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Determination of "Resonant Frequency" for Current-Fed Power Amplifier Us-ing Multifrequency Modeling Technique
Fig. 1. Simulation waveforms of power amplifier operating at natural frequency and resonant frequency
The Time-Invariant Multifrequency (TIMF) Modeling technique is utilized to account for the DC and first harmonic of the power amplifier. The corresponding steady-state circuit model is also derived for easier comprehension and use (as shown in Fig. 2). Based on the circuit model, the sine and cosine coefficients of the currents and voltages can be determined. When switching at resonant frequency, tank voltage is purely sine wave (see Fig 1) and the cosine part of vCr(t) should be zero. Based on the modeling results, the resonant frequency is determined. Such frequency depends not only on the tank's inductor and capacitor, but also on the filter inductors. With sufficiently large Lf, the resonant frequency can be approximated by natural frequency. Whereas, with decreasing Lf, the deviation of the resonant frequency from natural frequency will increase. If still switching at natural frequency, the effective resonant tank is "inductive" and causes a large current spike.
Fig. 2. Steady-state circuit model of power amplifier
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