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Common-emitter amplifier

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

Open this experiment in the circuit simulator

Objective

Amplify a 10 mV, 1 kHz signal with a voltage-divider-biased common-emitter stage and measure the gain and phase inversion.

Procedure

Run and compare the input and output waveforms in the Waveforms tab.
Remove the bypass capacitor (set Ce to 1 pF): the gain drops to about −Rc/Re = −2.

Theory

Ic ≈ 1.45 mA gives gm = Ic/VT ≈ 56 mS. With the emitter bypassed, Av ≈ −gm·(Rc ∥ RL) = −56 mS · 1.8 kΩ ≈ −100. The output is inverted (180°).

Questions to answer

Why does the bypass capacitor raise the gain?

Components used

Battery / DC source, 3 × Electrolytic Capacitor 100uF 25V, Function generator (sine), Oscilloscope (2-channel), 5 × 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.