Design of Step-Down Broadband and Low-Loss Ruthroff-Type Baluns Using IPD Technology

Ruthroff-type transmission line transformers (TLTs) and baluns prevail over their broadband and low-loss performance. A typical Ruthroff-type balun with a 1:4 step-up impedance transformation ratio was successfully presented using integrated passive devices (IPDs) process. Moreover, two proposed baluns with step-down impedance transformation ratios of 1:1 and 9:4 were developed by modifying the combination of one Ruthroff-type TLT and one Ruthroff-type balun. The two proposed impedance step-down baluns whose balanced impedance is lower than unbalanced impedance make Ruthroff-type balun more flexible for applications; meanwhile, fabricating the baluns using IPD process can help to enhance the low-loss performance. The measured results show that the proposed 1:1 balun exhibits an insertion loss of 0.46 dB with 1-dB fractional bandwidth of 138.9%, and the proposed 9:4 balun exhibits an insertion loss of 0.75 dB with 1-dB fractional bandwidth of 72.4%. The chip areas of the proposed 1:1 and 9:4 baluns, including the pads, are 0.6 and 0.64 mm², respectively. The two proposed baluns are the first on-chip step-down Ruthroff-type baluns and having an option of center tap, which is highly contributive to wideband and high-efficiency power amplifier design.