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The material properties of GaN and the AlGaN/GaN heterostructure such as high breakdown field and high sheet charge density, allow AlGaN/GaN HEMTs to be operated at significantly higher drain bias voltages as compared to other III-V compound semiconductor FETs. As expected, larger RF voltage and current swings result in higher normalized output power at microwave frequencies. AlGaN/GaN HEMTs are capable of generating output power density in excess of 10 W/mm in the X-band, which is at least an order of magnitude larger than what is obtainable with GaAs FETs. In this paper, we discuss the effect of the load impedance on measured output power (Pout) and efficiency (η) at various drain bias conditions in class B mode. Dynamic loadlines; extracted at the device's output are used for analysis of the trade-off between voltage and current swings at different load resistances and its effect on output power and efficiency.