Define differential gain,common mode gain and CMRR in terms of circuit parameters.?

Question

Define differential gain,common mode gain and CMRR in terms of circuit parameters.?

Answer 1

Differential gain is the actually desired gain of the amplifier when different signals is applied to the two inputs of the amplifier. You could for instance apply a sine signal to the positive input (v+) and an inverted signal to the other input (v-). The (momental) differential gain is=output voltage/((v+)-(v-)).

Common mode gain is the undesired gain when the same signal is applied to both inputs. Ideally it should be zero.

CMRR (Common Mode Rejection Ratio) is a measure of the quality of the amplifier and is=differential gain/common mode gain. Usually it´s expressed in dB however.

The simplest way of calculating CMRR is to tie one input to the ground and input a sine at the other and measure the voltage (v1) at the output. Then input the same sine at both inputs and measure the voltage (v2) at the output. CMRR can then be calculated in dB as:

CMRR=20*log(v1/v2)

CMRR varies with frequency. Usually its good at low frequencies but gets worse at higher frequencies.