A Sub-2-nW, 69-dB Gain Inverter-Based OTA for Energy-Harvesting Biomedical Sensor Nodes

This paper presents an ultra-low-power, high gain operational transconductance amplifier (OTA) based on a novel inverter-based local feedback architecture, specifically targeting energy-harvesting biomedical sensor nodes. The proposed design employs digital inverters biased in the subthreshold regime, ensuring minimal power consumption while maintaining strong analog performance. The local feedback network senses common-mode variations through replica branches and dynamically adjusts the source voltage of the main inverter pair. By modulating the transistor VGS according to the exponential current–voltage dependency in the subthreshold region, this mechanism robustly stabilizes the output common-mode level without requiring a dedicated CMFB circuit, while simultaneously enhancing the effective transconductance and gain. The OTA achieves a high DC gain of 68.7 dB and consumes only 1.6 nW from a 0.3 V supply. Employing TSMC 180nm technology, the compact layout of the OTA occupies 540 µm², and the design achieves a competitive figure-of-merit (GBW·CL/Power) of 187.5, demonstrating its potential for next-generation ultra-lowpower bio-signal processing applications.