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Do I have to design two CMFBs for two stage differential opamp?

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QT_GIRL

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Hi guys,

I am designing a two stage differntial opamp. I am not sure if i have to design two CMFB or just use one for both. Im not really sure what the difference is. Does anybody know?
 

opamp and CMFB-

Hi,
Generally speaking, one CMFB is enough for the whole opamp, you can detect the output common mode voltage, which is compared with a reference, then the output is feedback to the first stage of the opamp.

Hope it helps!
 

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    QT_GIRL

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opamp and CMFB-

so i have to detect the stages that uses both pmos and nmos current sources and control one of the sources using a CMFB?
 

opamp and CMFB-

Hi
It depends your circuit.
see Rabii's thesis or paper.
it can show when we need two CMFBs.
regards
 

opamp and CMFB-

Hi,
If you post your core circuit here, maybe we can figure out how to add the CMFB for your circuit.

BR!
 

Re: opamp and CMFB-

It depends on your circuit. If the gain of both two stages is very high, you need two CMFB. If one stage's gain is very low(<0.5), you don't need CMFB in this stage.
 

opamp and CMFB-

dear hr_reazee:
could you post the "Rabii's thesis or paper"? thanks
 

opamp and CMFB-

Hi
A 1.8V Digital-Audio Sigma-Delta Modulator in 0.8u CMOS

regards
 

opamp and CMFB-

Hi
Can reference "INTRODUCTION TO CMOS OP-AMP AND COMPARATORS" this book.
 

Re: opamp and CMFB-

neter said:
Hi
Can reference "INTRODUCTION TO CMOS OP-AMP AND COMPARATORS" this book.
Click to expand...

who is the author for this book? any link to download this from ?

thanks
 

Re: opamp and CMFB-

Usually, one CMFB is enough EXCEPT that the output common mode voltage of the second stage is independent of that of the first stage.
 

Re: opamp and CMFB-

hi ,guy.
it depends on your circuits as upstairs said .


the less cmfb you use,then the circuit works well;the better your design is.
 

opamp and CMFB-

if common mode Vs of two stages are different or need to control independly
 

Re: opamp and CMFB-

It depends on whether one CMFB can do the trick.
If it can't, then you need to use 2 CMFB.
You can run some simulation & check op poits to help you decide.
 

Re: opamp and CMFB-

QT_GIRL said:
Hi guys,

I am designing a two stage differntial opamp. I am not sure if i have to design two CMFB or just use one for both. Im not really sure what the difference is. Does anybody know?
Click to expand...

you can use just one CMFB for two stage,but it is slow sometimes .most time i think use two CMFB for each stage
 

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