Common mode feedback circuit

[IvanTheTerrible]

I need help explaining this circuit (taken from R.Allen)

How does it set the output common voltage?

 

[dgnani]

The grayed area is a (fully) differential amplifier acting as CMFB. Its inputs are a reference voltage Vcm and the average of the outputs of the controlled differential amplifier. The controlled amplifier provides negative feedback to the CMFB amp so given a sufficient open loop gain of the CMFB its inputs will be forced to very close voltages. That is the common mode voltage is forced to Vcm.
The negative feedback operates as follows, starting from equilibrium, assume MC2 gate goes above MC1 gate, less current will flow in MC1 hence the gate-drain of MC3 will decrease, the gate voltage of M3 M4 is decreasing but their total current cannot decrease (fixed by the tail current M5) so the drain voltages of M3 M4 have to decrease to keep it constant; MC2 gate -the average of M3 M4 drains- is lowered.

 

[santom]

Thanks for explaining.I really appreciate that but I kindly suggest...you could probably explain a bit more detailed if you find some free time as its quite hard for many to get the principle_of_operation from your description in my opinion.

 

[dgnani]

it is hard to know what someone else already knows or is confused about; if you want to ask specific questions I will be happy to elaborate

 

[ic98]

the same idea is used in a CMFB circuit for FD opamp in johns&martin book, see there for more info.