Efficiency Enhancement of 250W Doherty Power Amplifiers Using Virtual Open Stub Techniques for UHF-band OFDM Applications

We propose a new efficiency enhancement method of Doherty power amplifiers (PAs) using virtual open stub techniques. In this method, we consider a peak amplifier's output line as a virtual capacitive open stub only seen at low power level. Using this method, we can transform the low power load impedance seen from the carrier amplifier into higher than the impedance of classical Doherty PA (>100ohm). The relations of the load impedance and the length of the virtual open stub are derived analytically. The developed UHF-band 250W 2-way Doherty PA has been achieved a drain efficiency of 42.1% and an intermodulation (IM) distortion of -27.3dBc at an output power of 44.1dBm (10dB output backed-off) under an integrated services digital broadcasting-terrestrial (ISDB-T) OFDM signal. To the best of our knowledge, these results are the highest backed-off efficiency ever reported among the over 40dBm saturation high power Doherty amplifiers