A CMOS op amp with fully-differential gain-enhancement

A fully-differential folded-cascode op amp design is presented which achieves both high dc gain and high unity-gain frequency. The high dc gain is attained using an active cascode circuit to increase the output resistance of the amplifier. In contrast to previous work, the gain-enhancement is implemented using only two fully-differential op amps instead of four single-ended amplifiers. Implementation of common mode feedback for these internal amplifiers is achieved with a single device made possible due to the local unity-gain feedback around the amplifiers. Simulation results indicate that the fully-differential technique results in superior performance over the conventional technique in terms of area, power, and settling behavior