LearnTronics
Emitter Follower — Analog Buffer
Recipe #8 — emitter follower / analog buffer
An emitter follower usually gives very little voltage gain. That sounds unimpressive until a circuit needs to drive a load without dragging down the signal source.
The output follows the input
The emitter voltage follows changes at the base, typically about one base-emitter junction drop lower in DC level for a silicon transistor.
For small AC changes, the voltage gain is usually somewhat less than 1.
Real-world use: buffering a high-impedance signal source, driving a lower-impedance load, isolating amplifier stages, and increasing available current drive.
This should sound familiar
The digital buffer introduced fan-out: a signal may need help driving several following inputs. The emitter follower is an analog version of the same broad engineering idea — preserve the signal while improving drive capability.
Buffer does not mean “free power”
The extra load current comes from the power supply through the transistor. The input signal controls that power; it does not create it.