The front stage is usually a fully differential amplifier dedicated to rejecting high input common-mode voltage so that a clean and gained-up version of the differential input is passed to the output stage, which is effectively isolated from the high input common-mode voltage. This is a huge advantage in achieving exceptional CMRR.

Matching resistor networks is important in both stages to achieve excellent CMRR and GE performance. This is especially true for the input stage, where the resistor network is sometimes in-package trimmed at final production. The output of the input stage is fully differential, and the common-mode voltage is defined by the Common Mode Feed Back (CMFB) circuitry. It is worth noting that this common-mode output voltage is the same as the common-mode input voltage of the output stage. For compliance, the common-mode voltage is set between ground and supply voltage. It is normally set to somewhere close to mid-supply. With this arrangement, the output stage is decoupled from the high input common-mode voltage seen at input of the device.

The transfer function of the amplifier shown in Figure 3-3 can be written as: