Paul W9AC Christensen:
"If the output stage is broad-band tuned and biased in Class A, it will absorb some
or all of the reflected wave which may increase heat losses. In this case, the reflected
wave is not re-reflected and will not show up as an increase of the forward reading."
Karl and all ...
Does anyone know of a computation that predicts the level of power absorption
in an amplifier (or classic generator) based on operating class and reflection
coefficient? This is an area of transmission line mechanics that is
universally treated as a footnote rather than a detailed explanation with
mathematics -- possibly because it's in a gray area at the intersection of
amplifier and transmission line theory.
Some of you may recall an ongoing debate on a related matter between Walt
Maxwell and Warren Bruene where each intensely defended their position as to
whether a conjugate match existed in a power amplifier when tuned for maximum
available output level. Class of amplification entered into that debate and
was acknowledged by both parties, as I recall. I am more interested in getting
to a number that shows power loss due only to reflected wave absorption in the
exact situation that Karl describes above.
Since for all practical purposes there is no conjugate match or
another kind of match between the output of any amplifier and 50 Ohms or
any other impedance that you wish to run up the flagpole there is no
measurable absorption of any "reflected power."
Transmitting amplifiers must only supply the necessary Voltage to
develop the required output power. (helped by impedance transforming
devices and circuits) The class or kind of amplifier is irrelevant.
On test benches the output of some test generators must have a 50 Ohm
low reactance resistor (sometimes within the instrument) to create a
controlled "source impedance," possibly because the circuit has more
than one source port.
Can you imagine a 240,000 Watt TV transmitter requiring a 50 or 75
Ohm resistor dissipating 240,000 Watts and a 480,000 Watt TV
transmitter? It does not work that way.
The maximum power transfer theorem has never applied to the generator.
--
Ron KA4INM - Youvan's corollary:
Every action results in unwanted side effects.
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