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True, but only in a linear system representable by a Thevenin source having
a resistive component that is dissipative. But an r-f amplifier is a non-linear system with a non-dissipationless "internal resistance" and cannot be modeled with a Thevinin dissipative source. 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. The latest QEX has a revealing examination of impedance and conjugate matching matters. Bottom line seems to be that one concentrates upon the specific load *resistance* specified for all other specs to be met. Such a load permits the amplifier to deliver maximum power within specifications and as such is conjugately matched to its load. -- 73, George W5YR Fairview, TX http://www.w5yr.com "Richard Harrison" wrote in message ... John Woodgate wrote: "The problem is that people say "output impedance" when they mean "load impedance". Quite right. I`ll use "source" and "load". Current through a load depends on the voltage. Ratio of volts to amps is the impedance. A source with the same resistance and offsetting reactance to the load enjoys a Goldilocks relationship with its load. The source`s volts and amps perfectly match the demands of the load. It`s just right. There`s no surplus of either volts or amps when source and load are connected. Its a match. Only a matched source and load deliver all the power available in a source. If we have too much resistance in our load, it doesn`t take as much power as it could. If we have too little resistance in our load, too much power is lost in our source. The perfect match of equal source and load resistances, with the reactance neutralized, is the only condition permitting maximum power transfer. Somme amateurs want all the power they can get from their transmitters. Best regards, Richard Harrison, KB5WZI |
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 |
"Richard Clark" bravely wrote to "All" (27 Feb 05 23:49:44)
--- on the heady topic of " Say what you mean." RC From: Richard Clark RC Xref: aeinews rec.radio.amateur.antenna:26138 RC On Mon, 28 Feb 2005 06:28:24 GMT, "George, W5YR" RC 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. RC Hi George, RC I know you won't appreciate this, but it is a telling, simple test of RC a practical situation with a practical Amateur grade transistor model RC 100W transmitter commonly available for more than 20-30 years now: RC 1. Presuming CW mode into a "matched load" (any definition will do); RC 2. Report the DC power consumed before hitting the key; RC 3. Report the DC power consumed while holding the key. RC Concurrently note: RC A. Report Heat Sink Temperature for a previously idle/rcv condition; RC B. Report Heat Sink Temperature after 10 minute key-down. RC For a hypothetical "100W" model (again, a contemporary, common example RC for Amateur use) available through standard commercial venues: RC 2. About 20W - 30W RC 3. About 200W - 250W RC A. About 20 degrees C (or room temperature) RC B. Well above 37 degrees C (or skin temperature) RC Now, if we are to be any judge of efficiency (Thevenin does not have RC to be invoked, condemned, or venerated); then it runs close to 50% RC (110%). Others can invoke their favorite deity to explain. The maximum power transfer 50% efficiency Po/Pin figure is only valid for a "linear" amplifier termed Class A (current flows through the whole cycle). Other amplifier classes were invented which improved on that 50%. i.e. Class B (push-pull) can approach just shy of 70% (current flows through 50% of cycle), and Class C close to 90% (current flow less than 50% of cycle). Did I misunderstand the problem? A*s*i*m*o*v .... "Beware of all enterprises that require new clothes." -- THOREAU |
Richard Clark wrote:
To forestall any armchair engineers, yes, this efficiency is System efficiency... .... which is not the definition of "plate (anode) efficiency" from the IEEE Dictionary: "The ratio of load circuit power (alternating current) to the *plate* power input (direct current)." For amplifier efficiency calculations, only the DC power input to the plate-collector-drain is considered, by definition. The power dissipated in the surrounding support circuitry is not included in the definition of "plate efficiency". Reference: _Electronic_Fundamentals_and_Applications_ by John D. Ryder, Copyright 1954, by Prentice-Hall, page 348, Section 10-6. Plate-Circuit Efficiency -- 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 =--- |
Richard Clark, KB7QHC wrote:
"However, I would be surprised if a practical common Amateur grade transistor model transmitter commonly available for more than 30 years now has any configuration that does not supply voltage directly to the final transistors---." My Kenwood TS-130S manual has specifications on page 2. Transmitter output impedance is specified as: 50 OHMS! Om page 30 is a level diagram. Output from the transmitter`s low-pass filter is measured as 70.7 volts at 14.25 MHz, which is 100 watts into 50 ohms (square root of PR). D-C power is fed to the center-tap of a push-pull output transformer to the final transistors. From the specifications page also, the power reguirement is TX: 18A 13.8V DC. It`s a linear amplifier. Only 40% efficiency. The designer probably was more interested in low harmonics than efficiency. The final by itself only takes part of the 18A ao its efficiency is more than 40%. Kenwood says its transmitter presents an impedance of 50 ohms at its "OUT" terminals in the diagram on page 30. 100 watts at 70.7 volts. Best regards, Richard Harrison, KB5WZI |
