Stabilizing a power amplifier feeding a high Q resonant load

A grounded grid power amplifier for the booster cavity of the TRIUMF cyclotron, operating at 92 MHz is found to be very stable when operating into a 50 Ω load. However, when connected to a high Q cavity via a long transmission line it can become very unstable. Even though a phase shifter (trombone) enables one to adjust the resonant frequency of the cavity to be centered between two transmission line resonances, the amplifier tends to oscillate near the operating frequency due to insufficient isolation between input and output circuits. An expensive but easy solution to such a problem is to decouple the amplifier from the cavity and transmission line by employing a circulator. However, the solution that is presented in this paper uses external feedback from anode to cathode to neutralize the internal feedback inherently present in the tube. The neutralization is adjusted such that the isolation over the amplifier´s bandwidth is increased by at least 15 dB. This along with input and output resistive damping has further stabilized the amplifier, which now operates routinely with no parasitic oscillation. Results of computer simulation of the amplifier response are reported