A Bandwidth Improved Broadband Compact Lumped-Element Balun With Tail Inductor

A novel broadband compact lumped-element balun with a tail inductor is proposed in this letter. The employment of tail inductor improves amplitude and phase balances for better broadband performance. Furthermore, compared with baluns utilizing conventional micro-strip approaches or second-order lattice topology, this proposed topology offers significant circuit size reduction. Theoretical analysis of the optimized tail inductor value is presented to improve operation bandwidth. The concept is realized and validated by a 2.45 GHz balun with surface mounted devices (SMD). The measured results illustrate an amplitude balance less than 0.5 dB and a phase balance less than ±6^° in a 36.7% bandwidth. The insertion loss is around 3.6 dB while the return loss of the unbalance port is better than 15 dB in the whole working frequency range. The core circuit occupies only 2 mm × 2.5 mm limited by SMD size.