A lossless microwave (dielectric resonator (DR)) active filter is presented. Equivalent circuits for a dielectric resonator coupled to a microstrip line or to another dielectric resonator are developed from an electromagnetic analysis. With the overall electrical model of the passive and active filters described by a microwave CAD program, simulations and sensitivity analysis can be obtained. In the active filter circuit a feedback active loop (composed of a variable gain amplifier and a phase shifter) is coupled to the central DR of the passive filter to compensate for its losses. Measurements of temperature performance, power handling, third-order intermodulation product and noise level are included.