The OPA97x is a 12V, true rail-to-rail input operational amplifier with an input common-mode range that extends to both supply rails. This wide range is achieved with paralleled complementary N-channel and P-channel differential input pairs, as shown in Figure 6-1. The N-channel pair is active for input voltages close to the positive rail, typically from (V+) – 1V to the positive supply. The P-channel pair is active for inputs from the negative supply to approximately (V+) – 2V. There is a small transition region, typically (V+) – 2V to (V+) – 1V, in which both input pairs are on. This transition region can vary modestly with process variation. Within this region, PSRR, CMRR, offset voltage, offset drift, noise, and THD performance can be degraded compared to operation outside this region.

For more information on common-mode voltage range and PMOS/NMOS pair interaction, see the Op Amps with Complementary-Pair Input Stages analog design journal.

Figure 6-1 Rail-to-Rail Input Stage