0.6-V, μW-Power Four-Stage OTA With Minimal Components, and 100× Load Range

A four-stage operational transconductance amplifier (OTA) for ultralow-power applications is introduced in this letter. The proposed circuit inclusive of frequency compensation requires minimal transistor count and passives, overcoming the traditionally difficult compensation of four-stage OTAs and bringing it back to the simplicity of three-stage OTAs. At the same time, the proposed circuit achieves high power efficiency, as evidenced by the > $3.7\times $ (> $11.3\times $ ) improvement in the large-signal (small-signal) power efficiency figure of merit ${\mathrm { FOM}}_{L}~({\mathrm { FOM}}_{S})$ , compared to prior four-stage OTAs (sub-1 V multistage OTAs). Thanks to the lower sensitivity of the phase margin to the load capacitance, the proposed OTA remains stable under a wide range of loads (double-sided as in any three- and four-stage OTA), achieving a max/min ratio of the load capacitance of > $100\times $ .