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A transistor may be connected into a circuit in one of three ways, or configurations-namely, the common base, common emitter, and common collector. Of the three, the common-emitter configuration is by far the most versatile and widely used.
However, since the other two configurations have overriding advantages for special applications (such as impedance matching) , it is essential that you be able to recognize each of the three circuit configurations when you encounter them.
In the conventional three-terminal transistor, a signal is normally applied to one of the three elements (base, emitter, and collector), and the resulting signal extracted from one of the two remaining elements. Another way of saying exactly the same thing is that an input signal is applied between two terminals of the transistor and extracted from two terminals, one of which is different and one of which is common to both input and output circuit. The terminal which is common to both input and output circuit will prove, in actual practice, either to be connected directly to ground (grounded, in other words), or to be separated from ground by some fixed voltage source only, such as a battery or power supply. A review of the many circuit diagrams used in this series will confirm that, in the vast majority, the input-signal voltage is developed across some type of impedance between the input terminal and ground. Also, the output-signal voltage is developed across an other impedance between the output terminal and ground.
("Ground" in each instance is the point of neutral, or reference, voltage.) Every circuit must have a reference voltage against which all other voltages in the circuit-whether positive or negative, or alternating, pulsating, or direct--can be compared.
The most convenient such voltage is that of the earth, or ground, which is normally taken to be zero volts.
It is frequently easier to identify the input and output terminals of a transistor circuit than the terminal common to both circuits. However, the common terminal can be identified by the process of elimination. Once the common terminal has been recognized, it is usually a simple matter to satisfy one's self that it is grounded either directly or through a fixed voltage source.
VOLTAGE, CURRENT, AND POWER GAIN
When the "configuration" of a circuit has been established, it is usually desirable to know what gains in voltage, current, and power the circuit can be expected to deliver. These values of gain are tabulated in Table 1, along with the input and output impedances for each type of circuit, since both voltage gain and power gain are directly related to the input and output impedances.
Table 1. Typical Gain and Impedance Values.
Type of Circuit
Common Base Common Emitter Common Collector Current gain Less than unity Medium-about 50 Medium-about 50 Voltage gain More than l 00 Several hundred Less than unity Power gain Medium High Low Input impedance Very low
Low Very high Output Impedance Very high Medium Very low
THE COMMON-EMITTER CIRCUIT
Figs. 1 and 2 depict two successive half-cycles in the operation of a typical phase-inverter circuit which uses two PNP transistors, the first in a common-emitter and the second in a common-base configuration. This circuit ( and the common collector configuration discussed later) will substantiate the characteristics listed in Table 1. Before discussing these characteristics, it is desirable that each circuit operation be understood; and this requires a detailed discussion of the electron currents which flow in each circuit.
Identification of Currents
The following electron currents are flowing continuously in the two-stage phase-inverter circuit of Figs. 1 and 2.
1. Input driving current for transistor X1 ( dotted green).
2. Voltage-divider and biasing current (solid green).
3. Base-emitter current for transistor X1 (frequently referred to as base current) (also in solid green).
4. Collector-emitter current for transistor X1 (frequently referred to as collector current) (red).
5. Base-emitter current (base current) for transistor X2.
( dotted blue) .
6. Collector-emitter current (collector current) for transistor X2 (solid blue).
Circuit Operation
The input driving current (dotted green) flows up and down through resistor R1 at the basic frequency, which is being amplified. During the half-cycle depicted in Fig. 1, this current is being drawn upward through R1. This tells us the applied input voltage is positive during this half-cycle. The positive component of applied voltage must be subtracted from the negative voltage created at the junction of R2 and R1 by the flow of the voltage-divider and biasing current (solid green). This current originates at the negative terminal of battery M1 and flows through R3, R2, and R1 (in that order) in order to reach ground and be able to re-enter the positive battery terminal, which is also grounded. Each point along this path is at a lower negative voltage than any preceding point. In other words, the voltage at the junction of R3 and R2 is less negative than the battery voltage, but more negative than the voltage at the junction of R1 and R2. Likewise, the voltage at the junction of R1 and R2 is less negative than the voltage at the junction of R2 and R3, but it is still negative with respect to ground.
