Class Ab Amplifier
A Class AB amplifier splits the output swing across two complementary devices — one sourcing current through the positive half-cycle, one sinking it through the negative half — and biases both with a small quiescent current so they overlap slightly near zero crossing, trading away Class A's perfect 360° conduction for 50-70% efficiency while keeping the crossover distortion that plagues true Class B small enough to stay inaudible.
The bias current is a tuning knob, not an afterthought. Set it too low and the amplifier behaves like Class B, with an audible notch of distortion every time the signal crosses zero because neither device is conducting; set it too high and the amplifier drifts toward Class A's heat and poor efficiency, so practical designs dial in just enough overlap current — often tens of milliamps in a solid-state power amp — to erase the crossover notch without sacrificing the efficiency gain, a balance that diode-based bias networks and thermal-tracking circuits exist specifically to hold stable as the output transistors heat up.
Efficiency scales with how hard the amplifier is driven, which is the whole appeal. Unlike Class A, where dissipation is nearly constant regardless of signal, a Class AB stage draws DC current roughly proportional to output power, so an idle or lightly loaded amplifier runs cool and a 100-watt AB amplifier might draw only 150-170 watts at full output instead of the 400+ watts a comparable Class A design would need, which is why essentially every mass-market audio power amplifier, from a car stereo module to a Crown or QSC PA amp, runs Class AB rather than Class A.
Crossover distortion is a geometry artifact, not a mystery. It happens because real transistors have a dead zone — base-emitter turn-on voltage for bipolar devices, threshold voltage for MOSFETs — that neither complementary device crosses instantaneously, so without bias current there's a brief window where output current is near zero while input voltage is not; the small quiescent bias current moves both devices just past that dead zone before the signal arrives, which is the mechanical trick that makes AB work.
It is the default choice for any application needing both power and fidelity. RF power amplifiers in cellular base stations, audio power stages from consumer receivers to professional touring rigs, and most motor-drive and class-D-adjacent linear stages default to AB because it recovers most of Class B's efficiency while keeping distortion low enough that, with modest feedback and careful bias tracking, measured THD can rival a Class A design at a fraction of the heat — the handful of audiophile holdouts who still prefer pure Class A are paying directly for the last fraction of distortion reduction that AB's compromise gives up.
Read a Class AB amplifier through a *managed handoff* lens rather than a *compromise* lens: the small bias current isn't a half-measure between Class A and Class B, it's the specific engineering control that converts a hard discontinuity at zero crossing into a soft one, which is why AB — not A, not B — ended up as the default topology everywhere both power efficiency and audible fidelity actually matter.