A dc-21 GHz low imbalance active balun using a 2 μInGaP/GaAs HBT process is presented in this letter for high speed data communications. A Darlington cell is adopted to enhance 3 dB bandwidth of the proposed active balun. A feedback capacitor is designed to compensate the phase error between differential output ports caused by the different number of stages. The proposed active balun achieves a broad bandwidth of 21 GHz, an average small signal gain of 2.5dB, a maximum amplitude imbalance of 1.2 dB, and a phase error of less than 5̊ The measured group delays of the balun are lower than 30 ps with low variation. Moreover, an eye diagram with a pseudorandom bit stream of up to 12.5 Gbps is presented. The active balun is appropriate for high speed data communications due to its low imbalance and group delay.