On Mon, 28 Feb 2005 06:28:24 GMT, "George, W5YR"
wrote:

IF the Thevenin source approach worked, we would have to be content with max
50% efficient amplifiers. We know we can do better than that substantially.

Hi George,

I know you won't appreciate this, but it is a telling, simple test of
a practical situation with a practical Amateur grade transistor model
100W transmitter commonly available for more than 20-30 years now:
1. Presuming CW mode into a "matched load" (any definition will do);
2. Report the DC power consumed before hitting the key;
3. Report the DC power consumed while holding the key.

Concurrently note:
A. Report Heat Sink Temperature for a previously idle/rcv condition;
B. Report Heat Sink Temperature after 10 minute key-down.

For a hypothetical "100W" model (again, a contemporary, common example
for Amateur use) available through standard commercial venues:
2. About 20W - 30W
3. About 200W - 250W
A. About 20 degrees C (or room temperature)
B. Well above 37 degrees C (or skin temperature)

Now, if we are to be any judge of efficiency (Thevenin does not have
to be invoked, condemned, or venerated); then it runs close to 50%
(±10%). Others can invoke their favorite deity to explain.

Now, if we are to be any judge of dissipation (no requirement for
advanced degree); then heat as a loss by virtue of less than 100%
efficiency is quite evident. Others can invoke photons to describe
why.

To forestall any armchair engineers, yes, this efficiency is System
efficiency. However, I would be surprised if a practical common
Amateur grade transistor model transmitter commonly available for more
than 20-30 years now has any configuration that does not apply supply
voltage directly to the final transistors; and instead adds a
significant current path outside of this load (citations to available
schematics would be compelling, but any argument without this would be
speculation). It takes very little effort to subtract out the power
drain of the receive mode (being very representative of the similar
power demand of supporting circuitry for transmit up to the driver
stage). Barring such amazing evidence of a significant power drain
not found in the finals, it follows that a simple computation of
efficiency has its merit and has been met.

73's
Richard Clark, KB7QHC