Common-mode rejection ratio of differential amplifiers

The common-mode rejection ratio (CMRR) of differential amplifier stages is treated by means of the matrix analysis of networks with the amplifier gain elements represented as four-pole devices. Two generalized CMRR equations are obtained in terms of imbalances in the transconductance and open-circuit voltage gain parameters of the composiste devices for both the resistor-loaded stage and the current-mirror-loaded stage, respectively. The nonlinear characteristic of the current mirror load has also been derived, and it is demonstrated that the high load sensitivity of CMRR in stages loaded by current mirrors is due to the mirror's nonlinearity. The effects of the second stage of an operational amplifier on its CMRR are considered. The CMRR expressions for a number of typical stages incorporating source resistances are given, and the experimental results with these stages are discussed in detail.