A low-voltage and low-power CMOS mixer with an active balun is designed for frequency down-conversion in ultra-wideband applications. The mixer contains CMOS amplifiers as the LO buffer and the modulating stage in a folded configuration to allow low-voltage operation. The RF input signal comes from a new low-power active balun that uses a pair of common-source NMOS and common-gate PMOS transistors. This balun gives an impedance transformation ratio of 1:2, and provides a differential signal within 0.5dB and 1.5° of gain and phase imbalance, respectively, from 6 to 10.6-GHz. The mixer and the active balun are designed in 0.18μ m CMOS technology to operate at 1.2-V supply voltage. The circuit performance of the mixer with the active balun achieves flat conversion gain of 17 dB within 1 dB variation, the maximum IIP3 of 0 dBm, and the minimum noise figure of 13 dB. The total circuit only consumes power of 2 mW. As compared to typical designs, it saves about 85% and 95% of power consumption in the balun and the mixer circuits, respectively.