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.

+1 vote
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

by (39.8k points)
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

⇒ 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

+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote
+1 vote