Design of Very Compact Bandpass Filters Based on Differential Transformers

This letter presents two compact differential transformer-based bandpass filters (BPFs) using the integrated passive device technology based on a GaAs substrate. The two designs are located at the 900-MHz center frequency and have a 3 dB fractional bandwidth of 14%. Good RF performances and miniaturizations are achieved in these two designs. The measured insertion losses of the type-A and type-B filters are less than 1.25 dB and 0.8 dB, respectively. The overall size of both BPFs are especially small compared to other works, which are only
$7.8\times 10^{-3}\lambda_{0}\times 7.8\times 10^{-3}\lambda_{0}$
and
$9.0\times 10^{-3}\lambda_{0}\times 7.8\times 10^{-3}\lambda_{0}$
. The second design provides a good solution for the limitations and issues of the first proposed design. To verify the proposed design, two prototypes are implemented, and the measured results validate the correctness of the presented design approach.