Design of an original K-band active balun with improved broadband balanced behavior

This letter deals with the design of an active balun exhibiting a single-ended input and balanced outputs. We propose an original method, based on the determination of specific impedance for connecting the differential transistor pair to ground, which greatly improves the balanced behavior over a broadband frequency range. The usefulness of this method is demonstrated through a 180 degrees power divider implemented on a 0.25-mu m SiGe BiCMOS process. According to simulations, measurements confirm a broadband well balanced behavior, since the power splitter exhibits phase and amplitude errors between the two channels less than 4 degrees and 0.5 dB, respectively, from 0.2 to 22 GHz. These errors are only 0.75 degrees/0.03 dB in a 2-GHz span around 20 GHz, in accordance with our optimization goals.