This article attempts to propose theoretical computing formulas for input offset voltage and common mode rejection ratio of common emitter differential amplifier

common emitter-common base differential cascode amplifier and common emitter-common grid differential cascode amplifier. It proves that Early effect plays an important role so far as input offset and common mode rejection characteristics are concerned.In the past analysis on differential amplifiers offset characteristic and common mode rejection characteristic have been separately dealt with as two different questions. And

in discussing offset characteristic the existance of Early effect

the essence of the matter has been neglected. However

if Early effect if analyzed as the leading factor

input offset characteristic and common mode characteristic of differential amplifier will fundamentally be unified in conception. Such an analysis will clearly show how differential amplifier circuits have been developed.The results thus obtained have led to the design of low drift operational module. The article analyzes the drift contribution to the input offset voltage from the nonideal characteristic of the following stage and gives the method to match low drift.In this operational module

open loop gain and common mode rejection ratio are no less than 120dB and offset voltage drift is no more than ?±0.5μV/℃. Hence

it can be used as high quality general purpose operational devices for automatic control and automatic detecting system.