The specified input common-mode voltage range of OPAx620 extends 100mV beyond the supply rails. This extended range is achieved with a complementary input stage: an N-channel input differential pair in parallel with a P-channel differential pair; see also Section 8.2. The N-channel pair is active for input voltages close to the positive rail, typically (V+) − 1.2V to 100mV greater than the positive supply. The P-channel pair is active for inputs from 100mV less than the negative supply to approximately (V+) − 1.2V. There is a small transition region, typically (V+) − 1.5V to (V+) − 0.9V, in which both pairs are active. This 600mV transition region vary ±500mV with process variation. Therefore, the transition region (both input stages active) range from (V+) − 2V to (V+) − 1.5V on the low end, up to (V+) − 0.9V to (V+) − 0.4V on the high end.

A double-folded cascade adds the signal from the two input pairs and presents a differential signal to the class AB output stage.