A 0.5V amplifier with adaptive CMFB compensation for a 2-1 cascade CT Delta Sigma modulator

This paper presents a 0.5V fully differential gate-input amplifier with adaptive common-mode feedback (CMFB) frequency compensation. The CMFB circuit employs a pair of transistors working in linear region as compensation resistor to trace the g_m variation of an error amplifier and thus to obtain a robust performance against process variation. This adaptive compensation method also applies to bias circuits and ensures stable bias against process variation. The amplifier is designed in a 0.13μm CMOS technology, achieves simulated performance of 63-dB open loop DC gain, 52-MHz unity-gain-frequency and 76° phase margin with a load of 31.25pF//64kΩ and 427μW power consumption at 0.5V supply. The amplifier is applied to a 2-1 cascade continuous-time Delta Sigma modulator with switched-capacitor-resistor feedback. The modulator achieves simulated performance of 102dB SNDR over a 25-kHz signal bandwidth at a sampling rate of a 6.4-MHz, and consumes 693μW at 0.5V.