(still me, N1JPR/I2 Filippo, different account...)


Thank you folks, lots of food for thought!

An update:

- I found 3 2SC3502 video finals in the PCB next to the tip of a CRT
in a PC VDU: fT 150MHz, but low max Ic 0.1A and about 1 watt
dissipation. Might be OK as oscillator or buffer, but I'm not sure it
would make a good driver. I'll try it.

- Decided to try a seemingly unsuitable device, a 2SK2545 low-
frequency power switching MOSFET from some industrial automation
contraption, with a humongous 1300pF input capacitance.

I drove the MOSFET DC coupled (!) at 0V DC with a common-collector
BD135 driver in order to swamp the base with current, and I am getting
out over 2W at 10MHz at about 12 ohm measured impedance, in class C,
with 12V supply.

Sounds cool considering that this MOSFET can switch 600V, handle 6A
and dissipate 40W. Actually, the MOSFET indeed STAYS rather cool,
while the dummy load of 3x 4 ohm 1 watt resistors get quite hot.


The common-collector driver + common-source final combination is quite
promising. My next challenges are

1) Making sure that no AF,LF, VHF spurs ('cept "clean" harmonics) and
clicks are generated across a range of voltages and frequencies. Now I
do get strange vibes at the lower DC supply voltages, probably via the
supply bus. Negative feedback seems to make a difference even in
class C.

2) Reduce driver current! For some strange reason I can't convince
drivers to run in class C, neither as common drain nor as common
source: even if I raise emitter voltage above base, gain drops (and
the MOSFET final goes to sleep), but the driver output still seems
undistorted and the driver dissipates like a stove, more so than the
final... more later when I get some proper data.


To those that advise putting tank circuits between stages: go see the
many 2- or 3-transistor QRP transmitters you find in magazines, kits,
etc.: putting all the filtering after the C-class final is very
common. If the VFO or VXO puts out just fundamental + harmonics, then
a LPF is all that's needed.


It would be cool to stitch together a 2-transistor thingie that works
on 5-15 MHz, gives 2W RF at 12V and ???W when fed say 30Vdc - into a
higher impedance, of course.


Stay tuned, & keep reminding me to be sane. :-)