Common-mode rejection of monolithic operational amplifiers

A simple expression is derived for the common-mode rejection ratio (CMRR) of monolithic operational amplifiers in terms of transistor processing parameters. The tradeoffs between transistor current gain and CMRR are investigated. The feasibility of 120-dB CMRR is demonstrated with a common-emitter differential pair. The addition of eight active devices can further improve CMRR, although at the expense of other parameters.