Design of j class power amplifier for wireless communication applications

Scientist are currently working on modern wireless communication systems to achieve maximum spectral efficiency using complex modulation techniques. Such systems require very high linearity power amplifiers to deliver the signal with minimal phase and amplitude distortion. In addition, the power amplifiers must be able to provide high power output to reduce DC power consumption, thereby extending the battery life and at the same time meeting heat dissipation requirements for cooling. In addition, multichannel communication systems such as software defined radios and cognitive radios tend to use multiple standards and multiple frequency bands such as WiMAX and LTE. The design of these systems is desired to be realized on a single hardware platform. These platforms require broadband power amplifiers that will reduce the need for multiple amplifiers within such systems, thereby speeding up development costs and implementation cycle. Until recently, in the power amplifier design for wireless communication systems, it focused on certain low RF bandwidths due to very narrow spectrum allocations. Future emerging technologies like WiMax and 4G, will require larger bandwidths not only because of the need for greater frequency spectrum allocation, but also because of the need to work with signals with baseband bandwidths of 100 MHz and beyond. Until now, other RF power amplifier applications, such as radar, have not benefited more from the work on power and efficiency due to the high RF bandwidth requirements. The results for very high efficiency power amplifiers, typically operating at over 75% efficiency, indicate sensitive multi-harmonic impedance terminations and very high gain-suppression levels at the same time. These factors give rise to narrowband frequency performance limitation and nonlinear operation, respectively. As a result, high linearity, efficiency and wide bandwidth are the main important three factors for RF and microwave power amplifiers used in modern communication systems. Among current RF transistor technologies, gallium nitride (GaN) semiconductor power transistors are superior semiconductor technologies for modern power amplifier applications because they can provide higher output power and efficiency than other existing technologies such as GaAs and Si. Given the above considerations, in this dissertation design, a GaN transistor was chosen to achieve high efficiency and output power with a single transistor. Until now, many topologies have been proposed for high efficiency power amplifier operations. Among these topologies, a good efficiency is obtained by controlling single-degree harmonic impedances in order to generate a rectangular voltage waveform at the output of the class F operation. However, to obtain the appropriate third order harmonic voltage, the output capacitor (Cout) must be set properly. Also, depending on the capacitance and operating frequency in some cases, the impedance of the capacitor may be shorted to the third harmonic frequency. In another class of high-efficiency operation E, high efficiency is achieved by acting as a ideal switch of the transistor. Neverthless, the actual ideal switching operation of the power transistor may vary depending on the operating frequency and the structural characteristics of the power transistor. Therefore, the efficiency drops due to the overlapping current and voltage swingings caused by the transition period during switching. These limitations diminish the efficiency of class E at high frequencies. As a result, despite the fact that over 70% efficiency is achieved in the F and E class operation, the performance of these power amplifiers depends on at very specific impedance values which forces them to operate at limited bandwidth. Therefore, the linear operation of class F and E is very weak. A demanding for exhibiting high efficiency and linear behavior in a wide bandwidth causes trade off in classical power amplifiers design. This problem have been tried to solve with J class power amplifiers that have been working on for the last few years. With this new approach, the J class, the theoretical side of the RF power amplifier design, which does not require a harmonic short circuit that exhibits the same efficiency and linear behavior of the AB class work, is shown. In the class J study, the phase shift between the output current and voltage waveforms is utilized to transform the second harmonic termination into a fully reactive regime. By this means, a significant contribution is made to the bandwidth-efficiency performance of the J-class designed using GaN (HEMT) power transistors. The linear J-class power amplifier, terminated with appropriate fundemental and second harmonic impedances, offers a promising solution for linearity, efficiency and bandwidth requirements in a single power amplifier design. A broadband J-class power amplifier design incorporating the advantages of a GaN transistor and J-class operation and taking into account the effect of external parasitic elements in a packed transistor is the focal point of this dissertation.