Control of the common-mode component in CMOS continuous-time fully differential signal processing

Abstract

A general description of fully differential (FD) amplifiers with common-mode feedback (CMFB) network is carried out in this article. The general requirements that any CMFB loop must satisfy allow us to deduce three suitable figures of merit (linear interaction or conversion gain, relative performances and nonlinear interaction between the common-mode and differential-mode loops) in order to compare the different approaches. The gain or transconductance, sensitivity to mismatching in the devices, and nonlinearity for every common-mode (CM) signal detector block have been derived. By identifying all the input-output signal paths in a generic FD amplifier with CMFB, the conversion signals are quantified. From the gain of the detector and the technique used for injecting back the CM correction signal, the performances provided by the CMFB loops in regard to the differential-mode counterpart are evaluated. From the CM signal detector nonlinearity, it is shown how the CM loop impacts on the distortion (nonlinear interaction) on the amplifier. Finally, all the considerations are verified by means of simulations, and the CMFB networks classified according to these figures of merit. A low-distortion CMBF loop based on the current steering principle of injection is proposed as well.