The analysis of balanced, linear differential circuits

The salient characteristics of differential electronic systems are overviewed, and a generalized low frequency analysis of these systems is presented. This analysis, which produces accurate Thevenin equivalent circuits for the input and output ports of balanced differential pairs, discloses a shortcoming pervasive of much of the electronic circuits textbook literature. In particular, an account is rarely made of the dependence of the differential input and differential output resistances on respective common mode input and output resistances. It is shown that the functional relationship of differential resistance to corresponding common mode resistance is easily factored into the generalized analysis of balanced differential pairs. The paper goes on to demonstrate that the failure to account for this dependence can lead to significant analytical errors when the common mode resistance at an input or an output port of a balanced pair is not substantially larger than the corresponding differential mode resistance. Such a situation often materializes when circuits are designed to deliver very low equivalent input noise and/or very wide passbands