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Emitter follower (buffer)

Transistors and thyristors · Intermediate · about 15 minutes · part of Zen’s Electronics Design Mastery

Take the output from the emitter and it follows the base, about 0.65 V lower: gain ≈ 1. The base draws only I_E / (β + 1), so a weak source (here a 10 kΩ pot) can drive a much heavier load.

Emitter follower (buffer): the circuit on the simulator's bench

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How it works

Unloaded wiper: VCC × position = 6 V, from a source resistance of 2.5 kΩ.

VE = VB − VBE = 5.928 V − 697.4 mV = 5.23 V; IE = VE / RE = 5.23 mA

IB = IE / (β + 1) = 28.9 µA. The pot sees RE multiplied by β + 1 ≈ 181 kΩ, so its voltage barely sags.

Try it on the bench

  1. Scroll over the pot: V_E tracks V_B, always about 0.65 V below.
  2. Lower R_E to 100 Ω: the load current rises tenfold, yet V_B hardly moves.
  3. Turn the pot right down: below about 0.6 V on the base the transistor switches off and V_E falls to 0.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “Transistors”.

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