| The downlink signal of LTE communication system is a Multi-Carrier signal which has a high peak-to-average ration. This kind of signal leads to higher requirement for linearity of base station power amplifier. At the same time, due to environment protection and energy saving, high efficiency of power amplifier is also very important. Based on the above requirement, how envelope tracking technology is applied to Doherty amplifier is studied in this paper, in order to achieving high efficiency while having a better linearity on high power input condition.Using envelope tracking signal to control grid voltage of peak amplifier in Doherty power amplifier is main work of this paper. In low power state envelope tracking circuit provides a low voltage to improve efficiency, and in high power state provides a high voltage to improve linearity in case of a small sacrifice in efficiency.In this paper, the research mainly includes:1) Studying the theory of power amplifier and operating principle of Doherty amplifier.2) The envelope tracking technology is analyzed and studied.3) The effect of envelope tracking technology to Doherty amplifier is analyzed. And the result of different bias voltage of Doherty amplifier is shown in this paper. A circuit structure is proposed to improved linearity of Doherty amplifier, which has a voltage controlling for gird of peak amplifier in Doherty amplifier circuit.4) The envelope tracking Doherty amplifier is designed, manufactured and tested. The test results show that IMD3 of ET-Doherty improved 3~5d Bc relative to traditional Doherty while PAE decreasing 2~4% in 8d B power back-off. |
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