A Wideband Isolated Real-to-Complex Impedance Transforming Uniplanar Microstrip Line Balun for Push-Pull Power Amplifier

In this article, a three-port uniplanar microstrip line balun is proposed to transform the balanced complex load termination to 50- Omega unbalanced port impedance. The proposed device is designed based on a symmetric four-port network with an open-ended transmission line inserted between branch lines in the middle of structure. To improve the isolation and match the complex termination, a resistive network is proposed and inserted between the two balanced ports. In addition, the theoretical analysis is carried out for the even and odd mode circuits to expand the bandwidth of the proposed realcomplex impedance transforming balun. With the proposed design theory, the impedance trajectory follower splitting and combining baluns are designed for wideband push-pull power amplifier (PPPA). To verify the design concept, first, a realcomplex impedance transforming uniplanar microstrip line balun is designed and characterized at 1 GHz. Then, a prototype for the wideband PPPA with impedance trajectory follower microstrip baluns is designed, fabricated, and validated in experiment. For the proposed balun, the simulated and measured results agree well with each other to prove the design concept. From the experimental results, the PPPA with the proposed balun shows a gain of 10.2-12.2 dB, output power of 42.7-44.7 dBm, and a drain efficiency of 59.8%-74.6% within the frequency range of 1.8-2.7 GHz.