Asimov wrote:
.. The maximum power transfer 50% efficiency Po/Pin figure is only valid for a "linear" amplifier termed Class A (current flows through the whole cycle). Other amplifier classes were invented which improved on that 50%. i.e. Class B (push-pull) can approach just shy of 70% (current flows through 50% of cycle), and Class C close to 90% (current flow less than 50% of cycle). Did I misunderstand the problem? What you missed is Richard C. trying to redefine the efficiency of a final amp. :-) -- 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 =--- |
On Mon, 28 Feb 2005 08:23:53 -0600, Cecil Moore
wrote: Reference: An obscure deity indeed.... |
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? -- 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 =--- |
Richard Clark wrote:
Cecil Moore wrote: Reference: John D. Ryder An obscure deity indeed.... He authored three of Prentice-Hall's Electrical Engineering Series books back in the 50's. He was Michigan State's Dean of Engineering at the time. -- 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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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 ----== 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 =--- |
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 ... 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. |
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 =--- |
On Monday, 28 Feb 2005 21:18:18 -500, "Asimov"
wrote: I'm sorry but it is an erroneous conclusion to think it cools. Don't you recall mentioning that the "average" voltage and current remains the same? Hi OM, One of those Class A characteristics. It is revealed by the rather more simple solution to the difficult problem I offered: What is power in? What is power out? This characteristic is part of the lore for those who built their own equipment as one of several NEVERs Don't let your amp run without drive; Don't let your amp run without bias. Both conditions can lead to melt down. Unfortunately this kind of Ham memory is fading to the point where many who witnessed this "internal resistance" are not here to point out the obvious errors in understanding exhibited by the credit card generation who build their rigs with plastic. 73's Richard Clark, KB7QHC |
Cecil Moore wrote:
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." This is why the load must be matched to the amp. Anyone who has fried a final will tell you so. Been there, Done that, Got the bottles to prove it. Dave WD9BDZ |
Richard Clark wrote:
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. You entirely missed the point. You don't know the output impedance because you don't have a way of determining it by swinging the output full-scale. Even for class A, large signals will/can have rail to rail swing. The device will not be linear for large swings: sinusoidal input swing will not result in a sinusoidal output swing. But "impedance" is a sinusoidal (s-domain) concept. So how can you define an impedance--a sinusoidal concept--when the waveform is not sinusoidal for an inputted sine wave? The point is that the output impedance is time dependent ("causes" the non-sinusoid output for sinusoid drive), which rather makes the concept questionable. As I wrote earlier, one might decide to consider a time averaged impedance, but I'm not clear on what the utility would be. 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. There is no "presumption." Linear parameters and theorems totally ignore practical limitations--this is a fact and you can look it up in just about any text on circuit analysis. The simple linear model is perfectly okay for small signal devices. It isn't okay for large signal devices. In any case, load pull equipment does not make the pretense of defining output impedance of an active large signal device. It does say what the load needs to be to acquire maximum power out of the device. To say pretending the device is similar to a linear small signal device is one of those assumptions forced into the argument. No, it isn't. Thevenins and conjugate matching (for maximum power transfer) are explicitly linear small signal device models. Their use in RF PA output design is a misapplication. There are any number of ways to do something wrong. We're talking about one of them. Misapplying small signal linear parameters to the output of a large signal device. 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 Who are you quoting and why? 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); Any definition won't do, and for this discussion the specific "won't do" is using conjugate matching which is a small signal (linear) model. 2. Report the DC power consumed before hitting the key; 3. Report the DC power consumed while holding the key. Hey, at least you're recognizing that DC power is important. Where in conjugate matching ideas or Thevenins theorem do you see any concern of DC power? That's right, you don't because they a simple small signal models where DC power and voltage have no bearing because the signals are so small, relatively speaking. 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. *You* brought up Thevenins and armchair philosophy regarding it, not me. I said Thevenins was irrelevent, and now you appear to agree with me. Ken effectively brought up conjugate matching, not me. The original comment I was challenging was: "...the antenna works as an impedance mathcing network that matches the output stages impedance to the radiation resistance." I simply wanted to make it clear that the "matching" done was not an issue of "output impedance" per se. It is an issue of how the transistor is to be loaded to extract maximum ouput power. 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. Exactly. It is about DC to RF efficiency, as I've been pointing out since my first post, and which you initially commented was "nonsense" but now seem to agree with. "Impedance matching" meant in the normal sense of conjugate matching for maximum transfer of power is a misapplied small signal concept/model. I think that is all I've really been saying. |
