Ultra broadband MMIC impedance transformer

The impedance transformer is an important, but little used, tool for MMIC designers. The Ruthroff style MMIC compatible broadside-coupled impedance transformer has excellent broadband performance, but has an inherent bandwidth limitation due to a half-wavelength resonance in the ground path. A novel but simple method is presented to mitigate the resonance and significantly extend the usable bandwidth of the transformer. A series of transformers are designed, fabricated and characterised to demonstrated the improved performance.