Close loop Stability question

[wangkes9]

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.

So If any one show a peak besides in and out nodes , and out node shows good
phase margin , is this circuits stable ?

I think we must set all node have a good phase margin, to ensure the circuits will
not oscillate. Is it right.

 

[LvW]

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.

 

[wangyufn]

Theoretically for a single loop you can open it at any node, analyze the phase margin and should get the same results of stability

 

[leehying]

Agree with wangyufn.