Rail-to-rail class AB output stage for operational amplifier with wide supply range

摘要

A rail-to-rail class AB output stage includes a P-channel pull-up transistor (4) having a source coupled to a first supply rail voltage (V+), a gate coupled to a first input conductor (2) of the output stage, and a drain coupled to an output terminal (6) of the output stage. An N-channel pull-down transistor (5) includes a source coupled to a second supply rail voltage (GROUND), a gate coupled to a second input conductor (3) of the output stage, and a drain coupled to the output terminal (6). A P-channel first bias transistor (20) includes a source coupled to the first input conductor (2) and a drain coupled to the second input terminal (3). A first bias circuit coupled between the first and second supply rail voltages produces a first bias voltage (21) on a gate of the first bias transistor (20). A P-channel second bias transistor (10) includes a source coupled to be first input conductor (2). An N-channel third bias transistor (11) includes a source coupled to the second input terminal (3) and a drain connected to a drain of the second bias transistor (10) and to a non-inverting input of a servo amplifier (12) having an output coupled to a gate of the second bias transistor (10) and an inverting input coupled to a gate of the third bias transistor (11) or a suitable reference voltage. A second bias circuit coupled between the first and second supply rail voltages produces a second bias voltage (16) on the gate of the third bias transistor (11) and the inverting input of the servo amplifier.