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

Abstract

Purpose The purpose of this study is to discuss the effect of hybrid cascode compensation with quality factor (Q-factor) control module for the three-stage amplifiers driving ultra-large load capacitors. Compared to the present frequency compensation solutions, it extends the amplifier bandwidth by establishing an extra AC feedback pathway besides the primary pathway through the Miller capacitor, increasing the loop gain at the gain–bandwidth product (GBW) frequency by pushing to the higher frequencies the nondominant poles. Design/methodology/approach A Q-factor control block is used to improve the damping factor of the compensation loop with no power or area overhead, thereby reducing the frequency peaking and the undesired oscillation in the time response for small load capacitors. The Q-factor control module is realized by a tiny-size on-chip capacitor, and provides an extra feedback loop to feed the damping current back to the input stage. A left-half-plane (LHP) zero is also introduced to further improve the stability. Findings A prototype of the proposed amplifier is simulated in 180-nm CMOS with a quiescent current of 24-µA from 1.80-V voltage supply. It achieves a 3.98-MHz gain–bandwidth product for 500-pF load capacitor, while the overall compensation capacitor is limited to 0.5-pF and the DC gain is extended beyond 100-dB. Originality/value The proposed amplifier is absolutely stable for the load capacitors ranging between 80-pF and 100-nF.