The voltage at the base of X1 is the instantaneous sum of the negative voltage created by the voltage-divider action, and of the input voltage across R1. The latter is positive during the first half-cycle shown in Fig. 1, and negative during the second half-cycle of Fig. 2. This instantaneous voltage is one of the two important biasing voltages of any transistor, the other being the voltage at the emitter of X1. This is a negative voltage, the result of four separate electron currents flowing downward through R4 and every one varying some what during a single cycle of operation. Consequently, an exact computation of emitter voltage at any instant is a difficult process.
However, in the PNP transistor the voltage at the emitter of X1 must always be more positive than the base in order for any electron current to flow from base to emitter. This base emitter current is known as the "biasing current." Another way of saying this is that the base must be more negative than the emitter in order for biasing current to flow. During a positive half-cycle, the base is made less negative, and this restricts the flow of base-emitter biasing current.
Transistor action is such that a slight reduction in base emitter biasing current generates a much larger reduction in the other electron stream flowing through the transistor namely, the collector-emitter current, shown here in red. This phenomenon accounts for the decreases in the two currents indicated in Fig. 1.
During a negative half-cycle, the base voltage is made more negative. This drives more base-emitter biasing current through the transistor, and thus a much larger collector-emitter cur rent is generated. This phenomenon accounts for the increases in the two currents indicated in Fig. 2.
With this somewhat elementary understanding of current and voltage actions around the transistor, we are in a position to explain the meanings of the various characteristics listed in Table 1 for the common-emitter configuration-namely, current, voltage, and power gain, and input and output impedance.
Change in collector current
Current gam = -~-=-----------
Change in base current
Values of 50 or even 100 are not uncommon for common-emitter configurations. The current gain of a common-emitter circuit is closely related to the beta factor of the transistor discussed in Section 1. However, the amount of current gain achieved in any particular circuit will depend not only on the beta factor, but also on the values of resistors external to the transistors.
Change in collector voltage
Voltage gam = -==-~----=---~.—
Change in base voltage
During static operation of the circuit of Fig. 1, when no input signal voltage is applied (meaning zero driving current is flowing) , the difference between emitter and base voltages is normally only a fraction of a volt. Consequently, changing the base voltage by a small fraction of a volt will cause a substantial change in the amount of collector-emitter current.
Since this current must flow through a large load resistor, R3, it develops a large voltage drop across this resistor. This voltage drop is always directly proportional to the amount of collector current, in accordance with the Ohm's-law relation ship between voltage and current.
Fig. 1. Operation of a two-stage phase inverter using common-emitter and
common-base configurations-positive half-cycle.
Fig. 2. Operation of a two-stage phase inverter using common-emitter and
common-base configurations-negative half-cycle.
Because the lower end of resistor R3 is tied to a point of fixed voltage (the negative battery terminal), the voltage at the top of R3 will become more negative when the collector current decreases (Fig. 1), and less negative when it in creases (Fig. 2) . Since the upper end of R3 is connected directly to the collector terminal of the transistor, large changes in voltage are occurring at the collector as a result of very small changes in voltage applied to the base. Thus it is easy to visualize how a large voltage gain is achieved in the common emitter configuration, as indicated in Table 1.
Power gain = Voltage gain X current gain Al
Power delivered at load so power gain = ~--~~.-------
Power delivered to input
Since we have already satisfied ourselves that high volt age and current gains are both available from the typical junction transistor in a common-emitter configuration, the pro duct of these two gains will obviously give a high power gain.
Another way of looking at power gain is provided by the second formula. This leads naturally to a consideration of the two other important characteristics of any transistor circuit namely, input impedance and output impedance.
Impedance, in the broadest sense, represents opposition to electron flow. It can always be expressed as a ratio between any applied voltage and the resulting current flow. This ratio stems directly from Ohm's law, which states that: E=IX R
... where, E is the voltage applied across a component in volts, I is the resulting flow of electron current through the component, in amperes, R is the resistance or impedance of the component, in ohms.
