CM in fully differential circuit

[nxing]

Hello everybody,

I am just wondering anybody can shed some lights on my following question:

say for a fully differential opamp with feedback (for example, resistor), it says that the feedback is for differential signal but common-mode signal. Can anybody give me an intuitive explanation, why is that?

Thanks!

 

[ygu_sanjose]

It depends on where the feedback tied to. If it is tied to the inputs, it is for differential mode feed back. If it is tied to the current source, it is for common mode feed back.

 

[pfd001]

For differential amplifier, the transfer function is vp-vn=A(vinp-vinn). It is for differential signal. so you need common mode feedback to control the common mode signal

 

[gingerjiang]

it depends on the input and output signal, these signals' common-mode components decide the common-mode of circuit, and these signals' differential- mode components decide the differential-mode of circuit