Design of a broadband low imbalance active balun using Darlington cell technique in 0.35-µm SiGe BiCMOS process

Design of a broadband low imbalance active balun in a 0.35-μm SiGe BiCMOS process for microwave applications is presented in this paper. The proposed active balun employs Darlington pairs in a differential pair to archive broad and flat bandwidth with good amplitude/phase matches. In addition, a current mirror with a small inductor is realized as an ideal current source to further reduce signal loss and amplitude/phase imbalances at high frequency. Between 0.2 and 14.6 GHz, the proposed active balun demonstrates a gain variation of within 3 dB, an amplitude imbalance of within 1 dB, and a phase error of within 10°. Moreover, this circuit can be further applied to broadband microwave circuits, such as frequency divider and balanced mixer, due to its broad and flat bandwidth with good amplitude/phase matches and compact area.