6.7 Load Network with Transmission Lines

At ultra-high and microwave frequencies, generally all inductances in the load-network circuits of the power amplifier are normally replaced by the transmission lines to minimize power losses and effects of the parasitic capacitances. The load-network circuit can be composed with any types of transmission lines including open-circuit or short-circuit stubs to provide the required matching and harmonic-suppression conditions. In some cases, for instance, for compact small-size power-amplifier modules developed for handset wireless transmitters, it is advisable to use series microstrip lines and shunt chip capacitors. Using a lumped-distributed Class-E load network structure with short-length transmission lines can be very effective and helpful to increase the efficiency of a microwave power amplifier [24]. Generally, the transmission-line modeling (TLM) technique in the time domain can be used to simulate the Class-E power amplifier. By means of the TLM technique, the load network elements can be modeled by the open-circuited and short-circuited stubs [25]. However, the TLM technique cannot be used to evaluate the required optimum parameter values directly to obtain the desired performance and the optimum conditions cannot be ensured. Idealized calculation of the collector voltage and current waveforms in the frequency domain, required for an optimum Class-E mode, implies the availability of an infinite number of harmonics with their optimum amplitudes and phases in the output spectrum.

First, consider a contribution of any harmonic in close approximation to a parallel-circuit Class-E mode similar to that for a Class E with shunt capacitance. The Fourier-series expansion...