Question

At what condition differential amplifier function as a switch

(a) 4VT < Vd < -4VT

(b) -2VT ≤ Vd ≤ 2VT

(c) 0 ≤ Vd < -4VT

(d) 0 ≤ Vd ≤ 2VT

The question was asked in an interview for internship.

My question comes from Operational Amplifier Internal Circuit topic in section Operational Amplifier of Linear Integrated Circuits

Answer 1

Right option is (a) 4VT < Vd < -4VT

Delve into insight with: For Vd > 4VT, the output voltage are VO1 = VCC, VO2= VCC-αF IQRC. Therefore, a transistor Q1 will be ON and Q2 will be OFF. Similarly for Vd> -4VT, both transistors Q2  &  Q1 will be ON.