Rail-to-rail input/output operational amplifier and method

Abstract

A rail-to-rail differential amplifier includes first and second input terminals, and an output terminal and an input stage including differentially connected N-channel first and second input transistors, and differentially connected P-channel third and fourth input transistors. A P-channel first cascode transistor has a source coupled to a first supply voltage to the drain of the first input transistor. An N-channel cascode transistor has a source coupled by a second resistive element to a second supply voltage and to the drain of the third input transistor. A first gain boost amplifier has an output coupled to a gate of the first cascode transistor, a first input coupled to the source of the first cascode transistor and the drain of the first input transistor, and a second input coupled to a drain of the second input transistor and a bias control circuit. A second gain boost amplifier has an output coupled to a gate of the second cascode transistor, a first input coupled to the source of the second cascode transistor and to the drain of the third input transistor, and a second input coupled to a drain of the fourth input transistor and the bias control circuit. An output stage includes a pull-up transistor coupled between the first supply voltage and the output terminal, a pull-down transistor coupled between the second supply voltage and the output terminal, and a class AB bias circuit coupled between drain electrodes of the fist and second cascode transistors and between gate electrodes of the pull-up and pull-down transistors.