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analogrf

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Common mode Gain in feedback loop
Nov 18^th, 2009, 4:56am

Hello all,

While designing the macromodel of a fully differential op amp, i wanted to decide about the 'loop gain' of CMFB circuit.

What factors would determine the loop gain (Acm)?
I initially thought, with a CMRR= 50dB, Adm = 60db, i could simply find it as
Acm= Adm/CMRR. ==> Acm = 3.16. Would it be the optimum value for my two pole opamp macromodel?
Could there be other factors, determining it, considering im at the macromodelling level (ideal sources) ??
Thanks in advance for sharing ideas,

AF

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Common mode Gain in feedback loop Reply #1 - Nov 20^th, 2009, 7:45am hi, in macro model whats the need of CMFB? Thanks Rajasekhar.
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aaron_do

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Re: Common mode Gain in feedback loop
Reply #2 - Nov 23^rd, 2009, 4:01am

Hi,

from my understanding, CMRR is not equal to Adm/Acm, but is equal to Adm/Acm-dm where Acm-dm is the common-mode to differential mode gain. In an open-loop amplifier, Acm-dm can be directly related to Vos, or the amplifiers mismatch. Either way reducing the common-mode gain of the first stage will improve the overall CMRR of a multi-stage amplifier.

I don't have my texts with me but I believe you can read the same definition in Razavi's Analog IC design book.

cheers,
Aaron

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Tlaloc

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Posts: 81
Dallas, TX

Re: Common mode Gain in feedback loop
Reply #3 - Nov 24^th, 2009, 9:42am

Realize that there are two Acm's; one through the input diff pair, which ideally is zero, and one through your CMFB circuit. The CMRR is Adm/Acm where Acm is the common mode gain through your diff pair, but that is not the gain that you get for your CMFB. That path generally controls the tail source or the loads, which is a different path through your input devices. Since your CMFB shares many of the same transistors as your diff path, it can be tricky to set a vastly different CMFB gain and phase response than the main amp. Also, because of the different path, the CMFB gain is unrelated to CMRR. The CMFB gain determines the common mode voltage error at the output, which normally is not critical.

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