Common-mode rejection

The common-mode rejection ratio (CMRR) of a differential amplifier (or other device) is a used to quantify the ability of the device to reject common-mode signals, i.e., those that appear simultaneously and in-phase on both inputs. An ideal differential amplifier would have infinite CMRR, however this is not achievable in practice. A high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong electromagnetic interference (EMI). An example is audio transmission over balanced lines in sound reinforcement or recording.

Ideally, a differential amplifier takes the voltages, ${\displaystyle V_{+}}$ and ${\displaystyle V_{-}}$ on its two inputs and produces an output voltage ${\displaystyle V_{\mathrm {o} }=A_{\mathrm {d} }(V_{+}-V_{-})}$, where ${\displaystyle A_{\mathrm {d} }}$ is the differential gain. However, the output of a real differential amplifier is better described as

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