7.3.2 Rail-to-Rail Input

The specified input common-mode voltage range of the OPAx354-Q1 family of devices extends 100 mV beyond the supply rails. A complementary input stage (an N-channel input differential pair in parallel with a P-channel differential pair) achieves this extension. The N-channel pair is active for input voltages close to the positive rail, typically (V+) – 1.2 V to 100 mV above the positive supply, while the P-channel pair is on for inputs from 100 mV below the negative supply to approximately (V+) – 1.2 V. A small transition region exists, typically (V+) – 1.5 V to (V+) – 0.9 V, in which both pairs are on. This 600-mV transition region can vary ±500 mV with process variation. As a result, the transition region (both input stages on) range from (V+) – 2 V to (V+) – 1.5 V on the low end, up to (V+) – 0.9 V to (V+) – 0.4 V on the high end.

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