For the common-mode gain
analysis, the circuit is redrawn in Figure 6.
Due to the fact that the common-mode gain
[A.sub.cm] for all frequency filters equals to 0, the CMRR equals to infinite, if ideal active elements are considered.
Due to the tail current source in true differential amplifier, the common-mode gain
is reduced by increasing the output resistance of the bias current source.
where [v.sub.d] - [v.sub.in1] - [v.sub.in2], -([v.sub.in1] + [v.sub.in2])/2, [A.sub.d] is the differential-mode voltage gain, and [A.sub.cm] is the common-mode gain
. It is similar for the expression for [V.sub.02].
The CMRR is the ratio of differential gain to common-mode gain:
Amplifiers can have good common mode rejection at their inputs, or possibly even have common-mode gain. Some differential filter and attenuator topologies act on the differential component but not at all on the common mode component.
whereas the common-mode gain [A.sub.cm] is zero, and therefore the common-mode rejection ratio (8) is infinitely high.
where [A.sub.dm] and [A.sub.cm] are the differential and common-mode gains, respectively.