Common-mode rejection limitations of differential amplifiers

The common-mode rejection ratio (CMRR) equations of the differential pair and differential cascade amplifiers are reformulated in terms of imbalances in the saturation current. Early voltage, and current gain parameters of the transport model of the bipolar transistor. The effect of source resistance on CMRR is included in the formulation. The CMRR of this class of amplifier is shown to be limited by the magnitude and imbalance of the open-circuit voltage gain of the active devices. Also, a design characteristic, common to a wider class of differential amplifiers, is shown to produce a lower bound on the attainable CMRR.