This paper considers one type of common-mode feedback (CMFB) circuit that uses two differential pairs to sense the common-mode output voltage (V_oc) of a fully differential op amp. Analysis of the circuit and simulation show that even powers of the differential-mode output voltage (V_oc) affect the common-mode control signal generated by the CMFB circuit. As a result, undesired tones appear in V _oc and V_oc due to transistor nonlinearity in the CMFB circuit. SPICE simulation results show that adding degeneration resistors to the differential pairs reduces the undesirable tones significantly