CONTROL OF THE COMMON-MODE COMPONENT IN CMOS CONTINUOUS-TIME FULLY DIFFERENTIAL SIGNAL-PROCESSING

A general description of fully differential (FD) amplifiers with commo n-mode feedback (CMFB) network is carried out in this article. The gen eral requirements that any CMFB loop must satisfy allow us to deduce t hree suitable figures of merit (linear interaction or conversion gain, relative performances and nonlinear interaction between the common-mo de and differential-mode loops) in order to compare the different appr oaches. The gain or transconductance, sensitivity to mismatching in th e devices, and nonlinearity for every common-mode (CM) signal detector block have been derived. By identifying all the input-output signal p aths in a generic FD amplifier with CMFB, the conversion signals are q uantified. From the gain of the detector and the technique used for in jecting back the CM correction signal, the performances provided by th e CMFB loops in regard to the differential-mode counterpart are evalua ted. From the CM signal detector nonlinearity, it is shown how the CM loop impacts on the distortion (nonlinear interaction) on the amplifie r. Finally, all the considerations are verified by means of simulation s, and the CMFB networks classified according to these figures of meri t. A low-distortion CMBF loop based on the current steering principle of injection is proposed as well.