Rail-to-rail input stage of an operational amplifier

摘要

An input stage of an operational amplifier uses current sources to allow first and second differential input transistor pairs to operate near the power supply rails. The output stage of the operational amplifier also operates within a saturation potential of the power supply rails. The first differential input transistor pair operates when the input signal is less than a predetermined threshold, while the second differential input transistor pair operates when the input signal is greater than the predetermined threshold. A detection circuit at the input terminals prevents phase inversion of the output signal should the inputs be driven beyond the power supply rails. A current cancellation circuit removes current variation induced by voltage changes at the output of the input stage and provides high gain and low input offset voltage.