On Monday, 28 Feb 2005 09:21:38 -500, "Asimov"
wrote:

Did I misunderstand the problem?

Hi Isaac,

Probably. It only asked two things (both power readings).

73's
Richard Clark, KB7QHC

On Mon, 28 Feb 2005 10:57:24 -0600, Cecil Moore
wrote:
An obscure deity indeed....
He authored three
What an impoverished disciple of an obscure deity. Did he get his
ankles wet measuring supply current - or are we talking water level
above the knees?

Asimov wrote:
"The maximum power transfer 50% efficiency figure is only valid for a
"linear" amplifier termed Class A (current flows through the whole
cycle)."

Maximum power transfer does not care how many degrees of a cycle the
power flows. Nor does it care whether the source is electronic. Whether
it applies depends entirely on the behavior of the source and load.

It`s true that current in a linear Class A amplifier flows continuously
and does not vary in average amplitude over a complete cycle.

When there`s no signal input to a Class A amplifier, its power input is
volts x amps and its efficiency is zero. Maximunm undistorted output
requires 1/2 the d-c input to the stage. As this power exits the stage,
it cools as the remaining dissipation is only 1/2 the no-signal
dissipation.

An amplifier can be a linear source without operating under Class A
conditions. Higher efficiency is the main reason for not operating in
Class A. By turning the amplifier devices off
for part of each cycle, efficiency can be raised significantly beyond
50%.

Best regards, Richard Harrison, KB5WZI

Cecil Moore wrote:
George, W5YR wrote:

The latest QEX has a revealing examination of impedance and conjugate
matching matters.


Hi George, what's the title and who's the author?

It's RF Power Amplifier Output Impedance Revisited, by
Robert L. Craiglow. Take it with a grain of salt, Cecil.
There is more than one conceptual mistake contained in
the article.
73,
Tom Donaly, KA6RUH

Rich Grise wrote:

No, just trying to make the point that it does, in fact, _have_ an
impedance. (even if it's running class E.) What that exact impedance is,
of course, is left as an exercise for the reader. :-)

And like any impedance, is a function of frequency.

And another thing - in a transmitter, the impedance matching only happens
at the one frequency, which is a lot different scenario from, say, a
stereo. This could be a confusion factor here.

There may be more similarity than difference over the respective 20 KHz
bandwidth.

ac6xg

Tom Donaly wrote:
It's RF Power Amplifier Output Impedance Revisited, by
Robert L. Craiglow. Take it with a grain of salt, Cecil.
There is more than one conceptual mistake contained in
the article.

Shirley, you jest. Conceptual mistakes in QEX? :-)
Unfortunately, I don't have a way to read it. Today
I got my 2004 ARRL Periodicals on CD-ROM but am not
a subscriber to QEX since they refused to publish
my article that would have ended all arguments. :-)
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:

Rich Grise wrote:
Evidently, the guy's never tuned up a 40 meter pi-net output transmitter. ;-)

If that's not impedance matching, I don't know what it is! (Oh, "Load line"
matching? What are the two parameters of the load line? Voltage and Current,
right? What's the slope of the load line? Impedance!)

And there's the catch. If the load line is the source
impedance, the load (not the designer) effects the source
impedance.

Exactly. Pure coincidence.

Allan Herriman wrote:

On Sat, 26 Feb 2005 04:53:03 +0000 (UTC), "Reg Edwards"
wrote:

The phrase "output impedance" in connection with amplifiers is ambiguous and
likely to result in arguments.

The correct description is "internal impedance" or "internal resistance" and
should always be used.

S22 is fairly well defined.

Just to bring back to the original discussion and reiterate:

S22 is a small signal (linear) parameter, by definition. It does not apply to
the large signal environment.

This idea is somewhat related to the idea that power amps should be tuned for
"maximum transfer of power," which is a small signal (s-param) issue, and
requires conjugate matching. The idea is incorrect because it ignores the
practical large signal non-linearity and *any* consideration of DC to RF
efficiency (which is prime for PA design). Linear parameters provide *no*
recognition of things like DC to signal power efficiency and therefore practical
issues like supply rails.

First order matching of an RF PA to a load involves transforming the load to the
optimum point on the AC load line (for example, more or less equal positive and
negative swing limits for class A). That's what "matching" is for an RF PA. It
makes no statement about actual "output impedance" of the source. What is said
is that "such and such RF PA will deliver X power into some specifed impedance
within some VSWR circle." That's all. The concept of output impedance begins
to break down for large signal devices.

Tam/WB2TT wrote:

"Rich Grise" wrote in message
news
On Fri, 25 Feb 2005 17:59:56 -0500, Tam/WB2TT wrote:


"gwhite" wrote in message
...
Richard Clark wrote:

On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote:

RF transmitters are not ....

Sorry OM,

This was all nonsense.

Nice articulation. I don't know who OM is, but RF transmitter power
amps
are
not "impedance matched." Neither are audio power amps for that matter.

My stereo amp has a spec on output impedance. As I recall, it was around
0.16 Ohms. Intended load is 4 - 16 Ohms.


That works because the transmission line is less than 0.01 wavelength.
So impedance matching becomes moot. If the speaker line were 1/4
wavelength
long, there would be almost no signal transferred at all.

Cheers!
Rich


There is nothing wrong with driving a transmission line/antenna from a zero
impedance source. It does NOT change the SWR. The point is that an audio
amplifier with a damping factor of 50 is NOT conjugate matched.

Somebody mentioned Motorola Application note 721. This is what it says:

************************************************** **************************************
" ..the load, in first approximation, is not related to the device, except
for VCE(sat). The load value is primarily dictated by the required output
power and the peak voltage; it is not matched to the output impedance of the
device. "
************************************************** *****************************************

When device people talk about "matching", they mean matching the load to
what the transistor wants to see, which is not the conjugate of the output
impedance. The way this is done is to build an amplifier, and vary the load
until maximum output power is reached. The transistor is then removed, and
the impedance looking into the coupling network is measured. The conjugate
of this is sometimes listed as "output impedance" on data sheets. Newer data
sheets will have an asterisk * next to that, and a note explaining what it
means. If you look at Philips literature, you will see exactly the same
explanation.

Nice. Exactly: "what it wants to see" is perfect. Of course, "varying the
load" requires load pull test equipment and that can be expensive. When load
pull equipment is not available, we're stuck with other methodology. In that
case, my first order cut is the AC load line, a harmonic short at the device,
and enough flexibility in the layout to pull it in by cut and try. I haven't
used ADS or Microwave Office's Harmonic Balance simulators. I suppose with good
behavioral models and a good simulator, a good deal of cut and try could be
circumvented.

Obviously people don't have 100 W (or more!) network analyzers looking into the
output and pretending the device is similar to a linear small signal device.

Cecil Moore wrote:

gwhite wrote:
The strongest argument for dropping the impedance matching concept is PA
efficiency, and therefore maximum signal swing. Obtaining maximum swing is a
load line issue.

So what impedance does the reflected wave encounter?


Don't know. Whatever mismatch there is, at whatever moment in time, it simply
results in reflection back towards the load. Reflections could also cause
additional non-linearity in the PA. Therefore reflections for higher powered
PA's are directed off to a dummy load via a circulator.