A Fast and Low Settling Error Continuous-Time Common-mode Feedback Circuit Based on Differential Difference Amplifier.

A high-speed and high-accuracy continuous-time common-mode feedback block (CMFB) is presented. To satisfy speed and accuracy requirements, some modifications have been applied on differential difference amplifier (DDA) CMFB circuit. The proposed method is applied to a folded cascode op-amp with power supply of 3.3 V. In order to verify the proposed circuit, simulations are done in 0.35 μm standard CMOS technology. In the worst condition when the output common-mode (CM) voltage is initialized to VCC or GND, only 1.1 ns is required to set the output CM voltage on the desired level. Also in a wide range of input CM voltage variations, the deviation of the output CM voltage from reference voltage is less than 6 mV, so simulation results confirm the expected accuracy and speed while simultaneously the proposed CMFB circuit preserves other characteristics of DDA CMFB circuit such as unity gain frequency, 3-dB bandwidth, phase margin and linearity.