A simple scheme to extend the linearity of the continuous-time CMFB circuit for fully-differential amplifier

A linear common-mode feedback (ALCMFB) circuit which combines the modified version of the DDA CMFB with source degeneration using MOS transistors is presented. A 1V 50-M sample/s multiply-by-two (X2) circuit with this ALCMFB circuit has been developed to verify our idea. Simulation result using TSMC 0.35 mu m CMOS process shows that this X2 circuit works smoothly. The variation range of the effective number of bits (ENOBs) of X2 circuit with our ALCMFB is tighter than conventional ones. Empirically, the signal-to-noise plus distortion ratio (SNDR) of the X2 circuit is related to the THD of the CMFB circuit. One can improve the linearity of the CMFB so as to obtain an X2 circuit with lower distortion.