In a Close loop circuits, we can test any node for frequency response for stability.
like Figure 1. If out node show a peak, the circuits will oscillate.
No, the circuit is becoming unstable only if the peak has an infinite amplitude.
So If any one show a peak besides in and out nodes , and out node shows good
phase margin , is this circuits stable ?
Yes, because in case of instability all nodes will come to the threshold simultaneously.
I think we must set all node have a good phase margin, to ensure the circuits will
not oscillate. Is it right.
How do you define the margin of single nodes ? Normally, the margin of the loop gain is calculated to verify stability/instability.