Compact GM-control for CMOS rail-to-rail input stages by regulating the sum of the gate-source voltages constant

A circuit for controlling the transconductance (gm) of the rail-to-rail input stage of an operational amplifier (opamp) operating in strong inversion, the input stage having at least an N-channel and a P-channel differential input pair of transistors each having a tail current and a gate-source voltage, one gate of each pair being connected to one input terminal, the other gate of each pair being connected to the other input terminal, the sources of the N-channel pair being connected to the common N-tail, the sources of the P-channel pair being connected to the common P-tail, the opamp having a common-mode input voltage range including low, intermediate, and high common-mode input voltages, the input voltage range having transition regions between the low and intermediate, and the intermediate and high common-mode input voltage values, the circuit comprising:

• control means coupled to the common tails of the differential input stage of the operational amplifier and the positive and negative supply rails for directly maintaining the sum of the gate-source voltages between the tails of the input pairs constant, said control means including active devices so as to provide the tail currents to the differential input pairs.