[Elecraft] k2 spurious

[David Woolley]

Surely the binocular transformer is converting the input impedance of 
the filter to the optimum load line for the PA transistors.  Typically 
that would mean such that the the voltage swing was just short of the 
full supply rail for the worst case complex impedance, at the worst case 
acceptable SWR, for maximum power, and with that power.

Also that transformer provides negative feedback.

I'd expect the actual PA collector impedance to be a lot higher than the 
impedance reflected through the transformer, as transistors and FETs 
are, to a first approximation, current sources, although the negative 
feedback would actually make the reverse termination impedance much 
lower than that from the transformer.

A hint that is the case is the option for different winding ratio if you 
are always going to use a lower maximum power.  Maximising the voltage 
swing increases the efficiency.  The alternative ratio doesn't transform 
the filter input impedance down so much.

I mention this as there is a common misconception that the "maximum 
power transfer theorem" actually applies in such cases, when applying it 
would produce less than 50% efficiency.

-- 
David Woolley
Owner K2 06123


On 25/10/2020 18:17, Don Wilhelm wrote:
> 
> The binocular toroid is a transformer to match the impedance of the PA 
> collectors to 50 ohms, which is the input impedance of the Low Pass 
> Filters.