| For the last few years, most of the satellites launched into orbit have been from the microsatellites category and the tendency seems to continue. The last two Canadian satellites, MOST and Scisat, were respectively of the microsat and smallsat category of spacecraft. These satellites are smaller in size and dedicated to missions that requires a less powerful and lighter vehicle. However, the reduction in weight of these satellites has a very important impact on the cost of the missions, since a good portion of the costs is dedicated to the launch, and the cost of launching a satellite is directly related to its weight. A bus of the microsatellite category can provide, on average, a hundred watts of power, limited by the dimension of the solar panels and the capacity of the batteries. For these reasons, the design of components for space applications, such as RF power amplifiers, requires special attention to the level of power consumption. The energy supplied onboard a space vehicle being limited, it is crucial to design RF amplifiers that are highly energy efficient. Moreover, the majority of modulation techniques used for satellite communication, such as QPSK, have a nearly constant envelope since this allows the amplifiers to operate near saturation where the efficiency is maximized. With a Doherty amplifier, amplitude modulated signals, such as QAM, can be used while keeping an adequate efficiency, which results in a higher transfer rate, for the same power consumption.; Multi-Chip Module (MCM) GaAs heterojunction bipolar transistors (HBT) technology is used for the development project of a high efficiency S band power amplifier in a Doherty configuration. This thesis states the objectives as well as the methodology of the development of the work, which is part of the Quicksat microsatellite project of the Canadian Space Agency (CSA). The Quicksat microsatellite could include onboard a Doherty amplifier, developed from the results of this work, for payloads that would require a data rate of about 1,5 Mbps. To better evaluate the performance of the Doherty amplifier, the design, manufacturing and qualification of two amplifiers in S band were implemented. The first amplifier uses a parallel standard configuration while the second is in a Doherty configuration. A detailed analysis of the theory of the high efficiency amplifiers is presented, as well as the detailed design steps, which comply with the rules of aerospace design.; For the characterization of all the circuits and the amplifiers, a complete test bench was set up and automated using the Labview(TM) software. This test bench makes it possible to measure the complete characteristics of the amplifiers, under various conditions of bias and temperature. The nonlinear characterization of the amplifiers is also possible with the option of two tones and gaussian noise input signals. Moreover, an automated analysis tool was developed to generate the graphs of the measured results. This tool also includes an interface to generate three-dimensional AM/AM and AM/PM curves that are very useful and provides a valuable aid for circuit tuning.; The final results show that it is possible to increase the global efficiency of an RF amplifier with a Doherty topology, while maintaining adequate linearity, if proper attention is given to the bias circuits. An important portion of the report is dedicated to the modular approach of the design. This modular approach makes it possible to characterize each circuit of the amplifier independently, in order to optimize the final performance. The amplifiers manufactured during the framework of this project are thus of the prototype level and the results will be used as part of a CSA internal research project, for the development of highly linear Doherty power amplifier, for the next generations of microsatellites. |
