Question

Show that the voltage gain of Emitter Follower is less than unity.

Answer 1

Voltage gain is

$$A_v=V_0/V_{in}$$

If we look at the emitter follower circuit, according to Kirchhoff's voltage law we can write:

$$V_{in}-V_{be}=V_0$$

Substitute and get:

$$A_v=\frac{V_{in}-V_{be}}{V_{in}}<1.$$

But why is that? Because for semiconductors,

$$V_{be}=(0.3...0.7)\text{ V}.$$