Question

Find Common Mode Rejection Ration, given gm =16MΩ^-1, RE=25kΩ

(a) 58 db

(b) 40 db

(c) 63 db

(d) 89 db

I had been asked this question in unit test.

This intriguing question originated from Operational Amplifier Internal Circuit topic in section Operational Amplifier of Linear Integrated Circuits

Answer 1

Correct option is (a) 58 db

In simpler terms: Formula for Common Mode Rejection Ration, CMRR= 1+2gmRE,

⇒ CMRR = 1+(2×16MΩ^-1×25kΩ)

        = 801 = 20log⁡801 = 58.07 db.