The input impedance of transistor X1 in Fig. 1 is the resistance between the input point ( the base) and the ground connection below emitter resistor R4. The internal resistance in the "forward" direction, between base and emitter of the PNP transistor, is only 20 to 40 ohms. Resistor R4 is in series with this junction resistor, but emitter resistors are normally held down to a few hundred ohms, or at most one or two thousand ohms. Additionally, R4 is shunted by the emitter base resistance of X2 and the other components ·connected to the base of X2. As a result, the input impedance at the base of X1 is low. This means that only a small voltage change is required at the base to cause a significant change in the base current. Input impedance might thus be defined as the change in input voltage required to produce a change in input current.
The output impedance of transistor X1 in Fig. 1 is the total resistance between the output point (collector) and the ground connection below emitter resistor R4. The bulk of this resistance is made up of the collector-to-base junction resistance within the transistor. This is the so-called "reverse" direction-meaning the electron flow from collector to base is opposite, or against, the normal flow direction for a PN junction. Consequently, the impedance to electron flow from collector to base (this is actually the direction in which collector emitter current does flow within the transistor) is as high as one or two megohms. We can look at the output impedance as a ratio between a change in collector voltage and the resulting change in collector current. Because collectors are biased in the reverse direction, so that batteries such as M1 are trying to drive electrons against the normal flow direction of the collector to-base junction, a relatively large change in collector voltage will have only an insignificant effect on the amount of collector emitter current.
Large changes in the collector-emitter current can be generated by changes in the base-emitter biasing current. These changes in collector-emitter current will cause substantial voltage changes at the collector, by virtue of the voltage drop which this current develops across R3. However, it does not necessarily follow that these changes in collector-emitter current can be brought about by changing the collector voltage by the same amount. In this respect, the collector terminal is analogous to the plate of a pentode vacuum tube. The plate voltage of a pentode has almost no control over the amount of plate current which flows; likewise the intrinsic value of collector voltage has almost no control over the amount of collector current. Just as the amount of grid-bias voltage regulates electron flow through the pentode, so does the amount of biasing current regulate the collector current through the transistor. Because of these considerations, the output impedance of a common-emitter configuration is very high.
THE COMMON-BASE CIRCUIT
Transistor X2 is connected in a common-base configuration (i.e., the base is common to both the input and output circuits). X2 like X1, is also a PNP transistor, which tells us the direction of electron flow through the transistor is from the base and collector, into the emitter. The arrows indicate the flow direction of the two transistor currents through X2; the collector emitter current is in solid blue, and the base-emitter current in dotted blue.
To understand the biasing conditions which regulate the currents flowing through X2, let us refresh our memories on the two important conditions which control the flow of transistor currents. These conditions are the voltages at the emitter and base, and of course the difference between these two voltages.
The base-emitter current of X2 begins at the negative terminal of battery M1, flows through resistors R5 and R6, and then through the very small "forward" resistance of the base-to emitter junction. From there it continues through resistor R4 to ground, where it has a free return access to the grounded positive terminal of M1. This is in every sense a voltage-divider action, and every point along the current path is at a lesser negative voltage than any preceding point.
We have already seen that during the positive half-cycles of Fig. 1, there is a decrease in both the base-emitter and collector-emitter currents through transistor X1. Since both currents flow through resistor R4, toward the ground connection at its lower terminal, any decrease in them will result in a smaller voltage drop across R4. This smaller negative voltage, at the upper terminal of R4, is applied directly to the emitter of X2. A smaller negative voltage at this emitter will increase the base-emitter biasing current through X2. This increase, and the inevitable increase in collector-emitter current, have been indicated in Fig. 1.
During the negative half-cycle shown in Fig. 2, both currents flowing through transistor X1 are increased, for reasons previously discussed. Since both must flow downward through R4, a greater voltage drop is generated across this resistor, and the upper terminal becomes more negative during this half cycle. This more negative voltage is applied to the emitter of X2; hence, as with any PNP transistor, less base-emitter biasing current will flow through X2 and automatically reduce the collector-emitter current. These decreases have been indicated in Fig. 2.
