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Class-B push-pull output stage (crossover distortion)

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

Open this experiment in the circuit simulator

Objective

Drive a 1 kΩ load with a complementary emitter follower and see the dead zone around zero (crossover distortion).

Procedure

Run and compare input and output near the zero crossing.
Reduce the amplitude to 0.5 V: the output vanishes.

Theory

The NPN conducts only above +0.65 V and the PNP only below −0.65 V, so inputs between ±0.65 V produce no output. Class-AB bias (two diode drops) removes the dead zone.

Questions to answer

How do two bias diodes remove the distortion?

Components used

2N3906 PNP transistor, 2 × Battery / DC source, Function generator (sine), Oscilloscope (2-channel), 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.