This paper proposes a fully integrated 5–12 GHz power mixer in tsmcTM 1P6M 0.18 μm CMOS technology. The power mixer is used to replace the conventional mixer and power amplifier (PA). The circuit consists of an up-conversion Gilbertcell mixer and a wideband and low loss 1:4 Guanella-type differential transmission-line-transformer (DTLT). The power mixer is biased at deep class A/B under a 3.5-V supply voltage and dissipates total DC power consumption of 172 mW. The power mixer achieves a conversion gain of 19.5 ± 1.5 dB from 5 to 12 GHz, an OP1dB of 13.1 dBm with a 3-dB power bandwidth from 4 to 12 GHz, a saturation power Psat of 15 dBm with a 3-dB power bandwidth from 4 to 12 GHz. The drain efficiency is 10.3% at 11-dBm LO power. The chip size including pads is 0.96 mm × 1.57 mm.