A 3-bit Digitally-Reconfigurable Driving-Capability nW-Powered Three-Stage Fully-Synthesizable Inverter-Based OTA for Multi-Load/Applications

This paper presents a fully synthesizable three-stage inverter-based operational transconductance amplifier (OTA) featuring a 3-bit digitally reconfigurable output stage for robust operation across a wide range of capacitive loads in ultra-low-voltage (ULV) and ultra-low-power (ULP) environments. Implemented entirely with standard-cell inverters and inverting-tristate (ITS) buffers, the proposed architecture operates from a 0.3-V supply while consuming less than 2.7 nW. The digital tuning mechanism enables near-constant gain-bandwidth product (GBW) and average slew rate (SRav) across a 10-110 pF load range, making the OTA suitable for both on-chip and off-chip applications in battery-less and energy-harvested systems. Fabricated in 180-nm CMOS, the OTA achieves > 65-dB DC gain, 3.5-kHz GBW, 0.16-V/ms SRav, and 94% rail-to-rail output swing, while demonstrating strong resilience to PVT variations. Measurements confirm > 2.1X improvement in small-signal efficiency establishing a new state-of-the-art for < 0.5-V digital-based OTAs.