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Class A, B and AB output stages

Transistors and thyristors · Intermediate · about 15 minutes · part of Electronics Engineering

Two emitter followers, an NPN for the positive half and a PNP for the negative half, push and pull the speaker. With the bases tied together (class B) neither conducts for the first 0.65 V either way: the output has a flat step at every zero crossing. Two diodes pre-bias the bases (class AB) and the step disappears.

Class A, B and AB output stages: the circuit on the simulator's bench

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How it works

Class B: each transistor needs V_BE ≈ 0.65 V before it conducts, so the output peak is V_in,peak − 0.65 = 3.35 V and the output sits at 0 V for |V_in| < 0.65 V (crossover distortion).

Efficiency at full swing reaches π/4 ≈ 78 %; class A, which runs current all the time, manages 25 %.

Try it on the bench

  1. Reduce V_in to 0.5 V peak: class B gives no output at all.

Parts on the bench

In the course

Electronics Engineering, module “Transistor amplifiers”.

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