A Parallel-Path Amplifier for Fast Output Settling

Pushing CMOS technology to the nanometer range is detrimental to analog circuits’ performance due to the reduction of gain and slew rate of amplifiers, so the classical approaches need to be revisited for adjustment in advanced nodes. This paper presents a parallel-path amplifier used as a switched-capacitor (SC) amplifier. The proposed amplifier includes a high bandwidth and slewing path parallel to a high gain path. The high bandwidth and slewing path, named the feedforward path, provides high charging/discharging currents to decrease the slewing time of the amplification phase, significantly $(\geq 60$%). In parallel, the high gain path provides sufficient open-loop DC gain for final settling (59 dB). The feedforward path is enabled/disabled by control signals provided through a hysteresis detector and by considering the status of the feedback voltage. The proposed amplifier is designed and fabricated in 65nm CMOS technology as a multiplying digital-to-analog converter (MDAC) in a pipeline ADC. The chip is under fabrication, and this paper covers post-layout performance of the proposed amplifier. The results reveal that enabling the feedforward path guarantees the amplifier to have a constant error $(\lt2$ mV) for an extensive range of input voltages (300 mV $\leq$ Vin $\leq 900$ mV) compared to its standalone high gain path. At the same time, the static current of the feedforward path is minimal $(\lt 100 \mu \mathrm{A})$, and it can drive large load capacitors.