Protection of transistor RF power amplifiers

Although more reliable than vacuum tubes when properly applied, transistors are considerably more sensitive to abuse. When used in RF power amplifiers, they are likely to be subjected to conditions which might cause failure by second breakdown or over-dissipation. Protection against second breakdown is generally afforded by operation with supply voltages sufficiently below the breakdown level. The brute force approach to avoiding over-dissipation by limiting maximum input power to a level which can never cause over-dissipation is effective, but prevents realization of more than a small fraction of the transistor´s capabilities. The transistor can be operated at a higher power level and remain protected by using circuitry which measures the power output and device temperature and adjusts the power input limit accordingly. In effect, the circuitry measures the temperature, computes the maximum permissible device dissipation at that temperature, and adjusts the power input so that the difference between power input and measured power output does not exceed the computed maximum permissible device dissipation. The use of this type of protection in one case has resulted in safe operation at 200% more power output than would have been permissible with the brute force approach.