Emitter Follower is also called as Negative current feedback circuit. This circuit exhibits a large Input Impedence, a small output Impedence, and a voltage gain of approximately Unity. Further the output voltage tends to be in phase with the Input Voltage hence the term FOLLOWER.
At first glance, one might be tempted to dismiss such a circuit as useless because of its unity Voltage gain. However, its Input and output Impedences promote its use as Input and output buffer stages in amplifier systems.
The circuit of an Emitter Follower is shown below:-
The main difference between an Emitter Follower and conventional amplifier is the absence of Collector Load and Emitter bypass Capacitor. The emitter resistance Re itself act as the Load and the ac output voltage Vout is taken across it. The biasing is provided either by base resistor method or potential divider method. Stabilization of operating point is exceptionally good because of high value of emitter resistance Re.
OPERATION OF EMITTER FOLLOWER
When input signal Vs is applied to the base, the resulting emitter current Ie develops an output voltage equal to IeRe across the emitter resistance Re. This voltage opposes the ac signal voltage Vs as it is in phase opposition to Vs. Thus it provides negative current feedback. Moreover, this voltage (Vout) feedback to the input is proportional to the emitter current hence this circuit is called NEGATIVE CURRENT FEEDBACK CIRCUIT.
It may be noted that when the input signal voltage Vs goes through its positive half cycle the output voltage Vout is also seen to go through its positive half cycle. Hence the output voltage is in phase with the input signal voltage.
ie; The output voltage (emitter voltage) just follows the input voltage and hence the name emitter follower.
ADVANTAGES OF EMITTER FOLLOWER
Emitter Follower's having a numerous merits some of them is depicted below:-
- High Input Impedence and Low Output Impedence, so it can be used for IMPEDENCE MATCHING. sometimes it is called as Impedence Transformer.
- Having 100% Feedback so the output is distortionless and Bandwidth is very Large.
- Relatively High current gain and Power gain.
- Output and Input ac voltages are in Phase and also approximately equal in magnitude. Emitter output voltage closely follows the input so it is called as Emitter Follower.