High current efficiency fast transient gain-boosted single-stage bulk-driven OTA with enhanced input stage suitable for heavy capacitive loads

Abstract

This article presents a 0.5 V operated, bulk-driven subthreshold OTA with up to 30 nF capacitive load driving capability. The input stage utilizing three unmatched pseudo-differential pairs is biased with two DC level shifters, especially bulk-driven flipped voltage followers in a direct and cross-coupled connection, exhibiting superior performance to existing bulk-driven OTAs. The input transconductance stage with local common-mode feedback (LCMFB) followed by partial positive feedback (PPF) contributes a large dynamic output current using three-successive amplification. As a result, the proposed OTA provides 24.2 times higher effective input core transconductance, 2.62 times higher DC gain, and 11.5 times more slew rate than a conventional class-A BD OTA with PPF at 5% lower static power. The post-layout results of the proposed OTA for a 10 nF capacitive load using UMC 0.18-µm technology in Cadence Virtuoso-XL yield a DC gain of 124.6 dB, a UGF of 183.47 kHz, a PM of 89.98°, and greater figures of merit (FOM) for small- and large-signal performance than the state-of-the-art bulk-driven OTAs.