Re: LTSpice simulation of uBitx Power Amp

Jerry Gaffke


I'm not assuming anything about drain loads, only that the load on the output of the transformer should be 50 ohms.
The LPF's on the uBitx and WA2EBY are symetrical, 50 ohms in and out, no impedance transformation there (at least for the fundamental).
Both amps are operating the IRF510's from a 24v supply, and the two drains of each amp
are bouncing about centered on 24vdc, out of phase.

The output should look very similar after we add in a low pass filter, 
though any harmonics will be knocked out to leave behind a clean sine wave.

On Sun, Feb 4, 2018 at 10:53 pm, John Backo wrote:
However, note that the output is NOT 50 ohms until AFTER the LPF.
The input to the LPF is somewhat lower. And the WA2EBY has a voltage advantage
over the 12V input to the drains, though the output of the drains is closer to 50 ohms.

So it is a bit of a misnomer to assume that the drain load is 50 ohms. This is true, incidentally,
of any AB linear amplifier (and is probably one of the sources of oscillation). I should think that the
true drain load in this configuration is closer to 10 ohms than 50 ohms. The LPF transforms it
to 50 ohms for the antenna.

But it is not unreasonable to assume an overall load of 50 ohms. But be aware though, that one of the functions of
the LPF is to produce this. It is not quite true at the drains. The actual impedance at the drains is a function
of the first capacitor/inductance stage of the LPF. Things are not quite as simple as they seem...

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