Differential amplifier having extended common mode input voltage range

摘要

A differential amplifier (10) having a circuit (18) which images or simulates the bias current flowing in an input stage (34) of the differential amplifier (10) is disclosed. This image circuit (18) resembles a differential amplifier input stage, couples to the signal inputs (28, 30) of the differential amplifier (10), and provides an output signal that reflects such bias current. One embodiment of the present invention feeds this output signal back to the differential amplifier input stage (34) to improve regulation of a constant current source (26). Another embodiment uses this signal to switch current from an external source (86) to the differential amplifier input stage (34) when a constant current source (26) within the differential amplifier input stage (34) fails to maintain a constant current supply.