Fully Differential Operational Transconductance Amplifier with Enhanced Phase Margin and Gain for Ultra-Low-Power Circuits

摘要

This paper presents a novel, low-power, subthreshold, bulk-driven single-stage class AB, fully-differential operational transconductance amplifier (FD-OTA) with self-cascode current mirror loads and cascode frequency compensation. The self-cascode composite load has enhanced its single-stage fully differential (FD) gain to 68.2 dB. The proposed OTA, when driving fully differential loads (2 × 15 pF) with cascode frequency compensation capacitor (C_C), has provided the gain bandwidth (GBW), phase-margin (PM) and gain margin (GM) of 7.07 kHz, 83 degree and 83 dB, respectively. Its fully differential structure provides enhanced single ended (SE) common mode rejection ratio (CMRR) and SE power supply rejection ratio (PSRR) of 110.6 dB and 69.2 dB, respectively. It has ensured a total harmonic distortion (THD) better than -45 dB with active MOSFET-degeneration at 200 Hz frequency even at rail-to-rail input signal swing of 1.2 Vpp in unity gain configuration. It consumed 90 nW quiescent power from a 0.6 V supply, delivering around 18.7 nA current in each branch of the proposed OTA. Its robustness has been verified performing process corners and Monte Carlo statistical simulations in consideration with process and device-mismatch effects its layout is drawn and post-layout results are very close to pre-layout results. Its fully differential class AB output stage has ensured the slew rates of 1200 V/ms for a 500 Hz, 1.2 Vpp pulse, while driving two fully differential capacitive loads each of 15 pF. This OTA has been employed to realize biquadratic low-pass and band-pass G_m-C filters to validate its application. The proposed FD-OTA has been designed in Cadence Virtuoso environment using UMC technology file in 0.18 μm standard n-tub CMOS technology.