Question: why cmrr is infinite for differential amplifier?
Answer: CMRR is the ratio of differential gain and the common mode gain. For ideal op-amp, the value of CMRR is infinite, but for practical op-amp's the value of CMRR used to be in the range of 80 to 100 dB. ... So, if both inputs are equal then the output of the op-amp in ideal condition should be zero.