hi a question about cmfb

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rail-to-rail cmfb

hello
now i am designing a fully differential charge pump,and now i have a problem:
we all know that we need a cmfb circuit for fully differential circuit ,but here because the output range is very wide ,nearly rail to rail ,how to design the cmfb's sensing circuit ,it is difiicult!! thank you for replying!
 

well , i think not , since the o/p diff. is rail-rail but the CM is fixed , so the cm sensing circuit will remove the diff. o/p and cmfb will deal only with the CM.
 

NOT sure, the cm of the output will change a lot ,it may be change from vss to vdd
and in other conditions, its cm voltage is vref, but the signal swing is so large that the sensing circuit failed to work .
 

well i dont understand , in resistive sensing for instance there is no condition on the voltage at the two terminals and the common terminal will provide the CM "sensing" , and the CM should be nearly equal to the Vref , if the cm changes from vdd-vss then we r not designing a cmfb as the cmfb function is to fix the cm of the o/p and not to let it change freely from Vdd-Vss.
anyway u can chack the CM circuits for CP in many pappers ,post them if u find something weired.
regards.
 

resistor divider, SC cmfb
 

Switched　Capacitor based CMFB is the nest option when high voltage swings are involved.
 

but i don't think the sc can be used in charge pump which is used in pll, because the nonideal character of switch will deteriorate the cp
 

using resistor or capacitor to sense th cm of output stage will be work.
 

use resistor and capacitor in parallel to eliminate sense circuit pole
 

may you explain more detail on why use resistor and capacitor in parallel can eliminate sense circuit pole? thanks!
 

CMFB means common mode feed back, that is whatever your signal swing, if the output is differential, the commond mode can be fixed on the constant voltage
 

mostly through a resistor divider,but the resistor should be large so that consume little current
 

use rail-to-rail CMFB can solve this problem, but take care of the stability of the loop.
 

resistor is alternative
 

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