With this preliminary understanding of voltage and current conditions around transistor X2, let us consider the common base characteristics listed in Table 1. The most important one is the current gain. In the common-base configuration, current gain is similar to the alpha factor of the transistor-it can approach but cannot exceed unity. It can be expressed by this ratio:
Since the base and collector currents of any transistor must also flow through its emitter, the emitter current will always be somewhat greater than the collector current. Also, to achieve any particular change in the quantity of collector current re quires a slightly greater change in the emitter current. The emitter is the input point in this type of circuit, and changes in the collector current can only be effected by larger changes in the emitter current. Consequently, the current gain in the common-base configuration, as expressed in the above formula, will always be a fraction less than unity-although in the average transistor, it will usually exceed .95 and will frequently be as high as .98.
V lta
Change in collector voltage o gegam- Ch . ·t 1 ange m emit er vo tage
Also, Voltage gain = Current gain X resistance gain
In the first formula, the voltage gain in reality is a comparison of output versus input voltages, since the voltage output is taken from the transistor collector, whereas the input is applied to the emitter. You have already seen that extremely small voltage differences between base and emitter voltages will cause significant changes in the amounts of the two currents through the transistor. In the common-base configuration, a small change in emitter voltage is either achieved or accompanied by a change in the base-emitter current flowing through the low-resistance input circuit. Simultaneously, a similar change occurs in the collector current flowing through the collector load resistance in the output circuit. Since this load resistance is usually many times larger than the resistance of the input circuit (through which the base-emitter current must flow), a large change in the voltage at the collector ( collector voltage) will occur as a result of a small change in emitter voltage. This is what is meant by voltage gain.
The second formula for voltage gain is based on the Ohm's law relationship between current, voltage, and resistance.
"Resistance gain" has nothing to do with the intrinsic properties of a transistor, but just a convenient way to express the ratio between the resistances of the output circuit and input circuit, respectively. "Current gain" has already been defined as the ratio between output current and input current. Since input current must flow through the small input resistance, and since output current must flow through the large output resistance, the product of current gain and the so-called resistance gain will give the same voltage gain as the first formula.
Because current gain is less than unity in the common-base configuration, the voltage gain achieved by the above formula will be slightly less than the resistance gain.
Table 1 tells us that the input impedance of the common base configured circuit is very low. Input impedance of any circuit can be expressed as a ratio between a given change in applied voltage (in this case, the emitter-voltage) and the resulting change in input current (in this case, the emitter current). We know that the two important biasing voltages of any transistor are those at the base and emitter, and that normally they differ by less than a volt. A small change in either voltage will cause a small change in the base-emitter biasing current, and lead to a much larger change in the collector-emitter current.
Thus a small change in the applied voltage at the emitter will eventually produce a large change in the collector-emitter cur rent, and the emitter current is composed largely of the collector emitter current. Therefore, a small change in input voltage leads to a large change in input current (emitter current). This is the definition of a low-impedance circuit.
The output impedance of the common-base circuit is very high. The output impedance is the ratio between any change in applied voltage at the output point (the collector) and the resulting change in output (collector-emitter) current. Because of the "reverse-bias" nature of the collector-base junction, the collector voltage normally has very little control- over the collector current. In the common-base circuit of Fig. 1, this lack of control is compounded by the fact that any change in collector current (brought about by a change in collector voltage) is largely nullified when the collector current flows downward through resistor R4, thereby developing a change in the important emitter biasing voltage which counter-acts the original change in the collector voltage. Perhaps an example will make this action clear.
Fig. 1 shows a half-cycle of operation during which the collector-emitter current of transistor X2 has been increased.
Let us forget momentarily about the normal operating conditions responsible for this increase in collector-emitter current, and imagine for a moment that it was brought about by a large increase in the negative voltage applied to the collector (a difficult feat in any transistor!). As this increased collector current flows downward through emitter resistor R4, it develops a larger negative voltage at the upper terminal of R4, and also at the emitter of X2. In a PNP transistor, when the emitter voltage is made more negative ( or less positive) , less base emitter biasing current flows and, in turn, less collector-emitter current. This is a form of degeneration. There are many examples of degeneration in tube and transistor circuits, and they all seem to be characterized by the following two conditions:
1. An increase in electron-current flow through the tube or transistor will change the biasing conditions of the device in such a direction as to reduce the current through the device.
