Differential amplifier with rail-to-rail input capability and controlled transconductance

An amplifier operable between first and second supply voltages which define a supply range consisting of (a) a first end range extending from the first supply voltage to a first specified voltage between the supply voltages, (b) a middle range extending from the first specified voltage to a second specified voltage between the first specified voltage and the second supply voltage, and (c) a second end range extending from the second specified voltage to the second supply voltage, the amplifier having:

• first differential means for amplifying a differential input signal by largely dividing a first operating current into first and second internal currents whose difference is representative of the input signal when its common-mode voltage (V.sub.CM) varies across the middle and second ranges;
• second differential means for amplifying the input signal by largely dividing a second operating current into third and fourth internal currents whose difference is representative of the input signal when V.sub.CM varies across the first and middle ranges;
• and summing means for selectively combining the internal currents to generate at least one output signal;
• characterized by control means for providing amplifier supply current to generate the operating currents and for suitably steering at least part of the supply current away from at least one of the differential means when V.sub.CM is in at least one part of the supply range so as to control the operating currents at values dependent on the steering of the supply current.