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A unique GaN power amplifier (PA) with an integrated evanescent-mode resonator as its output matching network is presented in this paper. Instead of matching the output of the GaN transistor to 50 Ω and connecting it to a 50-Ω bandpass filter, the GaN is directly integrated with a one-pole filter thus eliminating the conventionally-employed output matching network after the GaN transistor. This leads to a reduced circuit complexity and higher efficiency. The one-pole filter implemented by a strongly-coupled evanescent-mode cavity resonator with an unloaded quality factor of 320 is experimentally presented as a proof-of-concept demonstration. This design yields a very narrowband PA response from 1.24-1.275 GHz (2.8%), which is comparable to the bandwidth of typical communication signals. The PA exhibits a state-of-the-art performance of >; 70% efficiency, >; 10 dB gain, >; 30 dBm output power, and second and third harmonic levels of <; -70 dBc. The presented methodology has the potential to be further extended to a tunable design for multi-band applications.