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A K-band frequency doubler using InGaP HBT is developed, which features high fundamental frequency rejection, flat conversion loss over wide bandwidth, and high saturation output power. To obtain a balanced signal, a compact on-chip lumped rat-race hybrid is implemented. The circuit exhibits a measured conversion loss of 4 dB over the output frequencies from 14 to 22 GHz. The fundamental frequency suppression is better than 20 dB and the second harmonic saturation output power is higher than 7.3 dBm with a miniature chip size of 1 mm × 1 mm.