A low distortion CMOS continuous-time common-mode feedback circuit

In high-gain fully differential operational amplifier (FD op-amp) design, the output common-mode (CM) voltage of the FD op-amp is quite sensitive to device properties and mismatch. It is, therefore, necessary to add an additional control circuit, referred to as the common-mode feedback (CMFB) circuit, to stabilize the output CM voltage at some specified voltage. In this paper, we present a high linear CMOS continuous-time CMFB circuit based on two differential pairs and the source degeneration using MOS transistors. Theoretical analysis and SPICE simulation results are provided to validate our proposed ideas. Finally, we present two design applications of the proposed configuration, one is the FD folded-cascode op-amp and the other is the Multiply-by-Two circuit which is the key component in the popular 1.5 bit/stage pipelined analog-to-digital converter. Comparison with conventional topologies shows that the new configuration has attractive characteristics concerning their implementation in high linear analog integrated circuits. Copyright (C) 2010 John Wiley & Sons, Ltd.