The Common Collector Amplifier

The common collector amplifier is also called the emitter follower amplifier because the output voltage signal at the emitter is approximately equal to the voltage signal input on the base. The amplifier's voltage gain is always less than unity, but it has a large current gain and is normally used to match a high-impedance source to a low-impedance load: the amplifier has a large input impedance and a small output impedance.

A typical common collector amplifier is shown in figure 5.12.

 
Figure 5.12:  Basic common collector amplifier.

The voltage gain can be written as

The gain is thus in phase and slightly less than unity. The output impedance of the CC amplifier can be substantially less than the output impedance of the driving signal source.

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