Design of broadband lumped element baluns

This paper reports on a small size broadband lumped element balun suitable for integration in MMICs. The design is an extension of the well known lattice balun, which independent of frequency has 180° phase difference between the output ports, but suffers from a narrow amplitude bandwidth. It is shown how the amplitude bandwidth of the lattice balun may be improved by replacing inductors with low-pass T-sections, and the capacitors with high-pass T-sections. Scalable closed-form design equations for various bandwidths are derived. To validate the concept a prototype operating at 1 GHz has been fabricated with SMT chip components soldered on a PTFE laminate. It exhibits amplitude balance better than ±0.25 dB and phase balance better than ±8°, over more than one octave bandwidth. The effective chip-area is 7×9 mm².