This manuscript explores the output phase imbalance observed in the common-gate common-source transistors, and gives a mathematical formulation in determining the length of the compensation transmission line. To demonstrate the validity of this wideband phase-compensation technique, a DC-50GHz active balun is designed using commercial 90nm-CMOS process and then measured, where the phase imbalance is less than 5o while the magnitude imbalance is within 1dB. All ports are well-matched and positive gain has been sustained up to 50GHz; the output-referred 1dB compression point is around 3dBm. With 3 V and 32.5mA drain bias, the total power consumption is 97.5mW. To our knowledge, this is the widest active balun ever designed with such tight output phase requirement.