A Simulation-Driven Experiential Learning Framework for Teaching Ultra-Low-Power Analog Design Using Class-AB Recycling Folded Cascode OTAs

Abstract

This paper discusses the design and analysis of an ultra-low-power Class-AB Recycling Folded Cascode (RFC) Operational Transconductance Amplifier (OTA) for high-density biomedical sensing interfaces implemented with 45 nm CMOS technology. The proposed design tackles the trade-off between power use and driving capability found in traditional Class-AB operation. To address this issue, the RFC core features a dynamic Class-AB boosting mechanism, which improves the slew rate performance without significantly increasing quiescent current. The design method relies on the gm/ID approach, allowing for optimal transistor sizing in the subthreshold region. Furthermore, this methodology serves as a comprehensive educational case study for instructing students on subthreshold device optimization in modern nanometer regimes. This maximizes transconductance efficiency while meeting strict power limits. This makes the proposed OTA especially suitable for battery-powered and energy-limited biomedical applications. Simulations show that the OTA achieves a DC open-loop gain of 75.73 dB, a Common Mode Rejection Ratio (CMRR) of 136.36 dB, and a phase margin of 84.02° while driving a capacitive load of 20 pF. The circuit has a positive slew rate of 0.024 V/μs, with total power consumption of just 337.5 nW from a 1 V supply. Additionally, the design demonstrates strong robustness across variations in process, voltage, and temperature (PVT), ensuring reliable operation in real-world conditions. Thanks to its low power consumption, high gain, and stability, the proposed OTA is ideal for low-frequency bio-signal acquisition systems, such as wearable and implantable medical devices, where energy efficiency and signal quality are essential.