【正文】 kers one and four are at177。33 MHz distance from the fundamental frequency. The input power in this case is15 dBm and fundamental load impedance is at one of the found oscillation impedances(Г=,216。=).The increase of chip decoupling capacitors (4470 pF) in the case of amplifier A did lower the resonance from around100 MHz into 30 MHz region. Inside the177。30 MHz band there are also other oscillation tones which resemble a quasiperiodic solution and a chaos spectrum of quasiperiodic and chaos spectrums are shown for example in [11]. An interesting point of the resonance seen was that the large electrolytic capacitors applied to the bias supply did not attenuate the about 30 MHz resonance,due to its high inductance. Additional findingsSwitching amplifiers are dependent on sufficient amount of gate drive, and variations in the drive signal affect quickly the performance of the amplifier. In our case small variations in transistor pinchoff voltage resulted in 2–3 dB differences in gain between the amplifiers when bias voltage was kept constant. Such clear differences could beseen in vector network analyser measurements with a 0 dBm input drive. More constant results could be derived by adjusting the small drain bias currents to equal when no drive signal was applied. Now the measurements of gain were matched within dB. 6 Summary A tuned RF power amplifier has been designed for operation in a frequency band of – GHz. The amplifier was designed empirically to have nonoverlapping drain voltage and current pulses, and the required resonant circuit was implemented onchip. A new position of the DCblocking capacitor genereted a resonator trap that stabilizes the amplifier below the desired band, and a gate RCsink circuit was used to stabilize the amplifier at higher frequencies. Further, equal length input lines were implemented to equalize the timing of gate signals. The stability of the amplifier was evaluated through simulations of large signal Sparameters and stability circles. Additional resistances together with an RCsink circuit had to be applied to keep the stable output SWR range at more than :1. The amplifier was implemented onto a GaAs substrate with depletion mode high electron mobility transistors (FETs). The implemented amplifier delivers 2 W of output power while maintaining 56% drain efficiency. The frequency response is within dB at a frequency band of – MHz while the drain efficiency stays above 53% in this desired band. The amplifier was also measured with a load pull system at a spot frequency of GHz. With the help of tuners the amplifier achieved about W of output power and 62%efficiency. Load pull also revealed the amplifier’s sensitivity to oscillations at small drive levels. It was found out that the output matching network has a lowfrequency resonance that might contribute to the unstable implemented gate sink was verified to stabilize the circuit. It might be a beneficial idea to design an input LCtrap circuit tuned to the second harmonic, since there is second harmonic content at the input which widens the input waveform in the time domain, creating problems in terms of efficiency and power. Further, increasing the third harmonic content could make the input waveform more squarelike which is a desired feature in switching amplifiers.Acknowledgements： This work has been supported by The Academy of Finland, Infotech Oulu Graduate School, TriQuint Semiconductor Inc., Nokia Foundation, Tauno Tonning Foundation,Ulla Tuominen Foundation and The foundation of Riitta and Jorma J. Takanen. 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IEEE Microwave Magazine,7(5), 51–65. doi:Simo Hietakangaswas born in Alaha168。rma 168。, Finland, in 1980. He received the . degree in Electrical Engineering from the University of Oulu, Oulu, Finland, in 2005, and is currently working toward the at the University of Oulu. His technical interests lie in the field of analysis and modeling of switching RF power amplifiers.J