What is a negative phase margin; what does it mean?

Shreesh P. Mysore, Nov 18 2002

Let G(s) be the transfer function in question. A positive phase margin means that when |G(jw)|=1, the phase of G(jw) is greater than -180deg. This is a way of measuring the degree of stability of the system. A larger phase margin is better, in terms of stability. A negative phase margin means that the system is unstable.