Fully differential opamps and common mode feedback

[rtarbell]

Why is it that, fully differential opamps (opamps that have two inputs, two outputs, and one common mode votlage pin) require common mode feedback, but operational floating amplifiers (OFCs) do not need common mode feedback?

--> They both have two inputs and two outputs, but I don't understand why many of the OFCs and four terminal floating nullor (FTFN) circuits that I read about in literature do not have common mode feedback. Any ideas as to why this is?

 

[finfoun]

the reason for CMFB is to stablize the output op voltage, if there is no CMFB in differential opamp,the mismatch of current and high output resistance can cause output voltage to go ground or power supply

 

[jeffsky520]

so what

 

[rtarbell]

Thank you for your reply

--> I understand why fully differential opamps need CMFB, but
--> Then why DON'T floating nullor circuits require CMFB? They, too, have two inputs, and two outputs, like the differential opamp. But in most of the IEEE papers that I have read on them, the proposed circuits for floating nullors do not have any CMFB included.