Comparison Study of Averaging Resistor Implementations for Common-Mode Feedback Circuits

In this paper, a comparison between four different common-mode averaging resistor implementations for fully differential amplifiers is presented. Namely, these common-mode detectors are ohmic resistors, duty-cycled resistors (DCR), switched-capacitor-resistors (SCR), and pseudo-resistors (PR). One purpose of these averaging resistances is to keep the common-mode feedback error amplifier in a stable operating point when the differential amplifier has a large output swing. All resistor implementations are discussed concerning their implementation complexity as well as advantages, and disadvantages. The comparison focuses on the area consumption of each topology, the residual common-mode error from the ideal common-mode, the impact on the linearity of the main amplifier, temperature effects from $-40^{\circ} \mathrm{C}$ to 125° C, and finally, mismatch between the resistances. The comparison is carried out in a 40 nm CMOS technology. Post-layout results show that a PR-based implementation consumes the least amount of area while having the smallest effect on the linearity and achieving a competitive common-mode error.