1.2V constant-gm rail-to-rail CMOS Op-Amp input stage with new overlapped transition regions technique for ECG amplifier

The conventional technique of overlapped transition regions for producing a constant transconductance rail to rail Op-Amp input stage can only tolerate a limited amount of voltage shifting. This is limited by the minimum Vgs required for active mode operation of transistors. In this paper, we present a novel overlapped transition regions technique that overcomes the limitation of the conventional technique. This new overlapped transition regions technique has no voltage shifting limit. For both N-type and P-type conventional complementary differential input pairs, one source follower and one MOSFET are added to control the saturation point of current of input pairs. For 1.2V single supply voltage, simulation results demonstrate ±3.71% of overall transconductance variation. Cadence SPECTRE simulator and TSMC 0.25-μm CMOS technology are used to layout and simulate this work.