An analysis and implementation of common mode feedback in high-speed fully differential op-amp

A systematic analysis on the common mode (CM) component in fully differential operational amplifier is presented. It is revealed that there are two distinct CM closed loops active in a differential op-amp. After a general analysis on the stability of those two CM loops, a novel common mode feedback (CMFB) is proposed for a high-speed fully differential op-amp with a bandwidth of 350 MHz. CMFB is used for each stage of the op-amp. The chip was designed and implemented by using CSMC 0.5 mum CMOS mixed signal process. The testing results indicate that the high-speed op-amp works well and the CM loops are stable.