The emitter follower — also called the common-collector amplifier — is the third fundamental single-transistor topology. Unlike the common-emitter (voltage gain, inverting) or common-base (current gain of ~1, wideband), the emitter follower delivers a voltage gain just under unity, no phase inversion, high input impedance, and low output impedance. It doesn't amplify voltage. It amplifies current, which is exactly what you need for buffering.

The topology: input goes to the base, output is taken from the emitter, and the collector ties directly to V_CC. The emitter "follows" the base voltage, offset downward by one V_BE (~0.65 V). Any current the load pulls from the emitter is supplied through the collector — multiplied by β, so the base only draws 1/β of the load current.

Why it matters: impedance transformation. Input impedance is approximately β × R_E (in parallel with any bias network), and output impedance is roughly (R_source/β) + r_e, where r_e = 26 mV / I_E. A 2N3904 with β = 150 driving a 1 kΩ load at 5 mA emitter current gives you ~150 kΩ input impedance and ~5 Ω output impedance. That's a 30,000× impedance ratio through a 15-cent transistor.

Concrete example — driving a low-impedance load from an op-amp: A rail-to-rail op-amp with 50 mA output drive can't reliably feed a 32 Ω headphone. Wrap the op-amp's feedback loop around a complementary emitter-follower pair (NPN pulls up, PNP pulls down) and you get hundreds of milliamps of drive with the op-amp's precision. This is the output stage inside nearly every audio power amp ever built.

Rule of thumb: To pick R_E, set the quiescent emitter voltage to about V_CC/2 for maximum symmetric swing. With V_CC = 12 V and a target I_E = 10 mA, use R_E = 6 V / 10 mA = 600 Ω. Bias the base with a divider that sets V_B ≈ 6.65 V, and make the divider stiff — divider current should be at least 10× the base current (I_B = I_E/β ≈ 67 μA, so divider current ≥ 700 μA).

Watch out for:

• The V_BE drop eats your headroom — the emitter can't go higher than V_B − 0.6 V, so you lose ~1 V of positive swing.
• No level-shifting to ground — a single NPN follower can't pull the output below ~0.2 V. Use a PNP or a push-pull pair for full swing.
• Frequency peaking when driving capacitive loads. The output impedance is inductive at high frequencies (thanks to β falling off), and combined with load capacitance forms a resonant tank. A small series resistor (10–100 Ω) at the output damps this.