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. 1.9 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 (Vout) is taken 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 :
(i) There is neither collector resistor in the circuit nor there is emitter bypass capacitor. These are the two circuit recognition features of the emitter follower.
(ii) 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 ieRE 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 called emitter follower because the output voltage follows the input voltage.
The major characteristics of the emitter follower are :
(i) No voltage gain. In fact, the voltage gain of an emitter follower is close to 1.
(ii) Relatively high current gain and power gain.
(iii) High input impedance and low output impedance.
(iv) Input and output ac voltages are in phase.