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Emitter follower (common collector)

Analog Circuits Lab · free online experiment, runs in your browser with no install or sign-up.

Open this experiment in the circuit simulator

Objective

Buffer a 1 V sine with an emitter follower: voltage gain just below 1, in phase, low output resistance.

Procedure

Run and compare input and output.
Load the output with 100 Ω and see how little the gain changes.

Theory

Av = gm·RE/(1 + gm·RE) ≈ 0.99 and the DC level shifts down by VBE ≈ 0.65 V. The input resistance is high (≈ β·RE) and the output resistance is only re ≈ 1/gm.

Questions to answer

Where is an emitter follower used?

Components used

Battery / DC source, 2 × Electrolytic Capacitor 100uF 25V, Function generator (sine), Oscilloscope (2-channel), 4 × Resistor 220 ohm 1/4W, Transistor BC547 (NPN)

About the model

Time scale: for the browser simulator the source frequencies are divided by 100 and capacitors / inductors multiplied by 100; the waveforms keep their shape but run 100 times slower. Ideal wires; component models as listed in the catalogue (no parasitics, no temperature drift). Built without a breadboard so the schematic stays readable.