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!
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
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.
yxo said:
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
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.
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.