2. A decrease in electron-current flow through the device (usually on an alternate half-cycle) will change the biasing conditions in such a direction as to increase the current flow through the device.
"Phase inversion" of the input signal applied to the base of X1 has been accomplished by the complete circuit, because when the voltage at the collector of X1 becomes more negative, (as it did in Fig. 1), the voltage at the collector of X2 becomes less negative. Relating these important voltage changes to the electron currents which must actually flow in order to couple the voltage changes to the respective output circuits, we see electrons flowing into capacitor C1 in Fig. 1 and driving other electrons beyond C1, into the external circuit. This action "delivers" a negative voltage to that external circuit. Likewise, we see electrons being drawn out of capacitor C2. This action withdraws other electrons from the external circuit beyond C2 and, by so doing, "delivers" a positive voltage to that external circuit.
In Fig. 2 a half-cycle later, the increased collector current through X1 causes a smaller negative voltage to exist at the top of load resistor R3. Some portion of this increased current demand is supplied directly from C1, as electrons are withdrawn from its left plate. This action withdraws other electrons from the external circuit beyond C1, and thereby constitutes a positive voltage to the external circuit. The decreased collector current through X2 (and through R5) causes the voltage at the upper terminal of R5 to become more negative. This is symbolized by a flow of electrons onto the left plate of C2, and this action drives other electrons into the external circuit, beyond C2. The external circuit recognized the inflow of electrons as a negative voltage.
THE COMMON-COLLECTOR CIRCUIT
Figs. 3 and 4 show two successive half-cycles in the operation of a typical common-collector circuit. The common collector configuration has limited application and for this reason is not widely used. In this example it provides a high-impedance input circuit, in order to "match" this impedance with some equally high-impedance circuit which is providing the input voltage and current ( doing the driving, in other words). The common-collector circuit also provides a low-impedance output.
The primary purpose for including this type of circuit at this point in the guide is to convey some measure of qualitative under standing of the characteristics listed in Table 1 for the common collector circuit. Before these characteristics can be under stood, all the currents which flow in the circuit, and the resulting voltages and their changes, should be clear in your mind.
Identification of Components
This circuit includes the following components:
R1-Input resistor.
RVoltage-divider and biasing resistor.
R3-Voltage-divider, biasing, and input resistor.
R4--Emitter output resistor.
C1-Input coupling capacitor.
X1-PNP transistor.
M1-Battery or other DC power source.
Circuit Operation
This circuit may quickly be recognized as a common-collector configuration by the process of elimination. The input signal is obviously applied to the base ( or more accurately, between the base and ground). The output signal is obviously taken from the emitter (or between the emitter and ground). This leaves the collector, which is automatically "common" to both circuits. In the diagrams of Figs. 3 and 4, the collector technically is not grounded, but rather is connected to ground through the fixed voltage or power source M1. Power supplies, in practice, are usually bypassed by suitable filter capacitors, so that even at the lowest frequency of operation, signal frequency currents will be bypassed around the power supply to ground. This filtering process is known as decoupling and is discussed at greater length in other volumes of this series.
The presence of such a capacitor effectively places the collector at "ground" voltage to signal frequencies and thus enables us to say that it is a grounded-collector ( or the more accurate common-collector) circuit.
Four electron currents are at work in this circuit. They are:
1. Input driving current, usually called the "signal" (blue) .
2. Voltage-divider and biasing current (solid green).
3. Base-emitter biasing current (dotted green).
4. Collector-emitter current (red).
Let us momentarily ignore the presence of the signal current in Fig. 3, and consider only the currents which would flow during "static" conditions. First is the voltage-divider current, in solid green. It will flow continuously from the negative terminal of battery M1 and downward, through resistors R2 and R3, to ground. This current flow places a certain negative voltage at the junction of R2 and R3. Since the transistor base is connected directly to this point, this voltage constitutes the "base-biasing" voltage, and starts the initial flow of base emitter current through the transistor. The base-emitter current is shown in dotted green; it also originates at the negative terminal of battery M1. Base-emitter current is known as "biasing" current because it regulates the flow of the much larger collector emitter current (in red) through the transistor. When these two currents are established at their equilibrium values, they develop a voltage across resistor R4 because they must flow downward through R4 on their way to ground. This voltage, which is negative at the upper terminal, becomes the emitter "biasing" voltage. It must be less negative than the base volt age in a PNP transistor in order for any base-emitter biasing current to flow. The difference in the voltages at base and emitter normally is only a fraction of a volt, and only the tiniest change is required in either voltage to change the amount of base-emitter biasing current and consequently to bring about the much larger change in collector-emitter current.
