Research and design of CMOS fully differential telescopic operational amplifier with common mode feedback

Abstract

In the medium or low frequency cases, the performance such as the gain or bandwidth of the fully differential operational amplifier is not good. In this paper, a fully differential telescopic two-stage operational amplifier with two-stage common-mode feedback is designed by using TSMC 250 nm technology. The first stage operational amplifier is telescopic structure, while the second stage operational amplifier is common source structure. The inner structure of common-mode feedback is continuous-time common-mode feedback circuit, and the outer structure is switched capacitor CMFB structure. It has the advantages of high gain and good linearity. It overcomes the shortcomings of common mode feedback circuit in limiting the output swing. It can stabilize the DC operating point and improve the output swing effectively. Cadence Spectre simulation results show that the common mode feedback structure keeps the common mode level at 1.25 V, the open loop gain is 67.7 dB, the phase margin is 45°, and the unit gain bandwidth is 150.7 MHz under the 2.5 V supply voltage.