Controlling the quiescent current in class AB output stages has long been the Achilles heel of such amplifiers. Low crossover distortion depends critically on correct and stable quiescent current. Crossover distortion cannot be completely eliminated in AB designs due to the output devices switching in and out of conduction on alternate signal half cycles. The thermal stability of the bias control is also important for reliability of class AB amps with respect to thermal runaway. Daniel Joffe has designed a novel control circuit that prevents output devices to switch off completely, maintaining a small but critical quiescent current over the whole signal cycle. The circuit has no critical elements, uses only low-cost small-signal devices and can be grafted onto almost any amplifier.
Practical Electronic Control of Class AB output stage quiescent current