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A 45-Gb/s BiCMOS decision circuit operating from a 2.5-V supply is reported. The full-rate retiming flip-flop operates from the lowest supply voltage of any silicon-based flip-flop demonstrated to date at this speed. MOS and SiGe heterojunction-bipolar-transistor (HBT) current-mode logic families are compared. Capitalizing on the best features of both families, a true BiCMOS logic topology is presented that allows for operation from lower supply voltages than pure HBT implementations without compromising speed. The topology, based on a BiCMOS cascode, can also be applied to a number of millimeter-wave (mm-wave) circuits. In addition to the retiming flip-flop, the decision circuit includes a broadband transimpedance preamplifier to improve sensitivity, a tuned 45-GHz clock buffer, and a 50-Ω output driver. The first mm-wave transformer is employed along the clock path to perform single-ended-to-differential conversion. The entire circuit, which is implemented in a production 130-nm BiCMOS process with 150-GHz fT SiGe HBT, consumes 288 mW from a 2.5-V supply, including only 58 mW from the flip-flop.