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In this paper, a frequency doubler is developed for MMW applications in a GaAs 2-mum HBT process. This balanced doubler adopts the cascode devices to achieve a high conversion gain, with a built-in 180deg reduced-size Marchand balun for fundamental suppression. In addition, because HBT presents a high beta factor (current gain), an on-chip stable voltage source is desired to maintain the collector current in the doubler. Thus, a transistor biased in the saturation region can be used to fix the base voltage and sustains the doubler in the pinch-off region. Under 5-dBm input drive, this balanced doubler demonstrates a measured conversion gain of 6.1 dB at 31 GHz. The fundamental rejection is better than 23 dB.