A simple and accurate method for calculating the low frequency common-mode gain in a MOS differential amplifier with a current-mirror load

A simple and accurate analysis is presented for the low frequency common-mode gain of a metal oxide semiconductor (MOS) differential pair with a current-mirror load. The circuit is shown to be symmetric when the output conductance of the diode connected transistor is not ignored. This allows use of a half-circuit analysis and an accurate expression for the common-mode gain can be easily derived. The gain expressions in some previous textbook analyses are shown to be inaccurate.