classAB

Class AB is a category of electronic amplifier operation used primarily in audio power amplifiers. It lies between Class A and Class B: each output device conducts more than 180 degrees but less than 360 degrees of the input signal, providing overlap around waveform zero-crossings. This overlap reduces the crossover distortion typical of Class B while avoiding the continuous high idle current of Class A.

In a typical Class AB stage, a push-pull pair of complementary devices (bipolar transistors or MOSFETs) handles positive and negative halves of the waveform. The devices are biased slightly on, using diodes, a Vbe multiplier, or an adjustable bias circuit, so both conduct briefly near zero crossover. Proper biasing and thermal tracking are essential, because changes in temperature alter transistor or gate threshold and can reintroduce distortion or drift.

Performance characteristics balance efficiency and linearity. Class AB generally offers better efficiency than Class A and lower distortion than pure Class B, especially when global negative feedback is employed. Typical practical efficiency for audio power stages ranges from the upper tens to around 60–70 percent, depending on design and load. However, biasing complexity and thermal management add design challenges, and if bias or thermal stabilization is inadequate, crossover distortion can increase.

Applications include consumer and professional audio amplifiers, guitar amplifiers, and other high-fidelity power amplifier stages. Variants exist in vacuum-tube terminology (Class AB1 and AB2), which describe whether grid current occurs, affecting bias and distortion characteristics. Overall, Class AB remains a common compromise between linearity and efficiency in audio amplification.