Question

If the differential voltage gain and the common mode voltage gain of a differential amplifier are 48 dB and 2 dB respectively then its common mode rejection ratio is:
1. 46 dB
2. 25 dB
3. 48 dB
4. 50 dB

Answer 1

Correct Answer - Option 1 : 46 dB

Concept:

The output voltage of op-amp is given by:

V0 = Ad Vd + Ac Vc

Where, Vd = V1 – V2

\(V_C=\frac{V_1+V_2}{2}\)

Ad = differential voltage gain

Ac = common mode voltage gain

\(CMRR=20~log (\frac{A_d}{A_c})\)

Calculation:

Given, 

Differential voltage gain = 48 dB

Common mode voltage gain = 2 dB

\(CMRR = 20~log(\frac{A_d}{A_c})\)

CMRR = 20 log(Ad) - 20 log(Ac)

= 48 - 2 = 46 dBCM\(V_C=\frac{V_1 ~+~ V_2}{2}\)Vc=V1+V22Vc=V1+VGiven that Differential voltage gain = 48 dBCommon mode voltage gain = 2