Question

A differential amplifier has a transistor with β0= 100, is biased at ICQ = 0.48mA. Determine the value of CMRR and ACM, if RE =7.89kΩ and RC = 5kΩ.

(a) 49.54 db

(b) 49.65 d

(c) 49.77 db

(d) 49.60 db

The question was posed to me in an international level competition.

This interesting question is from Operational Amplifier Internal Circuit topic in section Operational Amplifier of Linear Integrated Circuits

Answer 1

Accurate answer is (b) 49.65 d

Unlock profound understanding with: Differential mode gain, ADM= -gmRC  and Common mode gain,

⇒ ACM= -(gmRC)/(1+2gmRE)

(for β0≫1).

Substituting the values,

⇒ gm= ICQ/VT =  0.48mA/25mv=19.2×10^-3Ω^-1

⇒ ADM= -gm×RC= -19.2×10^-3Ω^-1×5kΩ= -96

⇒ ACM= -(gmRC)/(1+2gmRE)= -(19.2×10^-3Ω^-1×5kΩ) /(1+2×-⇒ 19.2×10^-3Ω^-1×7.89kΩ)  = -0.3158

CMRR = -96/-0.3158= 303.976

    =20log⁡303.976

    =49.65db