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Common mode feedback voltage output

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simplsoft

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Hello

Can anyone tell me why a differential pair requires a common mode feedback circuit. Is the common mode output voltage of a differential pair a DC voltage only. Does both the output ends of the differential pair have different outputs i mean to say at the output of differential pair let say vout1 is 0.5V then vout2 is 0.47 volts. Do we have to set the change difference of the DC output voltage equal thats why we use CMFB? M i right.

Can anyone explain me I cant understand what Common mode output does the CMFB circuit sets.?

Regards
 

Hi,

can you show a circuit? So we can discuss about it..

Klaus
 

Can anyone tell me why a differential pair requires a common mode feedback circuit.
Click to expand...
A differential pair doesn't generally need CMFB. A high gain fully differential amplifier however does.

what Common mode output does the CMFB circuit set
Click to expand...
Ideally

Vcm = (Voutp + Voutn)/2 = Vref
 

yes I am talking about fully differential circuit output. why it requires common mode feedback.

In most of the research articles its mention that vref of CMFB circuit should be at the mid of the supply voltage i.e is I have used +/-1V then Vref should be 0V.
 

In most of the research articles its mention that vref of CMFB circuit should be at the mid of the supply voltage i.e is I have used +/-1V then Vref should be 0V.
Click to expand...

Sounds reasonable when looking at the fully differential amplifier as a building block. But the requirement might collide with asymmetrical swing of a specific amplifier topology.

The single stage amplifier in your previous thread has no symmetrical swing with respect to supply rails. If you want maximal output swing, you may want a different Vref than mid supply.

https://www.edaboard.com/showthread.php?t=369516
 

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