# Maximum current of the CMFB circuit

1. ## Maximum current of the CMFB circuit

Dear friends,

attached the two CMFB amplifier in which I want to compare,

the first one can give maximum cuurent to correct the output common mode voltage of 2Ib as it is a diode connected load to the control transistor M5.
in the second circuit the control transistor M5 is connected to the drain of M4 like of two stage ampliifer, thus the ouput controlling currrent can be more than 2 Ib if that was neccessary for correction.

Is that true ? if it then I can say regardless the gain and accuracy different of the two circuits, the second one can adjust the circuit for more mismattching if happeining

thnak you

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2. ## Re: Maximum current of the CMFB circuit

Yes, the second one has more gain and hence can adjust the CM voltage for less input difference with respect to the CM ref. But you have to see how many poles you accumulate in the CMFB loop and if the loop is stable.

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3. ## Re: Maximum current of the CMFB circuit

Originally Posted by Junus2012
... then I can say regardless the gain and accuracy different of the two circuits, the second one can adjust the circuit for more mismattching if happeining
Yes, I'd think so, even if you perhaps wouldn't need the higher gain: the common mode gain of the diff amp is (should be) rather low, in the order of 1, so a low CMFB (differential) gain in the order of 10..100 is sufficient in most cases.

But with higher gain, this also shifts the CMFB's second non-dominant pole (at M4) farther out, and you have to take more care about the stability of the CMFB loop - as sutapaniki remarked above. If necessary, you can always shift back the non-dominant pole(s) and gain phase margin e.g. by tripling the tail current (3Ib).

Why don't you simulate the whole circuit?

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4. ## Re: Maximum current of the CMFB circuit

Dear Erikl and Suta,

I understand well your explanation about the gain and stability, I have no doupt about it indeed.

My question was different but I might expressed it wrong,

I was saying that first CMFB can provide maximum current equal to 2Ib to M5 while the second one can provide more current. will that make difference in the presence of mismatching ?

Thank you
Best Regards

5. ## Re: Maximum current of the CMFB circuit

When you say mismatch, what mismatch do you mean?

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6. ## Re: Maximum current of the CMFB circuit

Originally Posted by Junus2012
... while the second one can provide more current.
Right: the 2nd circuit can adjust the common mode level faster than the 1st circuit by delivering more current to its output stage and its capacitive load.

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