A study on multi-octave GaN power amplifier using reactively matched gain cell

Abstract

In this thesis, a study on two-stage reactively matched gain cells are proposed to implement a high-gain multi-octave distributed power amplifier (DPA). The analytic analysis of proposed high-gain distributed amplifier (DA) is presented and supported with simulation and measurement results. Also, a shared bias network using simple microstrip line is introduced. The bias network not only enables the use of high-gain structures in DA configuration, especially in monolithic microwave integrated circuit (MMIC) with compound semiconductor process, but also has advantage on layout efficiency. To further enhance the RF performances and circuit reliability, the layout technique with reduced thermal coupling is applied. Finally, the high power amplifier module implemented with four MMIC dies, thanks to its high efficiency and low junction temperature.<br/> The DA analysis starts off with lossy m-derived configuration in artificial transmission lines, considering the effects of the gate coupled series capacitor used in DPA. After that, reactively matched cell, the proposed high-gain structure, is introduced and analyzed using Thévenin equivalent. The gain of two DPAs, conventional cascaded DPA and proposed reactively matched distributed amplifier (RMDA) is then compared to derive the design criteria of the reactively matched cell. The simulated results are presented to show the potential advantage on achieving high gain performance.<br/> The biasing of transistors is one of the common difficulties in MMIC design with compound semiconductor process. The standard compound semiconductor process only consists two metal layers, making the interconnection of the bias network to be difficult. Therefore, the most of the gain enhancement techniques for DA are implemented using CMOS process. The shared bias network is proposed to overcome the layout limitation. Simple microstrip lines are used for the proposed bias network, which behave similar to the ideal bias network: short at dc, open at radio frequency (RF). Precise analysis with active load modulation is done to design the shared bias network. The shared bias network is verified with the RMDA structure, and could be employed for other topologies.<br/> The electro-thermal effect of GaN high power amplifier is also studied. Power amplifiers (PAs) operating in a wide bandwidth have limited efficiency and GaN power amplifiers dissipate large amount of dc power. Therefore, the electro-thermal effect of a multi-octave GaN power amplifier should be considered for circuit performance and stability. Thermal coupling reducing approach and heat spreading approach are presented and verified by measured results.<br/> The implemented RMDA with the compact transistor layout has been implemented in a small die size of 10.7 mm2 and shows output powers reaching 40.3-43.9 dBm, power added efficiencies (PAEs) of 16–27% and small signal gains of 15.3–23.2 dB. The RMDA with the reduced thermal coupling achieves 40.6–43.4 dBm with a peak PAE of 29% in a slightly larger die size of 13.8 mm2.