K- and Ka-band miniature CMOS balun with a single spiral coupled structure

This paper presents a CMOS wideband miniature balun operating over the full K- and Ka-bands. The balun is based on a single spiral coupled structure where two symmetric spiral lines are stacked vertically (i.e. broadside-coupled) in an antipodal manner. The proposed structure provides ultra-wideband balun performance while occupying a minimal chip area compared with conventional spiral baluns. In order to offset residual inductance of the spirals, a compensation capacitor is shunted at the end of an unbalanced input line, which further improves the bandwidth. The proposed balun is implemented in a bulk 0.13-μm CMOS process. The measured insertion loss is less than 6.5 dB from 14 to 40 GHz. The amplitude and phase imbalances are less than 1 dB and 10°, respectively, over the entire bandwidth. The balun occupies an area of only 0.0095 mm2, which, to the authors’ knowledge, is significantly smaller than any previously reported CMOS baluns at similar frequency bands.