Emitter follower

The emitter follower (common-collector amplifier) is the BJT configuration with input at the base, output at the emitter, and collector at AC ground (usually tied to $V_{CC}$). Its voltage gain is just below 1, its input resistance is very high, and its output resistance is very low — it is the standard analog Buffer amplifier, the BJT analogue of the Source follower.

Why the output “follows” the input

In active mode the base–emitter drop is the roughly-constant 0.7 V of a forward PN junction. So the emitter voltage is

$v_E=v_B-V_{BE}\approx v_B-0.7\text{V}$

Whatever the base does, the emitter does the same thing offset by ≈0.7 V. The signal (the AC part) is passed through essentially unchanged: $v_e\approx v_b$, so

$A_v=\frac{v_e}{v_b}=\frac{R_E}{R_E+r_e}\lesssim 1$

Slightly less than 1 because the intrinsic [[Emitter resistance|$r_e$]] forms a small divider with $R_E$. There is no voltage gain — the value of the stage is entirely in its impedance transformation.

Common-collector / emitter follower: output at the emitter follows the input at the base; gain ≈ 1, very high input resistance, low output resistance — used as a buffer.

The impedance transformation

This is the whole point. By the Resistance reflection rule:

• Input resistance (high): the emitter load is reflected up to the base by $(\beta +1)$: $R_{in}=r_{\pi }+(\beta +1)R_E$ often hundreds of kΩ. The follower barely loads the source driving it.
• Output resistance (low): the source resistance at the base is reflected down to the emitter by $1/(\beta +1)$, and in series with the intrinsic $r_e$: $R_{out}\approx r_e+\frac{R_{source}}{\beta +1}\approx r_e\approx \frac{1}{g_m}$ typically only tens of ohms.

So a high-impedance signal source (which cannot itself drive a heavy load) connects to the follower’s base; the follower presents that source with an easy high-impedance load, and re-presents the same voltage at the emitter from a very low impedance, able to drive a heavy load. Voltage in ≈ voltage out, but the load is decoupled from the source. That is exactly the job of a Buffer amplifier, and it is why the emitter follower is the default output stage of analog circuits and op-amps.

Detailed analysis of the emitter follower: near-unity gain with a dramatic impedance transformation — the standard output buffer for analog circuits.

The three single-transistor BJT stages divide the labour: the Common-emitter amplifier provides voltage gain (inverting), the Common-base amplifier provides a low-input-impedance current buffer with wide bandwidth, and the emitter follower provides a near-unity-gain impedance buffer. They mirror the three MOSFET stages exactly.