common mode feebdack needed or not ?

[sofia_huze]

I am using a differential to single ended opamp. will it have any common mode problem.

common mode feedback required or not ?

 

[saro_k_82]

If you are not using any differential output stage with high impedance outputs you don't require CMFB.
You generally don't need such a stage to design a single ended output opamp. So you dont have to worry.

 

[wangh19]

hi saro,
if I design a differential out amplifier, I use cmfb to surpress the common mode noise, do I also have to design bias circuit to fix the input dc point voltage?

 

[saro_k_82]

CMFB is primarily used to fix the bias points. To suppress the common-mode noise, you need to improve CMRR. I don't see how CMFB can be used to 'improve' CMRR.

 

[wangh19]

thanks saro, I think what you say makes sense.

I design a single ended error amplifier as a cmfb circuit just like Razavi book describes, but I found that the dc point of differential out just about 50mv below my CM reference Voltage which is VDD/2. It seems the my cmfb circuit can not fix the bias point. What do you think about the problem? thanks.

 

[analog_match]

Where are you feeding back the cmfb...to the tail current source?

 

[mengcy]

wangh,I met the same situation with you.
You have to match the current of the cmfb stage and the feedback opamp stage.

 

[kspalla]

Can any one share the schematic or data sheets on CMFB please, I think it is interesting.

 

[kundahari]

Hallo,

I desgined a continuous time cmfb ckt for fully differential amplifier.if you want the ckt, i can send you,

thanks

 

[xulfee]

can u discribe the application where u want to use it?