In the above expression let's substitute N = 0 giving the closed-loop
1 + A f
Amplifiers with varying gains are obtained by adjusting the value of the
attenuation of the feedback block f. For a closed-loop gain of 1, f » 1 and this gives an open-loop gain A f = A. But for a
closed-loop gain of 10, f = [ 1/10] with the open-loop gain A f = [ A/10]. This means that the open-loop gain for unity closed-loop
gain is ten times larger than for a closed-loop gain of 10. The higher the
open-loop gain the lower is the gain margin, i.e., the margin against
system becoming unstable is ten times lower for a closed-loop unity gain
than for a a closed-loop gain of 10.
Please remember that Bode Plot or Nyquist Plot is plotted with open-loop
frequency response to predict the closed-loop behaviour. Please also
remember that higher the open-loop gain lower are the gain and phase margins.
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version 3.40. On 02 Nov 2004, 14:38.