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

摘要

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-mu m complementary metal-oxide semiconductor (CMOS) process. The core occupies a die area of 0.0053 mm(2) while consuming a static current of 10.97 mu A 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.