Instantaneous overdrive during transmitter turn-on takes an ultimate toll on the amplifier and power supply

     While most of us are aware that overdriving an RF amplifier is probably not a good thing, there is a subtle overdrive problem that is quite commonly encountered. Many transmitters use a feedback loop method to control RF output power. This feedback loop often controls the gain of one or more of the transmitter stages in order to limit the output power (see figure 5). In the "off" state, there is no RF output detected and  the stage gains are maximized. During the turn-on sequence, RF is applied to these gain-maximized stages and instantaneous high power (limited by the maximum capability of the output stage) results regardless of the power control setting. The feedback sensor senses this high power and sends a signal back to previous stages to reduce the gain and output power. Since this process takes finite time, the resulting RF applied to the input of the RF power amplifier is similar to what is shown in figure 6.

Figure 5: Feedback loop power control

Power Supply overshoots during turn-off can overvoltage the RF amplifier

     Most power supplies, when going from a full load to no load conditions will exhibit a voltage overshoot (figure 5). Ferro-resonant designs typically exhibit larger overshoots  of  longer durations than regulated supplies but even well designed regulated power supplies will overshoot with such a wild (100%) variation in load. If a transmitter unkeys and then rekeys 10-50 mS later, the amplifier may turn on at an elevated voltage. While this should not be a problem by itself (provided the overshoot is not too large!) but the RF transistors are much more fragile and susceptible to SWR and thermal damages when subjected to higher than rated voltages
     Another thing to watch is how a power supply will fail. Typical series pass regulated power supplies often fail with the pass transistor shorted. This allows unregulated (and much higher) voltage to be passed along to the output.