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  #101   Report Post  
Old March 1st 05, 12:06 AM
gwhite
 
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Richard Clark wrote:


No amount of armchair philosophies about
Thevenin's theorem will replace that loss.


Thevenin's is a linear theorem. Large signal devices are not linear. (Hey,
maybe triodes are, but I don't use them for PA's.)

There is no armchair philosophy about Thevenin's theorem because it does not
apply in the RF PA situation. Your's is a red herring.
  #102   Report Post  
Old March 1st 05, 12:58 AM
Ken Smith
 
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In article ,
keith wrote:
On Sun, 27 Feb 2005 17:12:46 +0000, Ken Smith wrote:

In article ,
John Woodgate wrote:
[...]
If the FM is what passes for music these days, it's MUCH better IMHO.


There is very little that is sent in the form of radio waves that is worth
the electrical power to send it.



Hmm, many advertisers would disagree. Since they have real money at stake
and you don't...


They say that 1/2 of all advertising money is just wasted. The problem is
no-one can say which half.

Besides the advertisers don't care about the value of what is sent. They
only care about the wallet of those who are listening.



The really sad thing is that much of
what is send via FM is really "voice grade" material. When FM was new,
the material for FM was specially produced to take advantage of the wide
bandwidth and large dynamic range.


Bandwidth, perhaps. Wide dynamic range? The FCC rules seem to
suggest otherwise.


How do you arrive at that? FCC rules don't specify the nature of the
music programming, really. If the "music" always has the modulation index
at least 30% with rap, the dynamic range actually needed is much less than
for something from BareNakedLadys

--
--
forging knowledge

  #103   Report Post  
Old March 1st 05, 02:16 AM
Richard Clark
 
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On Mon, 28 Feb 2005 22:53:17 GMT, gwhite wrote:

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.


Hi OM,

Well, it is more accurate to say that you don't, that is for sure.
Defining a solution by negative results can fill up a library without
any positive accomplishment. Obviously people don't have a nuclear
reactor, or lunar lander, or bank account to balance the national
debt. The joke of this, of course, is that no one needs a 100 W (or
more!) network analyzer, or nuclear reactor, or lunar lander, or bank
account to balance the national debt to explain a rather more trivial
problem. Which, by the way, has nothing to do with pretending at all.

The suggestion that
requires load pull test equipment and that can be expensive

does not negate its existence which commonly proves what you choose to
dismiss as impossible. I have calibrated this gear (called an
artificial or active load), and the gear (called transmitters) it
tests and there are no differences in Physics based upon your
presumption of low-power/high-power demarcations.

To say
pretending the device is similar to a linear small signal device

is one of those assumptions forced into the argument. There are any
number of ways to do something wrong. Trumping none of these straw
men validates another wrong impression passing as theory. This
returns us to the imposition of impossibilities to answer a rather
mundane concept, eg.
pretending the device is similar to a small nuclear device
pretending the device is similar to a mars rover
pretending the device is similar to the national debt of Lithuania


So to return to a common question that seems to defy 2 out of 3
analysis (and many demurred along the way) - A 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
  #105   Report Post  
Old March 1st 05, 03:20 AM
Tom Donaly
 
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Cecil Moore wrote:
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. :-)


Try to find a copy and read it, anyway, Cecil. It's
worth a pair of bloodshot eyes for its historical value
if nothing else.
73,
Tom Donaly, KA6RUH


  #106   Report Post  
Old March 1st 05, 03:58 AM
Wes Stewart
 
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On Fri, 25 Feb 2005 22:56:47 -0600, Cecil Moore
wrote:

Ken Smith wrote:
If you then
put in the output device protection they didn't include, you end up with
the matching as I explained elsewhere.


SWR foldback is part of impedance matching?


It is in the sense that it improves the source match by trying to hold
the forward power constant regardless of load. Most SWR foldback
systems overreact but a good ALC system, what we called a "leveling
loop" in waveguide reflectometers back in the mid-20th century
certainly improve the source match.

  #107   Report Post  
Old March 1st 05, 02:58 PM
Cecil Moore
 
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Asimov wrote:
RH An amplifier can be a linear source without operating under Class A
RH conditions.

I would really find it instructive if you finished up replying to this
message with an example of a "linear" source not operating in Class A?


Richard may be referring to Class AB push-pull where
the source is more than one device.
--
73, Cecil http://www.qsl.net/w5dxp


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  #108   Report Post  
Old March 1st 05, 03:29 PM
Richard Harrison
 
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Asimov wrote:
"I`m sorry but it is an erroneous conclusion to think it cools (when
signal exits)."

The Class A amplifier gets all its power from the d-c supply and it is
constant, signal or no signal. With signal power output, some of the
power in exits to the load.

I`ll use Cecil, W5DXP`s argument. Energy must be conserved. Energy in
equals energy out. If some goes to a load it does not stay within the
amplifier to make feat.

Asimov also wrote:
"---linear source not operating Class A?"

I`ll give an example. The Class B amplifier is biased near current
cut-off. Current is near zero when the signal is. Yet, output can
favorably vie with that from a Class A amplifier for purity. I learned
that nearly 60 years ago when I built my first 6N7 phonograph amplifier.

Best regards, Richard Harrison, KB5WZI

  #109   Report Post  
Old March 1st 05, 04:02 PM
Richard Harrison
 
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Cecil, W5DXP wrote:
"Richard may be referring to Class AB push-pull where the source is more
than one device,"

Push-pull works to eliminate harmonics without filters. In my Kenwood
TS-130S, the push-pull devices are 2SC2290*J`s, if my squint is right.
Pc=175 (W), VCBo=45 (V), IC=20 (A), etc.

Best regards, Richard Harrison, KB5WZI

  #110   Report Post  
Old March 1st 05, 04:21 PM
Cecil Moore
 
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Richard Harrison wrote:
I`ll use Cecil, W5DXP`s argument. Energy must be conserved. Energy in
equals energy out. If some goes to a load it does not stay within the
amplifier to make heat.


From "Electronic Fundamentals and Applications" by
John D. Ryder, regarding Class-A amplifiers:
"As the a-c output increases, the plate loss
decreases and the tube runs cooler."
--
73, Cecil http://www.qsl.net/w5dxp


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