Common mode feedback (CMFB) is a secondary feedback circuit required in fully differential analog designs to force the common-mode output voltage to a known reference level. Without it, unavoidable mismatches and leakage currents would cause the output nodes to drift toward the supply rails, saturating the amplifier. It measures the average of the two outputs, compares it to a reference, and adjusts the bias current accordingly.
Heard on the job
The op-amp is clipping because the CMFB loop isn't settling fast enough.
Insider tip
A poorly designed CMFB circuit is a classic cause of instability in differential amplifiers, often requiring its own compensation network.