Choosing a switchmode MOSFET that will amplify RF power isn't as easy as it sounds. A transistor that the manufacturer designed to do the job of RF power amplification will list things like power gain, bandwidth and RF output at different frequencies. The transistor's specifications are the launch pad for choosing component values in the rest of the amplifier circuits.
Just one problem. Both of these transistors were designed, sold and characterized to use as on/off switches. Period. The published specs say absolutely nothing about RF input/output impedances, bandwidth, or RF wattage. Nada.
I'm still learning to read between the lines, so to speak, and make sense of the numbers they do publish for these parts.
Just comparing published specs for the IRF520 and 24N50 should reveal the differences between them, if nothing else.
The spec for switching speed is all I know to use to predict how high the max frequency a MOSFET can amplify.
The specs "rise time" and "fall time" will provide some guide to how fast it can respond to RF drive power. For the IRF520, both those numbers list at 70 nanoseconds. Same as 0.07 microseconds. A 1 MHz signal has a period of 1 uS. A period of 70 nanoseconds corresponds to a frequency of abut 14 MHz. Clearly the IRF520 will function at about twice this frequency. Not a terribly scientific guideline, but there it is.
The 24N50 is a bit different. Rise time is shown as 170 nS, fall time is 320 nS. Corresponds to just under 6 MHz for the first number, and 3 MHz for fall time. Leads me to suspect that the power gain will be quite low. Makes it sound as if it will run out of steam somewhere between 6 and 12 MHz.
The rule you'll see with these switchmode MOSFET transistor like those two is that the higher the power ratings, the slower it will be. One hint can be seen in the fleabay sales listings for linears built with this kind of part. Some are listed as 7 MHz max, at least one I saw said 14 MHz.
There is one thing working in your favor in the long run. Higher and higher switching frequencies make switchmode power supplies a bit more efficient, and reduces the size of circuit components in a big way. This has created an incentive to make newer switchmode MOSFETs as fast as they can. And if what you want to do is amplify RF, faster is better.
I have not experimented with newer switchmode MOSFET transistors as RF amplifiers. The IRF parts are 30 or 40 year-old designs. The latest new twist is to make the transistor from silicon carbide. Maybe those will stand up to high SWR a little better. Switchmode MOSFETs are famously sensitive to high SWR in RF amplifier service. They tend to poof more quickly than the old-school bipolar RF transistors would. One mobile radio that has eight IRF520-type transistors in the built-in linear we nicknamed the firestarter.
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