Electromagnetic Stop-Band Network Improves Class F Amplifier Performance

In a class F power amplifier, producing the required short circuit at even harmonics and the open circuit at the odd harmonics at the active device output can substantially increase efficiency of the power amplifier [1, 2]. With such harmonic tuning, the voltage across the active device output and current through it do not contain harmonics of the same order simultaneously. In practice, it is very difficult to control impedances for an infinite number of harmonics, and the so-called ThirdHarmonic Peaking tuning is typically used. In this case, the input impedance of the output network is controlled up to third harmonic. A noticeable increase in efficiency can be achieved if a higher harmonics are also taken into account [3]. Recently, photonic bandgap (PBG) and defected-ground microstrip structures have been proposed as a novel way to accomplish filtering that provides a broad rejection band [4-11]. Such structures can be successfully used as the output networks of high-efficiency power amplifiers. As was suggested in [12], the terminology “electromagnetic stop-band” (ESB) is probably more appropriate than “photonic bandgap.” ESB seems to be more commonly used for microwaves, so it is used in this paper. To achieve high amplifier efficiency, an output network should also provide acceptable matching at the fundamental frequency. So, insertion loss in the pass band should be as small as possible. The weakness of the mentioned PBG (or ESB) structures, when used as amplifier output networks, is their imperfect characteristics in the pass band. This article presents an approach to improve the matching within the pass band.