Accurate Offset Line Design of Doherty Amplifier With Compensation of Peaking Amplifier Phase Variation

Accurate offset line design of Doherty amplifier is investigated to compensate the phase variation of the carrier and peaking amplifiers. The offset line at the peaking amplifier not only blocks the power leakage to the peaking amplifier at the OFF-state, but also assists in the load modulation like the offset line at the carrier amplifier. However, with the conventional offset line, the load is not properly modulated due to the phase variation of the class C peaking amplifier. Since the offset lines at the carrier and peaking power amplifiers (PAs) properly eliminate the reactive parts during the Doherty load modulation, the offset lines can handle the phase variation also. To compensate the phase variation, additional lines are added at the carrier and peaking amplifiers for the proper load modulation. With the new offset line, the load of the peaking amplifier can be properly modulated and the efficiency at the high power region is increased. In addition, the efficiency at the low power region is also increased due to the larger load of the carrier amplifier at the power region. The enhanced performance is validated by the Doherty PA employing CGH40045 devices at 1.94 GHz, applying drain bias voltage of 50 V. For the long term evolution signal with a 6.5 dB peak-to-average power ratio, the Doherty amplifier with the new offset line delivers a drain/power-added efficiency of 57%/54.8% which is 3.5%/3.1% higher than that with the conventional offset line at an output power of 45.4 dBm with the linearity -48.3 dBc using digital predistortion linearization.