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Broadband monolithic InGaP HBT frequency doublers for K-band application have been developed. The designs employ a Gm-boosted differential common-base configuration using either capacitive or transformer-based coupling between base and emitters. To the authors' best knowledge, these are the first frequency doublers utilizing the Gm-boosted configuration and the result demonstrate larger bandwidth and higher output power than any previously reported frequency doublers. The design with cross-coupled capacitors presents a fundamental rejection better than 20 dB over a 100% 3-dB bandwidth, extending from 6 to 18 GHz. The transformer-coupled design has about 15-dB fundamental rejection over a slightly narrower bandwidth extending from 7 to 16 GHz. Both doublers have conversion gain peaking at more than -0.8 dB and output powerPsat >; 13 dBm . The designs are also very compact with chip sizes less than 0.5 mm2.