| With the development of modern mobile communication technology, the high peak-to-average ratio non-constant envelope modulation signal is widely used, such as OFDM, 64QAM, QPSK, etc. In order to satisfy the linearity of the power, the traditional class A or class AB power amplifier must operate at a power back-off region from saturation when the signal is high peak-to-average ratio. However, the power amplifier efficiency reduces to very low when power is back off. Power amplifier wastes of resources, not only led to low efficiency but also reduces the life of the amplifier. Therefore in current power amplifier research the power amplifier efficiency improvement has become the hot and difficult issue when power is back off.This paper aims to find the way to solve this problem. Firstly, it introduces the heterojunction bipolar transistor (HBT) development and characteristics of the structure and then takes Doherty as the main subject, in theory, deduces the basic principle of Doherty amplifier, briefly introduces the basic principles of derivative structure of Doherty.Then, according to design specifications of the power amplifier system the implementation method is analyzed, and the circuits at all levels are simulated. The Triquint's Gen2 HV-HBT TG2H214220 is used for designing the Doherty amplifier. From the design of single-chip amplifier, input and output matching circuits are analyzed. Then, a symmetrical Doherty amplifier system with average output power 80Watt is presented.Finally, at all levels of the system amplifier circuit's debugging and testing are introduced, especially the final Doherty power amplifier. Test results meet the design objectives. When the Doherty power amplifier output power reaches to 49dBm(the power back-off 7.4dB from saturation), the efficiency is more than 43%. |
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