blackplasma wrote:I know it sounds high, but is package inductance really so much of an issue at 30MHz?
Sure, the TO-247 packages are tricky... but what about resonant-circuit drive, where one usually adds a wee bit of inductance anyway?
The highest I've worked at (so far) is 17MHz, and I very rarely use low-impedance squarewave drive into the FET -- I don't have the drivers, and stuff gets too hot, so I generally make drivers from discrete components, swamp the gate capacitance slightly with a ceramic cap to make it a little more predictable, and resonate it with a ferrite transformer. =P
What kind of power are you aiming for? If it's <~150W you could probably pull it off with a few IRF510s or IRF520s...?
The IRF 510 - 100V, 4A device. So, on AM you could run - with a not too big safety factor about 10 volts unmodulated DC. Figuring on 25 volts for 150% positive modulation - then 4x the DC for the peak of the class E waveform - that's 100 volts with NO SAFETY FACTOR. I usually recommend a 100% safety factor - but let's go 50% for the sake of the discussion. So, that's about 7 volts - times 10 for the peak at 150% modulation of 70 volts - not a great safety factor, but better than none!
Ok, let's take the current. The repetitive peak current is about 2x to 3x the DC current (depending on the gate waveform). It's lower for square wave drive. So, let's be generous and say 2x the DC. So, that's 2 amperes max current at the crest of the modulation waveform. Divide 2 by 2.5 (the peak voltage and current relative to the carrier) and we get about .8 amperes at carrier. 7 volts at .8 amperes = 5.6 watts per device ***** FOR AM SERVICE *****. It would be a LOT higher for CW / FM - at least 6 times the power or about 40 watts.
The input capacitance is about 200pF (250 max), so this is practical at 10 meters or even above - much more so than the 3500pF of the FQA11N90. These devices have a gate self-resonance of around 20mHz. The IRF510 will probably have a self-resonance of over 100mHz.
So, small die (like the 510) = low capacitance = low power.
That's essentially the design of my 10 meter rig. A bunch of devices that will run about 10 to 15 watts, put together to get about 100 watts.
Good discussion!
Regards,
Steve