High efficiency RF power amplifiers are increasingly needed in modern mobile communication systems to reduce the battery size and power supply consumption. Class-F RF power amplifiers offer improved efficiency over the conventional class-B power amplifiers by properly controlling the harmonic components of the voltage and current signals at the output terminals of the RF device while driving it to operate as an ON/OFF switch. To do this task, a suitable load network is to be synthesized in order to present the proper harmonic impedances at the output of the RF power transistor. In this paper, a new load network for class F power amplifiers has been introduced and derived analytically. The proposed network consists of a parallel short circuited λ/8 stub, parallel open circuited λ/8 stub, and a T-section lumped-elementtransformer. The benefits of this topology include simplicity of design, controllable bandwidth, and harmonic tuning and impedance transformation at the same time.
To confirm the approach of analysis, a 10 W class-F UHF power amplifier circuit has been designed and simulated using a typical Gallium Nitride high electron mobility RF transistor (GaN HEMT) to operate at 500 MHz with the aid of the Advanced Design System (ADS)computer package. The simulated results haveindicated that the circuit gives adc-to-RF efficiency of more than 84 % and a power gain of 11 dB at 500 MHz with an operating bandwidth from 440 to 540 MHz.