Class-AB amplifier

Amplifier 'classes' (A, B, AB, C, D…) describe how much of each signal cycle the output transistors actually conduct — a trade-off between sound quality and wasted heat. Class A keeps a transistor on the whole time: beautifully linear but it burns power even in silence. Class B uses two transistors, each handling one half of the wave, which is efficient but leaves an ugly 'crossover' glitch where neither is quite on.

Class-AB is the engineer's sweet spot: it biases the two output transistors just barely on, so they overlap slightly through the handover. That small idle current banishes the crossover distortion of Class B while keeping most of its efficiency — which is why virtually every hi-fi and headphone amplifier output stage is Class-AB.