about check stability of negative feedback loop

[Shano]

If a node "Vo" is incorporated in two negative loops,
the loop gain of loop1=-L1
the loop gain of loop2=-L2

when I want to check the loop1 stability,
I break loop1 to see the loop gain of the loop1.
The question is :
should I break the loop2,too?

If I break loop2,
loop gain of the loop1= -L1
If I DO NOT break loop2,
loop gain of the loop1= -L1/(1+L2)

which result could reflect the stability of the loop 1??

does anyone know it?
please tell me
thank you very much

 

[Analogworld]

Hi,

In order to check stability for multiple loops, one easy way to do is to use return- ratio(loop gain) analysis, which you have to find a point to break two loops simultaneously and inject a test source. Then you can look for phase margin of the return ratio.

For more details, you can take a look at the following paper

P.J. Hurst and S.H. Lewis, "Determination of Stability Using Return Ratios in Balanced Fully Differential Feedback Circuits," IEEE Trans. on Circuits and Systems II, pp. 805-817, December 1995.