Question

The polar plot of a transfer function passes through the critical point (-1,0).Gain margin is

(a) zero

(b) -1dB

(c) 1dB

(d) Infinity

Answer 1

Right answer is (a) zero

The explanation is: Gain margin is the point where the at phase cross over frequency gain of the system i 1 dB and  of a polar plot passing through the critical point is zero.