An ultra wide band double-balanced mixer with integrated LO buffer amplifier was developed for use between 18-45 GHz. The mixer employs a unique broadside-coupled balun for superior bandwidth over 18-45 GHz. It utilizes GaAs HEMT S-D diode mixing and integrates an LO power amplifier to reduce typical high drive requirements. The LO amplifier and balun allow the mixer to achieve approximately -10 dB conversion gain for up or down mixing across many frequency bands between 18-45 GHz using only 0 dBm of LO power.