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Loop stability question?

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huanchou

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Dear all:

I have a question. When a circuit includes two closed loops, which one will decide the loop stability and the circuit characteristics? High bandwidth loop decides it, or low bandwidth one?
Please help me to answer the question, thanks!
 

I don't think there is answer for this problem, it really depend on the application and how the two loops interact each other.
 

You have to analyze both loop stability. There is no confirmation that if you manage to stabilize one loop, the other loop will be stable as well. So, the best solutions is you have to ensure each loop to be compensated on depending type of application your circuit is designed for.
 

Try to find a common point between the two loops, if such a point exists. Then break the loops at that particular point to analyze the loop stability. If such a analysis is too difficult, run a transient analysis. Hopefully you won't see ringings in the waveform.
 

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    huanchou

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It depends on how the loops are connected each one.
Of course both have to be stable and you can analyze them separately, and the bandwidth is the smaller one if they are connected in series.
Max
 

questions for above two replys

why finding the common point and break it to analysis ???


why the BW will be the smaller on if series ? how about the case of in parallel ?
 

Because you need to break the loop to find Loop gain. In Rosenstark's theory(in the book with name of "Feedback amplifier princeples"), he gave a method of how the inject signal and the return signal can determined the loop gain: as following:
T=-1/(1/Toc+1/Tsc)
where Toc is the open circuit factor and Tsc is the short circuit factor
you can also easily determine it by pspice. by using a large inductor and a large capacitor to block AC and allow DC.
 

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    huanchou

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Every oscillation get the whole system oscillating ... this is from stability theory.
Regarding the bandwidth a simple explanation could be just look at the different loops as they would be simple filters.
 

Thanks brothers support !!!

i often confuse with feedback loop in series or/and in parallel cases.

The book Feedback amplifier principles, hope i can master this book and be familiar on feedback more
 

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