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Push-pull output stage

Transistors and thyristors · Advanced · about 20 minutes · part of Zen’s Electronics Design Mastery

An NPN emitter follower can only push current out and a PNP one can only pull it in. Put one of each back to back and the pair drives a load both ways, from either rail. That is the output stage of almost every audio amplifier and op-amp. Biased like this (class B), neither transistor conducts while the input is within about ±0.65 V of zero: crossover distortion.

Push-pull output stage: the circuit on the simulator's bench

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How it works

For vin > +0.65 V, Q1 conducts and vout ≈ vin − VBE. For vin < −0.65 V, Q2 does and vout ≈ vin + VBE. In between, both are off and vout = 0.

Output peak ≈ 2.985 V − 735.6 mV = 2.249 V. The flat step where the waveform crosses zero is crossover distortion, worst at small signal levels.

Class AB fixes it by biasing each base about 0.65 V apart with two diodes and resistors, so both transistors idle just on.

Try it on the bench

  1. Look at the blue output trace: a flat step every time it crosses zero.
  2. Lower the input to 0.5 V RMS: now the distortion is a large part of the signal.
  3. Raise it to 7 V RMS: the dead zone becomes a small fraction, which is why class B sounds worst at low volume.

Parts on the bench

In the course

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

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