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simulating common-mode feedback loop

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gingerjiang

gingerjiang

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hi all,

i have designed a folded-cascoded amplifier with sc cmfb
someone tell me that the bandwidth of common-mode loop is comparable to the bandwidth of differential-mode loop for the amplifier's stabilitation, it's right?
how do i simulate the bandwidth of common-mode loop?
thanks advanced!
 

I have designed a CMFB with less bandwidth than OPAMP,but it's stable.
is your circuit complete?
the ocm signal in the pic didn't connect to the loop.
 

renwl said:
I have designed a CMFB with less bandwidth than OPAMP,but it's stable.
is your circuit complete?
the ocm signal in the pic didn't connect to the loop.
Click to expand...

i mean that the bw of cm-loop may be less than the dm-loop', but it can't be too less
and in fact, the differential amplifier is a two-stage amplifier, the first stage is folded-cascoded, and the second stage is a class a outstage. the feedback signal of sc-cmfb circuit vc is feeded through a inversion stage.
 

Dear friend,
you have uploaded a FC opamp but then you said your opamp is a two stage one;
however, if your opamp is two stage I suggest to use two seperate SC CMFB network for each stage (not to need sign inversion).
but to understand how to measure CMFB loop stability specs. I refer you to read chapter 12 of Gray & Hurst's book on CMFB. they used one of their refernces to measure.
Hope to be useful!

Regards,
SAZ
 

zabihian said:
Dear friend,
you have uploaded a FC opamp but then you said your opamp is a two stage one;
however, if your opamp is two stage I suggest to use two seperate SC CMFB network for each stage (not to need sign inversion).
but to understand how to measure CMFB loop stability specs. I refer you to read chapter 12 of Gray & Hurst's book on CMFB. they used one of their refernces to measure.
Hope to be useful!

Regards,
SAZ
Click to expand...

in fact, my another opamp's 2nd stage is a class ab, so maybe i can't use the 2nd sc cmfb

i will read Gray's book, thanks very much!
 

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