# Determining stability of min phase system using GM and PM

by HasuChObe
Tags: determining, phase, stability
 P: 32 Alright, there's something I don't understand. The book says that the gain margin and phase margin must be non-negative for a minimum phase system to be stable. But the definition of a minimum phase system is one without right half plane poles or zeroes. Doesn't that inherently make it stable? Looking for a good explanation.
 P: 324 Your definition of minimum phase system is not correct: "[A] system is said to be minimum-phase if the system and its inverse are causal and stable." http://en.wikipedia.org/wiki/Minimum_phase So the definition is more restrictive than just stable. The definition also doesn't care how you get the stability. Also, I am not 100% sure about this but I think phase margin and gain margin only apply to systems with feedback. However there is nothing in the definition of a minimum phase system that requires feedback (i.e. the system could be an open amplifier).
P: 32
 Quote by es1 Your definition of minimum phase system is not correct: "[A] system is said to be minimum-phase if the system and its inverse are causal and stable." http://en.wikipedia.org/wiki/Minimum_phase So the definition is more restrictive than just stable. The definition also doesn't care how you get the stability. Also, I am not 100% sure about this but I think phase margin and gain margin only apply to systems with feedback. However there is nothing in the definition of a minimum phase system that requires feedback (i.e. the system could be an open amplifier).
Okay, maybe not the definition, but one of the properties as a result of what you just said. But that still doesn't answer the question. Even in your definition, it says the system is stable. How is there instability resulting from gain margins and phase margins in my stable system?

 P: 324 Determining stability of min phase system using GM and PM I guess I don't understand the question. Here is what I thought you were asking: System with such and such gain and phase margin = stable Minimum phase system = stable As both systems are stable, what makes them different?
P: 32
 Quote by es1 I guess I don't understand the question. Here is what I thought you were asking: System with such and such gain and phase margin = stable Minimum phase system = stable As both systems are stable, what makes them different?