Analog Electronics

Class-B Output and Crossover

In a push-pull output the NPN drives the positive half and the PNP the negative half; since each transistor works only half the time, efficiency is far higher than Class-A (theoretically 78.5%).

The price is crossover distortion: within the ±0.7 V band both transistors are cut off, and steps appear in the output at the zero crossings. For small signals the distortion ratio is large.

The cure is Class-AB: a small pre-bias is applied to the bases via diodes or a VBE multiplier; the transistors are held at the edge of conduction and the dead band closes. This is the standard output stage of power amplifiers.

Formulas

η(Class-B, max) = π/4 ≈ 78.5%

Dead band ≈ ±VBE

⚡ Open this lesson in the simulator — free

Test Yourself

What causes crossover distortion?

Answer: Both transistors being cut off inside the ±VBE band — While |Vin| < 0.7 V neither transistor conducts → the output sticks at 0.

How does Class-AB solve this problem?

Answer: By pre-biasing the bases near the threshold — A small quiescent current eliminates the dead band.

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