Design of a 20W GaN MMIC Doherty Power Amplifier for the Frequency Range 4400MHz - 5000MHz
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The Doherty Power Amplifier architecture is becoming increasingly popular for many RF producers because of its enhanced efficiency characteristics compared to traditional amplifier classes. One major constraint of practical Doherty designs is its limited bandwidth, which is due to its use of transmission lines. At the request of Kongsberg Aerospace, methods of implementing a 20W Doherty PA to cover the frequency range 4400 - 5000 MHz was explored. To achieve this, the theoretical frequency response of transmission lines and the Doherty system was deducted using Z-parameters, and lumped component equivalents of transmission lines is proposed. A practical Doherty PA using a Pi-equivalent of the transmission line was implemented in CAD using CREE's GaN MMIC technology. The main PA of the Doherty system was implemented as a class-B amplifier, and the auxiliary PA was implemented as a class-C amplifier. A practical method of finding passive MMIC components from ideal components using Z-parameters is given, and practical restrictions of MMIC design and general PA design is also explored. The method of using small-signal analysis to design the Doherty output network using Z-parameters with the Doherty equivalent circuit is also explored, with the aim of saving time and complexity. The finished Doherty design is given with results for the frequency band 4400 - 5000 MHz. The design was able to deliver above 18.9W RMS power across the frequency range, with a peak power of 20.3W at 4.7GHz. At saturation, the system delivers above 44% drain efficiency with a mean value of 50.6% across the frequency range. In 6dB backoff, the system delivers above 38% with a mean value of 42.2%. The system also has a power gain between 12.2dB and 13.3dB across the frequency band. A simple layout of the Doherty MMIC is given, using transmission lines as interconnects. The total area of the layout became 4.92mm2 with interconnects and 1.33mm2 without. Finally, the results are compared to other practical Doherty designs, and practical methods to improve the performance and frequency response further is given.