Global impedance attenuation network for multistage OTAs driving a broad range of load capacitor

Abstract

Summary Advanced multistage amplifiers suffer from load‐dependent stability issues, which limit the load capacitor range they can drive. In this work, the concept of global impedance attenuation (GIA) network is introduced to improve an amplifier's stability in the presence of significant load capacitor variations. Composed of multiple parallel resistor‐capacitor (RC) branches, the equivalent high‐frequency output impedance of gain stages is shaped by the GIA network such that a desired frequency spectrum is obtained over a wide range of load capacitor. The parasitic poles at the output of the gain stages are nullified by the proposed network, thereby simplifying the amplifier's transfer function and reducing the minimum load capacitor it can drive. The idea is applied to design a three‐stage operational transconductance amplifier (OTA) with cascode global impedance attenuation (CGIA). Small‐signal analysis shows that the OTA is stable regardless of the load capacitor, and it can drive very small to ultra‐large load capacitors. This feature is verified by the post‐layout simulations of a CGIA amplifier in 0.18‐μm complementary metal‐oxide semiconductor (CMOS) process. The core occupies a die area of 0.0053 mm2 while consuming a static current of 10.97 μA from 1.8‐V voltage supply. The unity‐feedback configuration is unconditionally stable for any load capacitor higher than 10 pF. To the best of our knowledge, this corresponds to the widest range of load capacitance reported for prior‐art three‐stage amplifiers.

Mathematical framework for three cross-over memristor and its realization employing OTAs

“…Operational transconductance amplifier (OTA) is a fundamental and popular active element, which has been used in various range of applications (Aminzadeh, 2019;Aminzadeh et al, 2020). Figure 3 represents the symbol and CMOS architecture of OTA.…”

A Methodology to Derive a Symbolic Transfer Function for Multistage Amplifiers

“…Since the early years of MOS analog integrated circuits, the compensation of two or more stage amplifiers was a key research area [1]- [28]. In this field several novel solutions were continuously proposed, especially for three-and fourstage amplifiers, and, despite several decades have been devoted to this subject, even in the last ten years the interest on this topic appears active [29]- [40]. Indeed, due to the continuous degradation of transistors' intrinsic gain and the low supply voltage of modern scaled technologies, more gain stages are required [35] and in any case the development of novel compensation networks remains an important research area in advanced CMOS technologies.…”

“…Despite from one side several multistage amplifier compensation networks were proposed and analyzed [1]- [40], and, hence, we know their voltage transfer functions which are typically reported in the original or successive papers, the continuous research on this domain maintains unchanged the need to evaluate the voltage transfer function especially in a novel more complex topology that could be adopted. Hence, methodologies to calculate the symbolic voltage transfer function simply and efficiently are still important.…”

Hybrid cascode compensation with Q-factor control module for three-stage OTAs driving ultra-large load capacitors

“…The Q-factor cannot be, however, reduced easily since it is related to the stages' transconductances and output impedances that cannot be controlled easily. Local impedance attenuation (LIA) network along with auxiliary feedback loops are used in conjunction with cascode Miller compensation to reduce the high Q-factor using the cascode local impedance attenuation (CLIA) (Tan and Ki, 2015), the cascode global impedance attenuation (CGIA) (Aminzadeh et al, 2020), the cascode Miller compensation with local Q-factor control (CLQC) (Cheng et al, 2019) and the transconductance-enhancement cascode Miller compensation (TECMC) (Dong and Zhu, 2018).…”

“…Still, the high Q-factor limits the OTA performance for the small load capacitors. Dual loop cascode Miller compensation with damping factor control unit (DLCDFC) (Aminzadeh and Dashti, 2018) and cascode global impedance attenuation (CGIA) (Aminzadeh et al, 2020) modify the high Q-factor of the HCFC amplifier using a serial RC network at the output of the second stage or both first and second stages, respectively. Significant silicon area is however, occupied by the required resistance and capacitance of hundreds kX and several hundreds fF.…”