A wideband frequency doubler based on AlGaN/GaN HEMT devices is presented. Rationale for the use of filters to improve the bandwidth performance of a frequency doubler is described. Input and output networks consisting of a harmonic reflector and bandpass filter are designed to provide adequate bandwidth response and high power with small conversion gain. A demonstration circuit achieves 13.2% fractional bandwidth at fo= 3.33 GHz with a maximum output power at 30.2 dBm.