Efficiency Enhancement of250WDoherty PowerAmplifiers Using Virtual OpenStubTechniques forUHF-bandOFDM Applications

We propose anewefficiency enhancement method ofDohertyPowerAmplifiers (PAs)usingvirtual openstub techniques. Inthis method, weconsider apeakamplifier's output line asavirtual capacitive openstubonlyseenatlowpowerlevel. Usingthismethod, we cantransform thelowpowerload impedance seenfromthecarrier amplifier intohigher thanthe impedance ofclassical Doherty PA (>lOOohm). Therelations of theloadimpedance andthelength ofthevirtual openstubare derived analytically. Thedeveloped UHF-band250W 2-way Doherty PAhasbeenachieved adrain efficiency of42.1%andan Inter Modulation (IM)distortion of-27.3dBc atanoutput power of44.1dBm(1OdBoutputbacked-off) underan Integrated Services Digital Broadcasting - Terrestrial (ISDB-T) OFDM signal. Tothebestofourknowledge, these results arethehighest backed-off efficiency everreported amongtheover4OdBm saturation highpowerDoherty amplifiers. IndexTerms - poweramplifiers, microwave Doherty amplifiers, efficiency, OFDM,MOSFETs.