Stability analysis for multiple fedback loops

[drabos]

Does anybody have experience how to do stability analysis if more feedback loops are in the circuit? (using cadence ade, spectre) does it possible with the aid of the stb analysis?

 

[LvW]

This, indeed, is a question which is not easy to answer.

1.) What kind of result do you expect from the analysis ? Only stable/unstable ?
Or do want to have a stability margin (magnitude or phase or a combination of both ?)

2.) In principle, it is quite clear: If you have three different loops, there will be three different loop gains and , hence, three different margins. And than the question arises: Is there any loop resp. margin which governs the system behaviour ?
In many cases the answer will be "yes", but one cannot give a general answer, because it depends on the loop arrangement.

3.) To avoid some errors: It makes no sense to reduce a four-loop system to a single loop since there will be exactly four different ways to do this - all with different results (thats perhaps surprising, but it is true !). Of course, in each case the closed loop transfer function will be the same (but not the loop gain function).

4.) Fazit: You will have more than one expression for loop gain and more than one margin; and than you have the choice to interpret this result.
That is the problem.

LvW

 

[ashish_chauhan]

If you have multiple loops in ur top level loop, Then for stability first check individual loop and then the top level loop. The bottom line is each loop should be stable in itself.

Indeed , as Lvm said the prblem is in opening only one loop at a time without others...

 

[LvW]

If you have multiple loops in ur top level loop, Then for stability first check individual loop and then the top level loop. The bottom line is each loop should be stable in itself.

Sorry, but I cannot agree. It is not so easy. In many cases it is not possible to discriminate between “individual” and “top level” loops. More than that, there are many examples in which one local loop shows instability, but can be stabilized by closing another loop. One simple example is an opamp exhibiting negative as well as positive feedback. To be stable the negative feedback must govern - and opening the negative FB loop makes the rest of the circuit unstable.
There is another simple example to illustrate the problem:
The circuit diagram in the attachement is one of the various alternatives for a composite amplifier. In fact, there are three different loops because you can open at three different points:
1. At the output of OP1 (loop with OP2 remains closed) or
2. at the output of OP2 (both loops are open), or
3. at resistor R2 (loop with OP1 remains closed).
And the question is: Is it possible to know – from the beginning – which loop is the critical one ? For my opinion this question cannot be answered in general without knowing the component values