Adaptive biasing technique to convert pseudo-class AB amplifier to class AB

Abstract

An essential modification to a conventional four-stage pseudo class AB amplifier results in a true class AB amplifier. As opposed to the conventional pseudo class AB scheme, which uses a current mirror in the third stage, this work uses an adaptive biasing circuit in the third stage to drastically reduce quiescent current. The pseudo class AB and new class AB amplifiers are designed in a 0.5 µm process with power supplies of ±1.5 V for a phase margin of 67° while driving a load of 32 Ω ‖ 500 pF. At a maximum negative load current of −12.5 mA, simulations show that both amplifiers consume 43.9 µA of quiescent current. However, at a maximum positive load current of +12.5 mA, the pseudo class AB consumes 1.22 mA of quiescent current, while the new class AB amplifier consumes only 140 µA: a reduction in quiescent current by more than a factor of eight at maximum positive load.