Nonlinear Adaptive Biasing for Low-Voltage Class-AB OTAs

An adaptive biasing circuit suitable for very low-voltage design of class-AB symmetrical ransconductance operational amplifiers (OTAs) is presented. A non-linear current mirror is exploited to enhance the maximum tail current as a function of the input voltage, achieving a super class-AB behavior. Robustness to process and voltage supply variations and advantages of the proposed solution are demonstrated by designing a 0.6Vclass-ABOTAwith adaptive biasing in 0.13-μm CMOS technology, and evaluating its performance in unitygain buffer configuration. The use of the proposed adaptive bias circuit provides a 20x slew-rate improvement with respect to the same OTA biased at constant current.