Common Mode Feed Back??

What is common mode feed back ??
What function does it have?

The Common Mode Feed Back is an extra circuit that fixes the common dc output voltage of a fully differential let's say amplifier.
You want for example that the two output voltage signals are symmetrical around a common fixed voltage.So this circuit will compare the dc output voltage with the actual voltage and will control for example a bias voltage of an amplifier in order to do this adjustement.

There are continuous and SC circuits depending on your applications needs.

See the book of johns & martin and ravazi for details.

Hi,

Almost every system will have this so called CMFB, which is realized through negative feedback mechanism. Basically the CMFB is needed in order to ensure the output signal is well symmetrical. This is because when there is a variation due to process and temperature, the so called dc common mode will change either shiting down or up from the intial CM let say 1.2V, which will cause signal not symmetry like clipping up to VDD or etc.

Thus, in order to avoid this situation, the common mode of a signal at the output stage is extracted through the resistor and that dc is compared to an error amplifier with a fix dc reference voltage which can be realized through a bandgap reference or pmos voltage divider.

Now, as the error amplifier output is either going low or high according to CM change, that output will be connected back to the tail bias of the differential amplifier in order to act as negative feedback. By doing this, the output common mode voltage can controlled.

Hope this helps you in understanding CMFB. You can get a clear picture of it from the Razavi book.

Thanks,
Suria

try this good reference
**broken link removed**

Also a good book where you will find a chapter dedicated to CMFB is Gray and Meyer 4th edition. I think is the best reference for trying to understand this type of circuit.

Hi.
CMFB is always necessary for fully differential circuits. Beside other reasons others mentioned, since almost always we use opamps in closed-loop form, in fully differential configuration, sometimes the negative feedback structure for differential signals acts like a positive feedback for CM signals and this will result in output saturation. So by the use of another negative feedback (more powerful than differential mode feedback) for CM signals, we will control the output CM level.

Regards,
EZT

check this topic

In an ADC fully differential that has vdd=2.5 V and vss=0 V, how do I define the vcm? I need to define this value externally or can I use the CMFB?

if the adc is the speicific , the output common is better VDD/2