A Start-up Assisted Fully Differential Folded Cascode Opamp

摘要

This paper explains the hidden positive feedback in a two-stage fully differential amplifier through external feedback resistors and possible DC latch-up during the amplifier start-up. The biasing current selection among the cascade branches has been explained intuitively, with reference to previous literature. To avoid the latch-up problem, irrespective of the transistor bias currents, a novel hysteresis-based start-up circuit is proposed. An 87dB, 250 MHz unity gain bandwidth amplifier has been developed in 65 nm CMOS Technology and post-layout simulations demonstrate no start-up failures out of 1000 Monte-Carlo (6-Sigma) simulations. The circuit draws 126 mu A from a 1.2 V supply and occupies the 2184 mu m(2 )area.