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Hi, all: this schemetic in attachment is a fully differential OPA which includes CMFB. Now, please look at this circuit, I do not know what is the role of M55 and M56. I think this two MOS transistor is unnecessary.
M55 & M56 only carry small amount of current. They are not active transistors, so they don't involve in ac analysis. I guess these two transistors just provide enough biasing current for the M13 & M14 to increase the gm of (M13 & M14). More system information are required (e.g. your circuit application, types of signal your circuit are required to process, supply voltage, current consumption requirement and etc...)before we can understand the whole circuit.