Question

When the magnitude of (AoL )×( ß ) = 0dB, state the condition at which system become stable?

(a) Phase angle is > -180^o

(b) Phase angle is < -180^o

(c) Phase angle is > +180^o

(d) Phase angle is < +180^o

I had been asked this question in an interview.

The doubt is from Closed-Loop Frequency Response and Circuit Stability topic in portion Practical Limitations of Op Amp Circuits of Linear Integrated Circuits

Answer 1

Answer: (b) Phase angle is < -180°

For stability, the Nyquist criterion states that when the magnitude of (AOL) × (β) = 0dB (unity gain), the phase angle must be less than -180° (more negative than -180°) for the system to remain stable. If the phase angle is >= -180°, the system becomes unstable or marginally stable. This represents the stability margin of the closed-loop system.