IV.5.5 Nyquist Stability Criterion
The Bode stability criterion is valid for systems where amplitude ratio and phase shift decreases monotonically with 
Nyquist stability criterion does not have any such limitation and is applicable in more general sense. The criterion states that:
If open-loop Nyquist plot of a feedback system encircles the point (-1,0) as the frequency 
varies
from 
to 
the closed loop response is unstable.
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Fig. IV.16: Example of systems for studying Nyquist stability criterion |
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(a) Stable process 
(b) Unstable process