A
Active8
hi:
just a note on an old thread where Kevin Aylward discussed stability of
systems with gain greater than 1 and positive feedback.
i checked out a paper on the Nyquist stability test, which shows he's
right about the possibility of a stable system with positive feedback
and gain greater than 1. not that it's demonstrated or even mentioned in
so many words, but it does at least agree with Kevin's statements
regarding the net encirclements of the (0, -1) point.
well, the same guy also wrote a paper on feedback (for the same class
he's teaching) in which he says that you can't have a stable system if
af >> 1 (af being loop gain) because "all real systems eventually
exhibit increasing negative phase shift with frequency."
at first this seemed like a contradiction, but it may be that "much
greater than 1" is the qualifier, not to mention the fact that the
system may be band limited.
gain and phase margin are a subset of the Nyquist test.
http://www.stanford.edu/class/ee214/Handouts/nyquist.pdf
http://www.stanford.edu/class/ee214/Handouts/HO17.pdf
http://www.stanford.edu/class/ee214/Handouts/
the last link is the index of the rest of the papers and that's how i
found HO17.pdf on feedback. note the simple trick used to get it.
enjoy...
br,
mike
just a note on an old thread where Kevin Aylward discussed stability of
systems with gain greater than 1 and positive feedback.
i checked out a paper on the Nyquist stability test, which shows he's
right about the possibility of a stable system with positive feedback
and gain greater than 1. not that it's demonstrated or even mentioned in
so many words, but it does at least agree with Kevin's statements
regarding the net encirclements of the (0, -1) point.
well, the same guy also wrote a paper on feedback (for the same class
he's teaching) in which he says that you can't have a stable system if
af >> 1 (af being loop gain) because "all real systems eventually
exhibit increasing negative phase shift with frequency."
at first this seemed like a contradiction, but it may be that "much
greater than 1" is the qualifier, not to mention the fact that the
system may be band limited.
gain and phase margin are a subset of the Nyquist test.
http://www.stanford.edu/class/ee214/Handouts/nyquist.pdf
http://www.stanford.edu/class/ee214/Handouts/HO17.pdf
http://www.stanford.edu/class/ee214/Handouts/
the last link is the index of the rest of the papers and that's how i
found HO17.pdf on feedback. note the simple trick used to get it.
enjoy...
br,
mike