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Difference between a single differential pair CMFB and double differential pair CMFB

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simplsoft

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Hello

Can anyone want help me in understanding what is the difference between the single differential pair common mode feedback circuit (CMFB) and double differential pair in CMFB. what is the advantage of using double differential pair? Can anyone explain the working of double differential pair attached . Double differential CMFB.jpgsingle differerntial pair CMFB.jpg
 

The first diagram which you are saying as double differential pair is slightly non-linear compared to the second one. The first one uses transistor as common mode detector and the common mode current is detected. In the second one, since only resistors are used to detect the common mode it might be more linear compared to first one. In the second one, the common mode voltage is detected.
 

Can you please explain which CMFB circuit is better. First one or second one. Please can you explain the working of the first double differential pair CMFB circuit. What happens if the output common mode voltage in the first transistor increases. and why two differential pair used. Is there any advanatge of using double differetial pair?
 

Everything depends on application...

DDA based is only a capacitive load for the diff-amp, so not decreasing it gain. However has reduced CMR and linearity and is not proper for rail-to-rail application.
Diff amp CMFB with averaging resistors is linear in wide range but provide resistive load for the amplifier - good for two (or more) stages amps especially with R2R output.

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Everything depends on application...

DDA based is only a capacitive load for the diff-amp, so not decreasing it gain. However has reduced CMR and linearity and is not proper for rail-to-rail application.
Diff amp CMFB with averaging resistors is linear in wide range but provide resistive load for the amplifier - good for two (or more) stages amps especially with R2R output.
 
Where's the input on the first amplifier? And the output, is being taken from the gate?
 

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