Could someone explain to me if there is a rule of thumb to determine the peak pulsed power of an (linear class a/b) RF amplifier if the peak cw power is known. For instance, if the amp is rated 300W (max power) in cw mode, the max power in pulsed mode is definitely higher, and surely depends on the pulse width (e.g. could be 1 kW for a 10us pulse). Is there a rough rule of thumb to figure this out without having to test it directly and risk blowing up the amp? The amp design is based on the RF power mosfet MRF151G (the tech specs do not say anything about pulsed mode ratings). thanks.
On a sunny day (21 Feb 2007 11:19:12 -0800) it happened "Pat" wrote in :
This is not so easy to answer. In all cases if you do not boost supply voltage then you are still limited by the max current the FET can do. If you stay in range of that you are limited by _thermal_ issues. In on transmitter I have, the amp is 30W CW and about 150W pep (SSB) with 2 junction transistors in push pull. The issues are also the losses in your coils etc.... I am sure I could peak it a bit more.... but things would start frying. Factor 5 should be possible, (SSB always assumed average of speech is 20% of max power, so if I talk up to 150W SSB I use about 30W average. It is _thermal_, watch the max temperature.
No, there is no rule of thumb. It depends on the design of the amplifier. Some are designed specifically for high peak pulsed power, where the ratio of peak to average may be quite high, and some are designed to run with a continuous unvarying output. I'd say it's common for a linear amplifier designed for AM service to be able to handle peaks at least twice the voltage (= 4 times the instantaneous power) of the carrier, but that's just one example. Amplifiers designed for a high peak to average power ratio are, I'd say, more commonly not linear, however.
You can look at the peak collector/drain current and collector/drain to emitter/source voltage ratings of the device; you can figure power dissipations; you can figure the peak output voltage for a given load...all those will go into finding the value you seek, but it also comes down to what you expect for MTTF, too. Eventually a stressed amplifier will fail--sooner rather than later as you increase the stress. The stresses include voltage, power, thermal cycling, i^2*t fusing currents, ... The power supply capabilities enter into it too, of course: the supply may be capable of delivering a high current for short periods, but a much lower average current--or it may not.