The input common-mode range may be defined as the maximum range of the input voltage that can be simultaneously applied to both inputs without causing cutoff, clipping, or saturation of the amplifier gain stages. The input stage must be capable of operating within its specifications over the dynamic range of output swing. If it cannot, the amplifier may saturate (or latch-up) when the input limits are exceeded.
Latch-up occurs most often in voltage-follower stages where the output voltage swing is equal to the input voltage swing and the operational amplifier is driven into saturation. The specified common-mode voltage range of the input stage must exceed the maximum peak-to-peak voltage swing at the input terminals or the input stage may saturate on peaks. When saturation occurs, an inverting stage no longer inverts. The negative feedback becomes positive feedback and the stage remains in saturation.