About CMFB for two sgage fully differential op amp

[tia_design]

fully differential amp cmfb

For two stage fully differential op amp, both first and second stages are fully differential, do both stages need their individual common mode feedback circuit? or just the second stage need common mode feedback circuit. Thanks!

 

[iamxo]

we can use just one stage cmfb from the second, but feedback to the first stage.

 

[yxo]

Before, Idesigned one CMFB for two stage fdif amp, But now, I'm thinking that the bestest solve it is two weak (with small gain ) CMFB for every stage. But I did not tested it

 

[faithful]

Why?
I do't think it is a good idea.
You may not be able to ultilize the miller capacitor as the first pole in the CMFB loop.

Before, Idesigned one CMFB for two stage fdif amp, But now, I'm thinking that the bestest solve it is two weak (with small gain ) CMFB for every stage. But I did not tested it

 

[spring1860]

i think one CMFB for the second stage is enough .

 

[mba7]

can someone explain to me exactly what's a CMFB please

 

[FvM]

I think overall CMFB (output to 1st stage) is also a means to achieve DC balance for the amplifier. Comercial differential amplifier, e. g. AD8132 are said to use this structure.

 

[teteamigo]

CMFB

Common Mode is related with max of signal swing.

We need that the common mode in output was in the middle of output voltage range of variation that permits active devices with gain.


In stages with active loads may be occur some kind of imbalances between DC operating point of the this load and the biasing active devices.


Because of this we need to regulate the output voltage by impose the operating point of active load or change value of biasing network.



e.g


a differential pair with a active load, and biased with a current source.


If by some reason the active load have a DC current different of I/2 (I-current source bias) then we obtain a imbalance that degrades the signal swing.


Don't forget push helped button!!!

 

[safwatonline]

if the two stages are identical (fully differential), then i think u must have two separate CMFB loops.

 

[teteamigo]

We need CMFB everytime that may have signal+common mode (input, output due to imbalances) to put any amplifier in region without gain.

Look to this example about opamps (in this case only common mode input matters)
**broken link removed**



For example: 2 stages amplifier

Stage 1:


Fully differential amplifier


Stage 2:


Fully differential amplifier wih CMRR infinite and CMFB


2 stages and only one with CMFB

 

[hbk2222]

use only one CMFB for the second stage(i test it)