# Electronic – Gain margin and Phase Margin Physical Meaning

bode plotcontrol systemphase marginstabilitysystem

I have been trying to understand the physical concept of Gain and Phase Margin.

What I understand about this is that a relative comparison around the critical point \\$(-1,0)\\$, which when converted to magnitude and phase form turns out Magnitude = 1 and phase = -180°.

Also for a negative feedback system the Gain and Phase Margin should be positive, i.e., a system is unstable under the following 2 cases:

1. When the System/OLTF phase is -180° but System Magnitude \\$>1\\$. Thereby making Gain Margin negative.
I was able to correlate a physical meaning to this condition as the same would lead to a positive feedback condition with Gain \\$>1\\$ thereby leading to Unbounded output and hence instability.

2. When the System Magnitude = \\$1\\$ but System Phase \\$>-\\$ 180°. I'm not able to get a physical understanding of this unstablility case.

My questions:

• How is after all phase used to comment about unstability of a closed loop system?

• In this case after accounting for the negative feedback inherently present due to negative feedback the net phase might turn out to be positive, so how does that make the system unstable?