Question

A differential amplifier with a common-mode input of 200 mV and a difference-mode input of 64 mV has an output of 2 mV due to the common mode input and 8 V due to difference-mode input. Find the common-mode rejection ratio.

• A. 82 db
• B. 70 db
• C. 10,000 db
• D. 12,500 db

Answer 1

The correct option is A 82 db

Difference-mode gain

$A_d=\frac{v_0}{v_d}=\frac{8}{64\times {10}^{-3}}=125$

Common-mode gain

$A_c=\frac{v_0}{v_c}=\frac{2\times {10}^{-3}}{200\times {10}^{-3}}=0.01$

$\therefore$ Common-mode rejection ratio CMRR

$=\frac{A_d}{A_c}=\frac{125}{0.01}=12,500=81.93db\approx 82db$