Question

Why the feedback circuit is said to be negative for voltage series feedback amplifier?

(a) Feedback voltage is 180^o out of phase with respect to input voltage

(b) Input voltage is 180^o out of phase with respect to feedback voltage

(c) Feedback voltage is in same phase with respect to input voltage

(d) Input voltage is in same phase with respect to feedback voltage

I had been asked this question in an interview for internship.

My doubt stems from Voltage Series Feedback Amplifier in section Operational Amplifier Characteristics of Linear Integrated Circuits

Answer 1

Right choice is (a) Feedback voltage is 180^o out of phase with respect to input voltage

Illuminate the topic with: Voltage series feedback amplifier have the difference voltage, Vid = Vin-Vf. Therefore, the feedback voltage always opposes the input voltage and is out of phase by 180^o with respect to input voltage. Hence, the feedback is said to be negative.