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

Abstract

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