The OPAx863 devices include a rail-to-rail input stage. The main stage differential pair using PNP bipolar transistors operates for common-mode input voltages from VS–– 0.2 V till VS+ – 1.6 V. The amplifier inputs transition into the auxiliary stage using NPN transistors for common-mode input voltages from VS+ – 1.6 V till VS+ + 0.2 V. The PNP and NPN input stages offer a gain-bandwidth product of 50 MHz and a voltage noise density of 5.9 nV/√Hz. The offset voltage for the two input stages is matched to lie within the device specifications. The auxiliary NPN input stage does not use the slew boost circuit during large-signal transient response. The input bias current for the PNP and NPN input stages is opposite in polarity, which adds an additional offset based on the values of the gain-setting and feedback resistors. A common-mode input voltage transition between these input stages will cause a crossover distortion which needs to be considered in high-frequency applications requiring superior linearity. Limit the common-mode input voltage to VS+ – 1.6 V (maximum) for main-stage operation across process and ambient temperature.
Since the OPAx863 devices are bipolar amplifiers, the two inputs are protected with anti-parallel back-to-back diodes between them, which limits the maximum input differential voltage to 1 V. The amplifier is slew limited, and the two inputs are pulled apart up to 1 V when the anti-parallel diodes begin to conduct in very fast input or output transient conditions. Care must be taken to use gain-setting and feedback resistors large enough to limit the current through these diodes in such conditions.