A two stage, fully differential operational amplifier in which each stage has a respective common mode feedback loop

摘要

A two stage, fully differential operational amplifier in which each stage has a respective common mode feedback loopA CMOS op-amp has a first differential gain stage DS1 having a first common mode feedback loop CMFB1, followed by a second differential gain stage DS2 having a second common mode feedback loop CMFB2. The first common mode feedback loop CMFB1 includes an amplifier 5 to monitor and correct the output of the first differential gain stage DS1 and the second common mode feedback loop CMFB2 includes an amplifier 15 to monitor and correct the output of the second differential gain stage DS2. There is an output buffer stage OS and the second common mode feedback amplifier 15 corrects the output of the second differential gain stage DS2 by monitoring the output of the buffer stage OS. The bandwidth of the op-amp is extended by the use of two gain stages, the first of which has a peak in its frequency response which compensates for the roll-off in the frequency response of the second stage. As a result, the overall 3-dB bandwidth is increased. The arrangement overcomes problems of common mode stability in prior art fully differential op-amps. Frequency compensation is discussed.