steve_qix
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« Reply #1 on: December 26, 2012, 10:41:57 AM » 

In theory, any MOSFET can be used for AM. You have to derate the device as follows:
1) Take the maximum VDS, divide it in half for a safety factor. Example: with a 900 V MOSFET, this value is 450 V. With a 200V MOSFET, this is 100V.
2) Figure the worst case maximum peak repetitive drain voltage under steady state RF conditions. For instance, with class E, the worst case condition is 3.5X the applied DC. This gives the maximum applied modulated DC that you can use. Example: take the 450V from step 1, divide by 3.5  you get around 130V. Another Example: Take the 100V from step 1, divide by 3.5 = 28V. Important: If you think you can get away without doing step 1 (in other words, you think you can operate reliably without the derating from step 1), don't bother. You will have repeated failures because things happen, like T/R switching anomalies, antennas falling down, mistuned loads, etc.
3) Now figure the unmodulated DC. This will depend on what percentage of positive modulation you desire. If you only desire 100% positive (NOT a good idea), you can divide the maximum DC (from step 2) by 2. If you desire something like 180% positive (this leaves a lot of headroom), divide the voltage from step 2 by somewhat less than 3. Example 130 / 3 = 43.3 V. I use 45 volts DC at carrier. Other Example from step 2: 28.5 / 3 = 9.5V (use 10 V).
An easier shortcut, at least for class E is to take the V DS max, and divide by 20. This gives the carrier voltage, assuming almost 200% positive modulation and a reasonable safety factor.
As can be seen, low voltage MOSFETs are not a win for AM unless you want to run really low power.
