circuits. The input and output of the emitter follower are the base and the emitter, respectively An emitter follower circuit shown in the figure is widely used in AC amplification, therefore this circuit is also called common-collector circuit.
DC operating point
Solving these equations, we can get , and
AC small-signal equivalent circuit
We assume and therefore can be ignored, and have
As , is smaller than but approximately equal to 1.
The input resistance:
The input resistance is in parallel with the resistance of the circuit to its right including the load , which can be found by . But as
Comparing this with the input resistance of the common-emitter circuit , we see that the emitter follower has a very large input resistance.
The output resistance:
The output resistance is in parallel with the resistances of the circuit to its left including the source, which can be found by , where
and we have
Alternatively, the output resistance of the emitter follower (treated as either a voltage or current source) can also be found as the ratio of the open-circuit voltage and the short-circuit current , where is the output voltage when the load is open-circuit , and is the output current when the load is short-circuit .
As the voltage gain of the emitter follower is close to unity, the open-circuit output voltage is approximately the same as the source voltage . The short-circuit current can be found as
Now the output resistance can be found as
The overall output resistance is
The emitter follower is a circuit with deep negative feedback, i.e., all of its output is fed back to become part of its input . The fact that this is a negative feedback can be seen by:
Due to this deep negative feedback, the voltage gain of the emitter follower is smaller than unity. However, many other important properties of the circuit, such as the input and output resistances, are drastically improved. In fact the emitter follower acts as an impedance transformer with a ratio of , i.e., the input resistance is times greater than and the output resistance is times smaller than .
Comparing this with the input resistance and output resistances of the common-emitter transistor circuit, we see that the emitter follower circuit has very favorable input/output resistances.
Although the emitter follower does not amplify voltage, due to its high input resistance drawing little current from the source, and its low output resistance capable of driving heavy load, it is widely used as both the input and output stages for a multi-stage voltage amplification circuit.
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