Dear Ken.
I can only guess that the "never in the same positiion twice" is an
indication that you have oscillation and not amplification.
I have used the L C L network with success on bipolar transistors but with
mixed results with FETs. You will get some very high voltages from
the inductor connected to the drain, especially if the design Q is too
high. Possibly this is killing the FET.
Can I suggest that a 1:4 broadband bifilar transformer would be a lot more
successful. I have built many of these for driving larger amplifiers
and also for driving the grids or cathodes of valve amps. They seem quite
reliable but would need a LPF after the transformer.
Regards
David G0MRF
Has anyone experienced problems
using an IRF530N in a 500kHz PA?. I am using an IRF530N into an L – C – L
output network the design parameters for the network are input:10 ohms,
output 50 ohms, centre frequency 502.5 kHz, Q = 12, giving, L1 38µH, C1
3.7nF, L2 84µH, C1 consists of a 3.3nF fixed capacitor plus a 2 gang
500pF variable, 3mH inductor in the drain. I have tried driving the MOSFET
with a 5v positive peak signal, and with a 3v rms signal sitting on a +ve
bias, I have managed to obtain 5w into a Marconi TF1152A/1 (50Ω) power meter
about three times, but when 12v is removed from the PA and reconnected there
is no output, C1 is adjusted for maximum power output, but is never the same
position twice, and there seems to be a very critical adjustment of the drive
to obtain, a) an output, b) anything that looks like a sine wave, although
when I have got it right there is a very good sine wave. Any ideas anyone as
to what may be going wrong?
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