Effect of nonlinearity in the CMFB circuit that uses the differential-difference amplifier

—This paper considers one type of common-mode feedback (CMFB) circuit that uses two differential pairs to sense the common-mode output voltage (Voc) of a fully differential op amp. Analysis of the circuit and simulation show that even powers of the differential-mode output voltage (Vod) affect the common-mode control signal generated by the CMFB circuit. As a result, undesired tones appear in Voc and Vod 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.

• Type: Conference
• Year: 2006
• Where: ISCAS
• Authors: M. M. Zhang, Paul J. Hurst
• Keywords: CMFB Circuit, Common-mode Output Voltage, ISCAS 2006, Output Voltage