On-chip transformer-coupled balun bandpass filter for 5-GHz applications

In this work, an on-chip transformer-coupled balun bandpass filter is presented. The proposed balun bandpass filter features a compact chip size and it can be designed according to the desired bandpass response and reference impedance. Specifically, a balun bandpass filter of 3rd-order bandpass response is demonstrated using the GaAs pHEMT process. The chip size is 1 mm×1 mm for a center frequency f₀ of 5.4 GHz and a bandwidth of about 28%. The corresponding electrical size is only about 0.018λ₀ × 0.018λ₀ at 5.4 GHz. Good balanced performance is also achieved.