Inverter-Based Subthreshold Amplifier Techniques and Their Application in 0.3-V $\\Delta\\Sigma$ -Modulators

Abstract

Subthreshold amplifiers suffer from the limited voltage headroom which leaves little space for conventional analog techniques to enhance performance and efficiency. This paper presents an evolution process of implementing conventional structures with inverters, allowing ultralow-voltage operation with increased flexibility in adopting traditional circuit techniques. Based on the proposed inverter-based elementary structure and CMFB, both the Miller-compensated (MC) operational transconductance amplifier (OTA) and the feedforward-compensated (FFC) OTA achieve significantly improved performance as compared to previous works. The proposed amplifier techniques are verified in <inline-formula> <tex-math notation= LaTeX >$\\Delta \\Sigma $ </tex-math></inline-formula> modulator (DSM) design, with MC-OTA for a DT-DSM and FFC-OTA for a CT-DSM, both fabricated in a 0.13-<inline-formula> <tex-math notation= LaTeX >$\\mu \\text{m}$ </tex-math></inline-formula> CMOS. The 0.3-V DT-DSM achieves 74.1-dB SNDR, 83.4-dB SFDR and 20-kHz bandwidth with 79.3-<inline-formula> <tex-math notation= LaTeX >$\\mu \\text{W}$ </tex-math></inline-formula> power, resulting in a Schreier figure of merit (FoM) of 158 dB. The 0.3-V CT-DSM achieves 68.5-dB SNDR, 82.6-dB SFDR, and 50-kHz bandwidth with 26.3-<inline-formula> <tex-math notation= LaTeX >$\\mu \\text{W}$ </tex-math></inline-formula> power, leading to a Schreier FoM of 161 dB. Both DSMs exhibit highly competitive performance among sub-0.5-V designs, validating the proposed subthreshold amplifier techniques.