Effect of Switched-Capacitor CMFB on the Gain of Fully Differential Op-Amp for Design of Integrators

Switched capacitor common-mode feedback (SC-CMFB) is a popular technique for stabilization of the output common-mode level of fully differential operational amplifiers. It provides advantages of excellent linearity across a wide amplifier output swing, lowest power consumption, and better feedback loop stability in contrast to continuous CMFB; and hence, are suitable for realization of high-gain wide-swing low-power opamps. But, its implementation demands careful consideration of some practical aspects, a number of which are well documented in literature. However, its detrimental effect on the amplifier's differential-mode gain is not quite explored. Equivalent resistive loading from the SC-CMFB is the reason for this effect, and is particularly important in op-amps meant to have large gain (like, the folded cascode). This SC-CMFB induced drop in amplifier dc-gain, and the consequent effect on the design of continuous and discrete-time integrators have been discussed together with pertinent analytical derivations and transistor level simulations. A few practical guidelines and circuit topologies for minimizing the gain reduction effect have also been provided.