6 Other BJT amplifiers

							6 Other BJT amplifiers
The common-emitter amplifier is perhaps the most frequently used of transistor amplifiers, but there are other kinds of BJT amplifier, each with its special features and applications. In this chapter we look at these and discuss their useful properties.

Common-collector amplifier
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A common-collector amplifier is the BJT equivalent of the FET commondrain amplifier and is used for similar purposes. This is why its alternative name is the emitter-follower. Fig. 6.1 depicts a typical common-collector circuit. As usual, capacitors couple the circuit to preceding and following stages. But these capacitors are not an essential part of the amplifier. If input or output quiescent voltage levels are suitable, the amplifier may be wired directly to adjacent stages.

Figure 6.1 A common-collector BJT amplifier is also known as an emitter-follower, because the output voltage at the emitter is always close to the input voltage. 68

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Other BJT amplifiers 69 RB1 and RB2 are used to bias the transistor into conduction. Normally the base current is such as to produce a collector current which generates a voltage across RE equal to half the supply voltage. This makes the quiescent output voltage sit nicely between the two extremes and allow the output signal to be as large as possible without risking its being clipped. The base-emitter junction is forward biased so the base current flows through the junction and becomes part of the emitter current. It flows first through the internal emitter resistance (re, Fig. 5.8) then through the emitter resistor RE. Together these act as a potential divider, so that the output voltage is less than the input voltage. However, re is small (typically 25 ) and RE is large (several kilohms), so the output voltage is generally 99 % or more of the input voltage. This is close enough to unity gain. Since output follows input very closely, the output is in phase with the input. In other words, this is a non-inverting amplifier.
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The input resistance of the amplifier is equal to the resistance of the biasing resistor (or resistors) in parallel with the load resistance (which includes RE). The biasing resistors typically have values rated in tens of kilohms. In the case of the load resistance, looking into the base of the transistor we see RE multiplied by the gain hfe , so RE appears to be several kilohms multiplied by 100 or more, that is, several hundred kilohms. As a rule, the amplifier is feeding its signal to a circuit connected in parallel with RE. The input resistance of the amplifier thus comes to be the parallel resistance of: the biasing resistors, RE multiplied by the current gain, and the input resistance of the load circuit.

• • •

Usually the resulting input resistance is high, so that we can think of a typical emitter follower amplifier as having a high input resistance. By contrast, the output resistance is approximately equal to re plus the output resistance of the source circuit (for example a signal generator or microphone) divided by the gain of the transistor. If the source has low output resistance, we may ignore it and consider only re, which is only a few tens of ohms. If the source has very high output resistance we may take RE to be wired in parallel with this, so reducing the output resistance of the amplifier. In most cases the result is a very low output resistance. With its high input resistance and low output resistance, the common-

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70

Other BJT amplifiers

collector amplifier is particularly useful as an impedance-matching (or resistance-matching) stage. That is to say, if we have a circuit or device (such as a sensor, for example) which has low output resistance and connect it to a monitoring circuit with low input resistance, we will probably find that the signal passing from one to the other is markedly reduced in amplitude. It may be almost lost. But if we put an emitter follower between the two, the follower draws very little current from the sensor, so maintaining the amplitude of its signal. At the same time the follower provides ample current to drive the monitoring circuit.

Bootstrapping
The concept of pulling oneself up by one’s own bootstraps is a comical one but is a lucid description of the function of capacitor C3 in Fig. 6.2. Buy Omitting R4http://www.download-it.org/learning-resources.php?promoCode=&partnerID=&content=story&storyID=1002 this file from and C3, we have a typical common-collector amplifier. Its output follows changes in the input but is usually six or seven hundred millivolts lower. Its input resistance, given the typical values quoted in the caption of Fig. 6.2, is the resistance of R1 and R2 in parallel. On a simulator, this is equal to 8.3 k . If we then insert R4 and connect C1 to the base side of R4, the input resistance becomes 17.9 k , the increase being due to the 10 k of R4 now in series with the paralleled resistances of R1 and R2. Apart from this, the action of the amplifier is unaffected. The final

Figure 6.2 Bootstrapping by the capacitor between nodes a and b gives this emitter-follower amplifier an input resistance of almost 500 kilohms, even though R1, R2 are as small as 15 and 18 kilohms, and R4 is only 3 kilohms.

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