Emitter Follower

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Emitter Follower

It is a negative current feedback circuit. The emitter follower is a current amplifier that has no voltage gain. Its most important characteristic is that it has high input impedance and low output impedance. This makes it an ideal circuit for impedance matching. Circuit details. Fig. 1 shows the circuit of an emitter follower. As you can see, it differs from the circuitry of a conventional CE amplifier by the absence of collector load and emitter bypass capacitor. The emitter resistance RE itself acts as the load and a.c. output voltage (V_out) is take across RE. The biasing is generally provided by voltage-divider method or by base resistor method.

The following points are worth noting about the emitter follower :

There is neither collector resistor in the circuit nor there is emitter bypass capacitor. These are the two circuit recognition features of that.
Since the collector is at ac ground, this circuit is also known as common collector (CC) amplifier.

Operation

The input voltage is applied between base and emitter and the resulting a.c. emitter current produces an output voltage i e RE across the emitter resistance. This voltage opposes the input voltage, thus providing negative feedback. Clearly, it is a negative current feedback circuit since the voltage fedback is proportional to the emitter current i.e., output current. It is call emitter follower because the output voltage follows the input voltage.