The OPAx994 is a 32V, 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 PNP and NPN differential input pairs, as shown in Figure 6-3. The NPN pair is active for input voltages close to the positive rail, typically from (V+) – 1V to the positive supply. The PNP 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 may be degraded compared to operation outside this region.

For more information on common-mode voltage range and complementary pair interaction, see Op Amps With Complementary-Pair Input Stages application note.

Figure 6-3 Rail-to-Rail Input Stage