How do you design a fully differential amplifier and what are the common-mode feedback requirements?

A fully differential amplifier has differential inputs and outputs, offering better noise immunity, doubled output swing, and even-harmonic cancellation. However, without common-mode feedback (CMFB), the output common-mode voltage is undefined. CMFB circuit senses output common-mode level (using resistive averaging or switched-capacitor sensing), compares to reference, and adjusts a current source in the amplifier. Design considerations include CMFB bandwidth (must be higher than main amplifier bandwidth for stability), settling time, and loop gain. CMFB adds complexity but is essential for proper operation.