A new low-power CMOS active balun is designed for ultra-wideband applications, using a pair of common-source NMOS and common-gate PMOS transistors. This balun gives an impedance transformation ratio of 1:2. Without compensation feedback, the circuit provides a differential signal within 2dB and 3° of gain and phase imbalance, respectively, up to 8-GHz. Total power consumption is only 1.44 mW at the supply voltage of Vdd=1.2V, much less than 12 mW of the traditional active balun. This saves 88% of power. The circuit can be fully integrated in RFIC for low power and low cost.