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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. |
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 |
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 |
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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 |
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. |
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 ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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 |
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 |
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 ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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