Amplification common collector

When a transisior is to be u ed in an electronic device, one of its terminals is eoimected to the inj>ut and the second servc as the oiitpui the third terminal is connected lo both and is ihc common tcrminal/riiree configuraiion.s are thus possible a common-emitter, a common-collector, and a common ba.se. The coiuinon-cmiiler coiiliguraiion has the widest application in amplification and is the one we consider in detail. 

Transistors are always operated so that amplification is obtained for a signal whose voltage is small on the input side (relative to the bias on the emitter) and small on the output side (relative to the collector voltage). Therefore small-signal theory applies. The designer obviously wants all signals to be amplified linearly, that is, by a common factor this makes the circuit behave more reasonably. 

The ratio of the input signal (either current or voltage) to the output signal is the "amplification factor," sometimes called "gain" or "transfer function." There are several ways to specify this, depending on whether the emitter or base or collector are "common." For common emitter circuits, the gain is also called the "hybrid forward emitter" ratio, or "beta," which is ... 

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