Abstract: Ultra-wideband power amplifier is the core，versatile and high value-added components in wideband communications，electronic countermeasures and radar systems.As the demand for communication systems continues to increase，the need for power amplifiers with wider bandwidth，higher efficiency，and more power is imminent.An ultra-wideband power amplifier is designed based on GaN（gallium nitride）high electron mobility transistor（HEMT）with operating frequency from 0.46GHz to 3.9GHz.Firstly，the input matching is optimized to achieve absolute stability and high gain in the full frequency band.Secondly，the high power and high efficiency output matching impedance space is extracted by load-pull design at different frequency points within the target frequency band，and then a four-stage Chebyshev impedance converter is applied as the output matching topology to achieve high efficiency and high output power in the wide frequency band.The test results show that within the operating band from 0.46GHz to 3.9GHz，the overall output power（P_out）is greater than 13.8W with the maximum output power of 24.1W；the full-band drain efficiency is greater than 50.1% with the maximum value of 67.2% at 1.5GHz.The measured results are in good agreement with the simulation results，and the design idea is intuitive and clear，which provides a direction for designing the currently required ultra-wideband power amplifier. 

Key words: ultra-wideband, power amplifier, GaN HEMT, Chebyshev converter structure, load-pull