Class AB two stage and folded cascode OpAmps based on a squaring circuit

This paper presents a novel adaptive bias technique based on the use of squaring circuits. Here, the tail current of an operational amplifier (OpAmp) is controlled using an auxiliary circuit. By design, it generates a well-controlled tail current which - to a first order approximation - is independent of the OpAmp's common-mode input voltage. As a result, parameters like common-mode rejection ratio (CMRR) and power supply rejection ratio (PSRR) are enhanced. However, when a differential input signal is applied, it generates a tail current proportional to the square of the OpAmp's differential input voltage. In this way, the currents of the OpAmp's input pairs are boosted, thus improving slew rate. Experimental results in 0.5 μm CMOS technology verify current and slew rate enhancement factors between 10 and 15 with less than 20% static current increase.