Gary,
With no load at the antenna port, the simulation shows an IRF510 drain current that stays near the quiescent 100ma
with perhaps 20ma pk-pk of RF riding on top of it.? (An initial spike that I'm ignoring here, as we would be increasing?
drive at audio rates.)? ? The drain current never goes toward zero, even when Q13 is driven with 400mv pk-pk.
Gate voltage is quite distorted, but does get down to 1.2v which should be enough to shut down the IRF510.
I'd guess that as the IRF510 shuts down the drain voltage rises, the gate-drain capacitance causes the gate voltage to rise,
and so the IRF510 continues to conduct.? ? ?When operating more slowly the IRF510 does indeed shut down as Vgs rises.
Jerry
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On Wed, Jan 31, 2018 at 09:40 am, Jerry Gaffke wrote:
One major conundrum:?
When the Bitx40 IRF510 turns off and the magnetic field collapses that energy needs some place to go,
the induced emf in T7 will create voltage spikes that rise as high as it needed till it finds a load,?
perhaps arcing across the transformer.? But my LTSpice simulation of the Bitx40 final with no load
shows no such spike, the output voltage remains pretty much determined by the turns ratio
as if T7 were a straight up forward transformer.
