An Ultra Low Power, 5nA Multi-Input Pseudo-Differential Bulk-Driven OTA With 92.97° Phase Margin for Biosignal Acquisition

This paper presents a low-power and low-voltage multi-input operational transconductance amplifier designed using 90 nm CMOS technology and optimized for biosignal processing. The OTA works entirely in subthreshold regions at 0.5 V supply voltage with bias currents as low as 5 nA, so that the total power consumption is in nanowatts. The design maintains a nearly $320 \text{mV}(100 \text{mV}$ to 220 mV) input commonmode range, a robust common-mode rejection below 1 kHz, a gain margin of 25 dB, a high phase margin (92.97°), and an achieved DC gain of $\mathbf{1 5. 4 4} \mathbf{~ d B}$. The bulk-driven input stage helps to overcome threshold voltage bottlenecks and further improves the input-linear characteristics of the circuit. On the output side, due to the presence of a self-cascode stage, there is a way to get high output resistance and symmetric source-and-sink behaviours for the short-circuit current at the output. When combined, all these design features ensure good gain and stability even with very limited voltage headroom and power budgets. The circuit demonstrates its suitability for portable and implantable biomedical systems, offering a compact, energyefficient analog front-end solution without compromising key performance parameters.