The function of the applied signal voltage is to provide these necessary small changes in the voltage at the base. In the negative half-cycle of Fig. 3, the signal current is being driven downward through resistors R1 and R3. The small component of negative voltage developed at the top of R3 by the signal current must be added to the fixed negative voltage created at that point by the voltage-divider current. The result is more negative voltage ~t the base and in turn, an increase in both the base-emitter biasing current and collector emitter current. These increases have been indicated in Fig. 3.
During the positive half-cycle of Fig. 4, the impressed signal voltage becomes positive and the signal current is drawn upward through resistors R1 and R3. This creates a small component of positive voltage at the upper terminal of R3, and a less negative voltage at the base. In a PNP transistor, the lower base voltage decreases the flow of both currents through the transistor, as indicated in Fig. 4.
The current gain of a transistor in the common-collector configuration approaches the beta value (10 to 100) of the transistor itself. Since only a small change in the base-emitter biasing current is necessary to achieve a much larger change in the collector-emitter current, the current gain is high.
Fig. 3. Operation of the common-collector configuration-negative half-cycle.
Fig. 4. Operation of the common-collector configuration-positive half-cycle.
The voltage gain is a comparison, or ratio, between the voltage change applied to the input and the resulting voltage change developed at the output. This gain cannot exceed unity in the common-collector configuration because the input and output voltages are also the two important biasing voltages for the transistor. It was previously stated that the voltages at the base and emitter are the biasing voltages, and that their difference is normally less than a volt. Also, it was stated that a very small change in either voltage will change the amount of the two currents flowing through the transistor.
Let us consider the example in Fig. 3, where the applied signal current flowing downward through R3 is of such a quantity that it develops -0.1 volt at the base. This bias is added to the negative voltage already there as a result of the voltage divider current. A more negative base in a PNP transistor causes additional base-emitter biasing current and, in turn, additional collector-emitter current, to flow. Both of these increased currents must flow downward through emitter resistor R4; and in doing so, they increase the negative voltage already at the emitter.
If this increase in voltage at the emitter were to exceed -0.1 volt, then the difference between base and emitter volt age would be a smaller negative voltage than it was before the signal was applied. Consequently, instead of increasing as it should, the base-emitter biasing current would now tend to decrease, and so would the collector-emitter current.
To sum this action up, the voltage change at the emitter is the output voltage, and the voltage change at the base is the input voltage. The output-voltage change cannot exceed the input-voltage change without completely nullifying the latter. Therefore, the voltage gain of a common-collector configuration will always be less than unity.
Even though no voltage gain is available, this is not true for power gain in the common-collector configuration. Power gain is current gain multiplied by voltage gain, and even though voltage gain is slightly less than one, current gain may be high, as previously stated.
Table 1 indicates that the input impedance of a common collector configuration is very high. This is due to the difficulty in effecting any significant change in base-emitter biasing current by making changes in the base voltage. We have already seen that this is true, because the collector current flows through the emitter resistor and acts to nullify any change in the base voltage. Impedance can always be thought of as a ratio between some voltage and an associated current.
Input impedance is a ratio between the amount of change needed in the input voltage (base voltage) to change the input current (the base current, which is also the base-emitter biasing current). Because of the "sell-compensating" nature of the circuit, it is extremely difficult to effect any change whatsoever in the biasing current. For this reason, the input impedance of the common-collector configuration is very high. On the other hand, the output impedance of the common collector is very low. The output impedance is a measure of the relative ease or difficulty with which the output current (collector-emitter current) can be changed by varying the output voltage ( emitter voltage) . Since the emitter voltage is one of the two important biasing voltages of any transistor, a change of only a volt or two at the emitter will cause a large change in the amount of base-emitter biasing current, and an even larger change in the amount of collector-emitter current.