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Common Mode Feedback circuitry is used to stabilise your output dc voltage in a 2 stage OTA circuit. This is achieved by comparing your output dc level with a reference voltage level and the resulting error voltage is fed back into your first amplifier stage. This error signal will correct your output voltage to the desired reference value.
CMFB(Common Mode Feedback) circuits are used at active loads OTA to stabilize the DC voltage of the common mode of the output .Consider the case of differential amplifier of load of MOS transistors with constant gate voltage .These transistors are equivalent to currnet sources .The sum of currents of these 2 sources should be equal to that of tail current which is practically impossible .Any mismatch at the currents will cause large voltage due to the large output impedance of the current sources (tail source or the active loads) eventually causing the transistors to go out of saturation into triode .To overcome this porblem, CMFb senses the common mode output level and compares it to a certain level (the required common mode output level) then uses the error signal to bias the active loads .For large gain CMFB loop, common mode level~=reference level .
Another way to think about it is for a fully differential OTA, a stable DC voltage must be determined for it to work. For the most basic OTA, a diff pair, there is a current source at the top and one at the tail transistor. They will be fighting each other, and so a stable DC operating point must be obtained.
Common Mode Feedback circuitry is used to stabilise your output dc voltage in a 2 stage OTA circuit. This is achieved by comparing your output dc level with a reference voltage level and the resulting error voltage is fed back into your first amplifier stage. This error signal will correct your output voltage to the desired reference value.
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