Guys,

Thanks for all of the insight - I appreciate the help.

Bruce Raymond/ND8I



"Roy Lewallen" wrote in message
...
The equation appears in Terman's _Radio Engineering_, where it's derived
with what looks like a bit of hand-waving. However, a paper is
referenced (Terman and Roake, "Calculation and Design of Class C
Amplifiers", Proc. I.R.E., vol. 24, April, 1936) which apparently has
more of an in-depth derivation. However, it appears to be depend
somewhat on characteristics unique to vacuum tubes. I recall having a
homework question in college that looked like it required a derivation
of the formula, and I struggled for several hours before giving up. The
professor told me that it was a somewhat emperical formula which
couldn't be rigorously derived.

Anyway, in Terman there's a graph (fig. 7-29 in the Third Ed.) which
does include the effect of conduction angle, but it's not immediately
obvious how the formula would be affected.

The formula commonly seen isn't generally used to predict power output,
but rather solved for R to give a nominal impedance that a class C
amplifier with output power Po would like to see. It's a good starting
point for designing output networks, but by no means universal.
High-efficiency amplifiers I've designed do require a different
impedance than predicted by the formula. For example, see Fig. 2.97 in
_Experimental Methods in RF Design_. The impedance seen by the collector
of that amplifier is about 18.7 + j8.5 ohms(*), while the formula would
predict about 9 ohms. There are more comments about that in the book.

I'm not aware of any rigorous formula that can be used to precisely
predict the optimum load impedance for a class C amplifier.

(*) I don't recall right off how much C the zener adds, but think it was
around 30 pF. So that's what I used for the calculation. Note that the L
values in the figure caption should be uH, not H and mH as shown.

Roy Lewallen, W7EL


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