On Sat, 14 May 2011 11:30:50 -0700 (PDT), K7ITM wrote:

FWIW, I've built audio amplifiers with output transistors with f-sub-t
around 50MHz. Those transistors certainly would provide decent power
gain at the lower frequency HF ham bands. They were used in the audio
amp to achieve very low distortion across the audio spectrum (a very
few PPM at full rated power output).

Hi Tom,

An Answer to one of my questions (and by inference two of them)!

The inference is there is NO transition frequency where source Z
becomes trivial/meaningful. Hence, statements alleging correlations
at one end of the spectrum must be observed at the other end.

If this is disputed, we (or at least I) would certainly like to see
what that transition frequency is.

One big difference between (typical) audio amplifiers and (typical) RF
power amplifiers is in the use of negative feedback. In an audio
amplifier, voltage-derived negative feedback yields a very low
amplifier output impedance. In almost all RF power amplifiers, little
or no negative feedback is used, so the output impedance is generally
much higher than with an audio amplifier running similar power and
supply voltage to the output devices.

Well, at the RF finals deck, before the Z transformer, there is slight
to no difference in output impedance. Both are sub-Ohm.

But again, the source impedance of a transmitter is seldom important
in the application of the amplifier.

This seems at odds with other writers - if it is an AF amplifier. And
yet none seem to be able to describe the transition frequency for
identical designs (such as yours above, or as close to it as
practicable) where it DOES become important (higher output power, as
if any Ham would snub that advantage if (s)he could snip out the Z
transformer in the rig).

One still unanswered question remains that if a low output Z is so
beneficial to achieving higher output power (a claim that none seem
ready to dispute), then why the step up in Z from the finals deck to
the output connector? I have read an explanation from Wim, but it
fails as an argument for AF where a similar mismatch between source
and load is observed - and yet this mismatch is the greatest quality
for an amplifier since sliced bread.

AF - RF A curious tennis match here where what going over the net in
this game is a golf ball in one direction, and a basketball in the
other.

What's important are things like
the optimal load impedance and the rated power output. I'd put things
like distortion specs for a linear amp far above source impedance in
importance. If you (the lurking reader) think source impedance is
important, please explain in detail _why_.

The self-same NEGATIVE feedback, lacking in retail rigs, would
accomplish any form of improvement at the cost of what H.W. Bode
called "noise gain." The dollar cost is not inconsequential. Not
much more detail is needed for the average Ham.

For the better than average Ham, the increase in NEGATIVE feedback,
goes directly to lowering:
Gain,
Power out (as a function of following gain),
Bandwidth (as a function of following gain),
Distortion,
Power supply disturbance,
Noise,
Input Z (or through clever design, raising it),
Output Z (or through clever design, raising it),
while increasing cost in near or GREATER proportion.

Every Ham knows the adage that there is no such thing as a free lunch,
and the marketplace enforces that without exception.

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

On Mon, 25 Apr 2011 18:07:59 -0700, Jim Lux
wrote:

ON9CVD made some simple measurements using a couple of resistors and
foudn that a TS440 has a Zout somewhere around 15-40 ohms (depending on
frequency and output power).
http://sharon.esrac.ele.tue.nl/~on9c...impedantie.htm

Let's see here, a reported source Z as low as 15 (let's just call it
20) Ohms with 100W into 50 Ohms. I have read an explanation that this
is impossible (or improbable) for RF from a retail HF rig (such as a
TS440), but achievable at AF.

Meanwhile, back at the tennis match....

73's
Richard Clark, KB7QHC