For inputs at either or both gates, there exists a common-mode and differential-mode voltage. These are, for applied voltages Vg1 and Vg2, common mode
Equation 8.48
and differential mode
Equation 8.49
Based on these definitions, the output, for example, Vd1, for a given set of inputs is
Equation 8.50
where avd1 and avcm are (8.23) and (8.26) (with RD1), respectively. For the output Vd2, avd2 is substituted for avd1. Vdm is a pure differential input and is not with respect to ground; the effect from the bias resistor is accounted for in the common-mode gain.
For the case of a single-ended input, for example, Vg1 = Vi and Vg2 = 0, the output is
Equation 8.51
This is identical to (8.20). The common-mode contribution can be significant, for example, in a resistance feedback amplifier (Unit 11). In this case, Vdm in (8.50) can be very small compared to Vcm.
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