Disclosed is microwave oscillator or amplifier circuitry which is tunable over a wide bandwidth and which includes an improved variable loading on a negative resistance semiconductor diode, such as an IMPATT diode. This loading is provided in mutually perpendicular directions closely adjacent the diode. In one direction, a DC bias connection is mounted together with an adjustable coaxial filter thereon, and this filter serves as a series tuning element for the diode. A movable short is mounted for adjustment in a direction perpendicular to that of the above coaxial filter and provides parallel tuning for the diode. These series and parallel tuning elements are operative to match the real and imaginary components of the diode impedance to the real and imaginary components of the load impedance, respectively, at a particular operating frequency, thereby maximizing the power transfer and the DC to RF conversion efficiency of the circuitry. For ease of frequency adjustment, all of the frequency dependent parameters of the circuitry are contained in a removable and replaceable insert which fits securely into the waveguide housing.