A novel approach to common mode gain in differential amplifiers

The analysis of common mode behaviour in monolithic active loaded differential amplifiers has been a complicated task that is beyond simplified models, and therefore designers have left it to simulation programs. A new and simpler method of analysis is presented in which the circuit is divided in two parts which are analysed separately. Calculations are notably simplified and a reasonable accuracy is obtained. Furthermore, the influences of finite offsets on common mode and differential mode gains are determined. The proposed method is illustrated with a practical circuit, which is simulated using PSPICE.