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.