A plasma lamp with a positive-loop feedback topology, having a resonating waveguide body and at least one amplifier critically coupled to the body which is stable under all operating conditions both before a plasma is formed and after the plasma reaches steady state. An iterative method for configuring the lamp circuit includes determining the load trajectory of each amplifier under all operating conditions, and overlaying it on a polar-plot showing regions of stability, conditional stability, and instability. If the load trajectory passes through an unstable region, circuit alterations are made to avoid that region.