Question

Mention the phase condition that leads to sustained oscillation

(a) ∠-Aß =0

(b) All of the mentioned

(c) ∠-Aß = multiple of 2π

(d) ∠Aß = odd multiple of π

I have been asked this question in quiz.

This intriguing question comes from Closed-Loop Frequency Response and Circuit Stability in section Practical Limitations of Op Amp Circuits of Linear Integrated Circuits

Answer 1

For sustained oscillation in a circuit using Barkhausen criterion, the phase condition is critical. The correct answer is (d) ∠AB = odd multiple of π. For sustained oscillation to occur, the loop phase shift must be an odd multiple of π (180°, 540°, etc.), which ensures that the feedback is positive. When ∠AB = odd multiple of π, the output signal is in phase with the input signal in a way that sustains oscillation indefinitely.