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A differential comparator that can sample 40-Gb/s signals and that operates off a single 1.2-V supply was designed and fabricated in 0.11-μm standard CMOS technology. It consists of a front-end sampler, a regenerative stage, and a clocked amplifier to provide a small aperture time and a high toggle rate. The clocked amplifier employs a bandwidth modulation technique that switches the feedback gain to reduce the reset time while keeping the effective gain high. We confirmed that the comparator receives a 40-Gb/s data stream at a toggle rate of 10 GHz with bit error rate less than 10-12 by laboratory measurements.