I have taken the liberty of crossposting this to alt.ham-radio.hf,
rec.radio.amateur.homebrew, and alt.ham-radio.vhf-uhf
If it has to be quick and easy you could look at ham radio solutions.
There are plenty of transistor amps described in books such as the
ARRL Handbook that do 3MHz-30MHz at 100W or more. They may need a
little mod for 40MHz or will operate sluggishly there but should be
able to produce 40W.
This seemed a good idea, and I have an 18" bookshelf of ham books,
handbooks, and article compendiums. But looking through them I
was unable to find much of use - I'm not interested in using tubes.
The power amp chapter in the 2002 ARRL Handbook (unchanged in this
respect from the 1998 issue) suggested the MRF464 bipolar RF power
transistor for a 30MHz 80W amplifier, but this is an obsolete part,
no longer available at the distributors I checked. They also don't
present a detailed design to go with this suggestion.
It might even be possible at less cost to modify one of those 'not
so legal' CB booster amps.
Surplus VHF TV driver modules are another option.
Interesting, where does one get these?
If it's just a tuned circuit you want the best bet may be to take a
large enough FET, a tuned circuit at the drain and drive it hard
from a digital oscillator, buffer and resonant step-up transformer.
Checking the MRF464 reminded me to look at Motorola's other newer RF
MOSFET power transistors. They still offer some high-frequency parts,
but most of their RF power MOSFET line was sold to M/A-COM. The data
sheets and app notes are unchanged from the Motorola versions.
In the case of an RF MOSFET, where the device is "on" for only a small
part of each cycle, say 30 degrees (which is 2ns at 40MHz), it's not
possible to "hard drive" the FET and turn it on and off, in the sense
we are used to. Instead the gate is presented with a sine wave wave
from a tuned matching stage (the gate's high capacitance looks like a
low RF impedance), and it's DC biased to be on for a small time at the
tip of each cycle. Of course it's not going on and off, instead its
channel conductance is just increasing and decreasing for a few ns.
These RF FETs are not made with the VMOS groove construction we're used
to in common power MOSFETs. With high capacitance and poor high thermal
resistance, not to mention high gate spreading resistance, ordinary power
MOSFETs are badly suited for applications other than on/off switching.
Instead the RF types are lateral FETs, sort of large versions of the old
small enhancement-mode FETs such as the 2n4351 we talked about in AoE.
This means that despite the RF power MOSFET's large die area, it has a
comparatively low-capacitance, which is well suited for RF work.
The M/A-COM parts are stocked by Richardson, and three of them look
interesting for my inventory. Does anyone recognize any of these FETs?
. type Vdss Pd Ciss Crss cost ea
------- ---- ---- ---- ---- -------
MRF134 65V 18W 7pF 4.5pF $21.42
MRF171A 65 115 80 8 (at 28V) 37.40
MRF148A 120 115 50 8 (at 50V) 35.70
By comparison, a modern VMOS type power MOSFET rated at 100V and 127W
has Ciss = 1150pF and Crss = 62pF (at 25V), which is about 10x higher.
Of course, it's able to handle much higher DC currents when fully on.