A 68.31-nW, 59.28-dB gain low-noise DTMOS current-reuse OTA in 65 nm CMOS for low-frequency biomedical applications

This paper presents a 0.5 V, 68.31 nW fully differential current-reuse operational transconductance amplifier (OTA) implemented in a 65 nm CMOS process, specifically targeted for ultra-low-power and ultra-low-voltage biomedical applications. Operating in the deep sub-threshold regime often limits transconductance and intrinsic gain due to reduced voltage headroom and restricted bias currents. To overcome these classical gain-noise-headroom trade-offs, the proposed architecture synthesizes four structural modifications over the conventional one: a Dynamic Threshold MOS (DTMOS)-assisted inverter-based input to enhance effective transconductance without increasing power, a cross-coupled positive feedback network for gain boosting via negative conductance, an inverter-based cascoded output stage that preserves the boosted gain while maintaining high differential output swing, and an optimized Miller compensation utilizing only two nulling resistors. Post-layout simulations demonstrate that the OTA achieves a DC gain of 59.28 dB and a unity-gain bandwidth of 18.45 kHz (13.92 kHz) while driving a 2 pF (10 pF) load. The use of multipath transconductance boosting mechanisms successfully reduces thermal noise, yielding an input-referred noise of 12.78 μ V r m s over a 1 kHz bandwidth. Furthermore, the design delivers high interference rejection, achieving 100.7 dB CMRR and 90.89 dB PSRR, making it well suited for low-frequency biopotential acquisition such as ECG in wearable and implantable nodes.