An amplifier package may include a transistor, an output impedance matching circuit and one or more radial stub harmonic traps coupled to a control terminal of the transistor or to an output terminal of the transistor. The output impedance matching circuit and the radial stub harmonic traps may be formed on a single substrate or separate substrates, which may be formed from gallium nitride. Each radial stub harmonic trap may provide a low resistance path to ground for signal energy above a fundamental operating frequency of the amplifier, such as harmonic frequencies thereof.