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Why CC amplifier is an emitter follower

Short answer

It is called emitter follower because the emitter voltage follows the base voltage with a gain just below 1 and no phase inversion. AC analysis yields Rin = R1∥R2∥(β+1)(re+RE∥RL) and Av = (RE∥RL)/(re+RE∥RL) ≈ 1.

Why common collector amplifier is known as emitter follower? Draw its ac equivalent circuit to find its input resistance and voltage gain.

Why it is called an emitter follower

In a common collector amplifier the input is applied to the base and the output is taken from the emitter. The emitter voltage is vE = vB - 0.7 V, so the output follows the input in phase with a voltage gain slightly below 1. The collector is connected to the supply (ac ground) and is common to input and output.

AC equivalent circuit (T-model)

 vs--Rs--+----B                    
         |      \  ie=(β+1)ib
       R1||R2    re
         |        \
        GND        E---+---- vo
                       |
                    RE||RL
                       |
                      GND

Collector and VCC are at ac ground. RB = R1 || R2 is from the base to ground. R'E = RE || RL.

Input resistance

vb = ie (re + R'E) = (β+1) ib (re + R'E)
Rib = vb/ib = (β+1)(re + R'E)
Rin = RB || Rib = R1 || R2 || (β+1)(re + RE||RL)

Example: β = 100, IE = 2 mA (re = 12.5 Ω), R'E = 2 kΩ: Rib = 101 × 2012.5 ≈ 203 kΩ.

Voltage gain

vo = ie R'E ,   vi = vb = ie (re + R'E)

Av = vo/vi = R'E/(re + R'E)  ≈ 1   (0.99 in the example)

So Av is just below 1, positive, with no phase inversion. With source resistance: Avs = Av · Rin/(Rin + Rs).

It acts as a buffer: high Rin, low Rout, voltage gain ≈ 1 but a large current gain.

✓ Answer checked by Claude Opus 5.5 (high effort)

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