| Switched-mode power amplifiers derive high efficiency by operating the active circuit element(s) between cut-off and the low resistance, triode, region. Simple models have been successful in describing this switching action for frequencies on the order of several megahertz. However, at higher frequencies (≥200 MHz), more sophisticated models are needed to accurately predict the performance of switched-mode power amplifiers.; A detailed analysis of field effect transistor (FET) Class E power amplifiers is presented in this work, where, some of the basic assumptions used in many earlier analyses are scrutinized. First, the choice of using a fifty percent duty-cycle simplifies analysis, but it is not required to maintain a theoretical efficiency of 100 percent. So, in the first part of this work, the original analysis of the Class E power amplifier is re-formulated to clearly show that, by varying duty-cycle, it is possible to trade-off power for higher operating frequencies. Similarly, additional design flexibility can be gained, while maintaining unity theoretical efficiency, by relaxing the zero voltage slope (soft-switching) constraint. Next, the effect of a transistor's nonlinear output capacitance is re-examined, based on charge conservation. Unlike the results of earlier publications, this analysis is not only physically meaningful, but it is also valid for any capacitance profile. This physically based analysis is, therefore, more viable for practical design, and is generalized for any duty-cycle and voltage slope. The final contribution of this work is a novel implementation of the Class E power amplifier, using a finite DC feed inductor. The validity of the analyses in this work are verified with computer simulations and measured data from a 200 MHz Class E power amplifier prototype exhibiting a maximum efficiency over 78 percent and maximum PAE above 74 percent. |
