass.

19.  In the circuit of Fig. 21-13, what purpose does component Y serve?

(a)  It keeps the input isolated from the output.

(b)  It keeps the input signal from “shorting out” through the power supply.

(c)  It keeps the emitter at signal ground, while allowing a DC voltage to exist there.

(d)  It helps to establish the proper bias at the base.

20.  In the circuit of Fig. 21-13, what purpose does component Z serve?

(a)  It keeps the circuit from breaking into oscillation.

(b)  It blocks DC to or from the external output device while letting the AC signal pass.

(c)  It matches the transistor’s impedance to the impedance of the external output device, or load.

(d)  It ensures that the output wave remains in phase with the input wave.