Re: CMFB
Since in OTA design the output resistance of MOSFET is very high, the output common-mode voltage is extremely sensitive to mismatches between n- and p-MOS current source. Only very little mismatches can caused the output common mode voltage go all the way up or down to the rail, forceing one of the output transistors in troide region and thus the gain drop significantly. In single-ended circuits, the output voltage can be stablize by external feedback loops (such as resistors and caps), however in fully differential circuits, external feedback can only stablize differential mode voltage but not common mode voltage, thus required extra common mode feedback circuit to stablize the output common mode. Without this circuit the fully differential OTA bascially cannot work properly.