Applications of Emitter Follower
The emitter follower has the following
principal applications :
(i) To provide current amplification with no
(ii) Impedance matching.
(i) Current amplification without voltage gain.
We know that an emitter follower is a current
amplifier that has no voltage gain
(Aν = 1). There are many instances (especially in digital electronics) where an
increase in current is required but no increase in voltage is needed. In such a
situation, an emitter follower can be used. For example, consider the two stage
amplifier circuit as shown in Fig. 1.16. Suppose this 2 stage amplifier has the
desired voltage gain but current gain of this multistage amplifier is
insufficient. In that case, we can use an emitter follower to increase the
current gain without increasing the voltage gain.
Impedance matching. We know
that an emitter follower has high input impedance and low output impedance.
This makes the emitter follower an ideal circuit for impedance matching. Fig.
1.17 shows the impedance matching by an emitter follower. Here the output
impedance of the source is 120 kΩ while that of load is 20 Ω. The emitter
follower has an input impedance of 120 kΩ and output impedance of 22Ω. It is
connected between high-impedance source and low impedance load. The net result
of this arrangement is that maximum power is transferred from the original
source to the original load. When an emitter follower is used for this purpose,
it is called a buffer amplifier.
It may be noted that the job of impedance
matching can also be accomplished by a transformer. However, emitter follower
is preferred for this purpose. It is because emitter follower is not only more
convenient than a transformer but it also has much better frequency response
i.e., it works well over a large frequency range.