Q.Define CMRR of an op-amp.

A good op-amp should amplify the difference between its inputs but ignore any signal common to both, and CMRR measures how well it does this. …

[!TLDR]

CMRR = differential gain $\div$ common-mode gain; a large value means good rejection of unwanted common signals.

An op-amp is meant to amplify only the difference between its two inputs. Any signal that appears equally on both inputs, such as noise or supply hum, is a common-mode signal and should ideally produce no output. The Common Mode Rejection Ratio quantifies this ability: it is the ratio of the differential-mode gain $A_d$ to the common-mode gain $A_c$. A high CMRR (for the 741 about 90 dB) means the common-mode signal is …