how to decide the value of cap in cmfb?

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mpig09

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dear all:

There are two kinds of caps in cmfb, one is big cap and another is small cap,
could anyone can give me some suggestions that how to decide the value of the
small cap?

Why big cap >> small cap?

thanks for your kind help.
 

why is the capacitors needed in the cmfb circuitry below anyhow? i know they are not for compensation, but what are their functions there exactly, please help.
 

henrywent,

Actually those caps are for compensation reasons. In their absence, the input capacitace towards the common-mode circuit creates a very low pole with the huge common-mode sensing resistors making the compensation almost imposible.
 

hi mpig09,

do you mean about the capacitances in the SC-CMFB circuit?

if so the big capacitance should be approximately the input capacitance of the transistor to which you are connecting for bias, if common mode gain is more then you can choose little lesser value,

the smaller cap should be 0.5 times or 0.125 times of the big cap.
 

Dear arunkumar446:

1. Yes, for the SC circuit.
2.the smaller cap should be 0.5 times or 0.125 times of the big cap
==>Is there a formula to decide the ratio?

mpig
 

so far i didn't see any formula for that, but depending on the smoothness we need at the ciruit output we choose the cap ratio, if we need more smoothness we can choose less ratio.
 

Dear arunkumar446:

I am glad that you give these information, they are good experience for design.
For the value of cap in cmfb, I wish to know detail principle or formula to prove it. Could anyone help me ?

mpig
 

You can read Paul Gray's book for SC CMFB, it will help you , it's actually a sc integrator
 

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