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a question with differential amp and cmfb

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wdd

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hello, guys,
I have designed a diff amp with SC-cmfb, the phase margin need to be opitimized,
when i do ac simulation, the SC-cmfb should be took out, so my question is whether the capacitor in SC-cmfb be one part of Cload??
thanks a lot.
B&R
 

hylas

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wdd said:
hello, guys,
I have designed a diff amp with SC-cmfb, the phase margin need to be opitimized,
when i do ac simulation, the SC-cmfb should be took out, so my question is whether the capacitor in SC-cmfb be one part of Cload??
thanks a lot.
B&R
Yes, because SC-cmfb introduce some loading effect to your diff amp. Actually SC-cmfb is a kind of switching load, which is not purely capacitive, its effect to your amp also depends on the switching frequency.
 

    wdd

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wdd

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thx hylas,
do you have some papers or document relevant?

B&R
 

hylas

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I don't have any references. But you may try in this way:

Assume the switching frequency of the SC-cmfb is much larger than the bandwidth of your amp. Eg. fsw > 10*UGF
1) Obtain a low frequency model of the input impedance of your SC-cmfb circuit. Input of the SC-cmfb circuit should looks like a resistive element at low frequency.
2) add the equivalent resistance to your amplifier and see the freq response.

Hope this help.
 

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