The most simple way to describe it is -
With a class-A amplifier the power taken from the DC supply remains constant regardless of signal power output. The device(s) internal dissipation decreases by the same amount as the signal power output (drive level) increases. If there's any change in power taken from the DC supply as the power output changes then the device is not operating under class-A distortion-less conditions. KISS. KISS. KISS. KISS. To slightly change the subject, anybody who mentions a conjugate match doesn't know what he's waffling about. And if I remember correctly, a 6N7, a class-B dual-triode, metal tube was specially designed to have a very high Mu and could be operated under very simple zero-bias conditions. Each of the two triodes handled one half of the complete audio sinewave. With grids in push-pull and the anodes in parallel, you could get 10 watts out of it as an HF frequency doubler. But it would still take the skin off your fingers. Ahhh! Happy days! ---- Punchinello. |
"Cecil Moore" bravely wrote to "All" (01 Mar 05 09:21:18)
--- on the heady topic of " Say what you mean." CM From: Cecil Moore CM Xref: aeinews rec.radio.amateur.antenna:26191 CM From "Electronic Fundamentals and Applications" by CM John D. Ryder, regarding Class-A amplifiers: CM "As the a-c output increases, the plate loss CM decreases and the tube runs cooler." harmonic generation... A*s*i*m*o*v .... Be nice to your kids. They'll choose your nursing home. |
Reg Edwards wrote:
With a class-A amplifier the power taken from the DC supply remains constant regardless of signal power output. I wish I had said that, Reg. thanks ... -- 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! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
On Tue, 01 Mar 2005 18:06:18 GMT, gwhite wrote:
It is about DC to RF efficiency, Put a number to it. as I've been pointing out since my first post, and which you initially commented was "nonsense" Hi OM, And so it remains with additional elaborations not quoted here. but now seem to agree with. Seeming is a rather insubstantial thing to hang your theories on. "Impedance matching" meant in the normal sense of conjugate matching for maximum transfer of power And this reveals the error of "Seeming" because the so-called meaning you ascribe is this same nonsense. Pay more attention to reading instead of writing. It has been pointed out more than once, and by several, that Matching comes under many headings. The most frequent violation is the mixing of concepts and specifications (your text is littered with such clashes). is a misapplied small signal concept/model. I think that is all I've really been saying. And I preserved this clash quoted above as an example. If there is any misapplication, you brought it to the table with this forced presumption. The misapplication of S parameters to a large signal amplifier is one thing, to project this error backwards into the fictive theory that there is some difference between large and small signal BEHAVIOR (not modeling) is tailoring the argument to suit a poorly framed thesis. None of your dissertation reveals any practical substantiation, hence it falls into the realm of armchair theory. We get plenty of that embroidered with photonic wave theory that is far more amusing. 73's Richard Clark, KB7QHC |
In article , gwhite wrote:
[...] You entirely missed the point. You don't know the output impedance because you don't have a way of determining it by swinging the output full-scale. You don't have to swing the output full-scale to measure the impedance. Any change in the load, no matter how small, will cause a change in the output voltage and the output current. From these you can calculate the output impedance at the current operating point. When a transistor is operating under large signal conditions into a tuned load, there is still an output impedance and this impedance still discribes what will happen for small changes in the load. -- -- forging knowledge |
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I read in sci.electronics.design that Ken Smith
wrote (in ) about '1/4 vs 1/2 wavelength antenna', on Wed, 2 Mar 2005: In article , gwhite wrote: [...] You entirely missed the point. You don't know the output impedance because you don't have a way of determining it by swinging the output full-scale. You don't have to swing the output full-scale to measure the impedance. Any change in the load, no matter how small, will cause a change in the output voltage and the output current. From these you can calculate the output impedance at the current operating point. When a transistor is operating under large signal conditions into a tuned load, there is still an output impedance and this impedance still discribes what will happen for small changes in the load. This incremental impedance is one of several different impedances that can be defined for a non-linear source. No one is more valid conceptually than another, but some are of more practical significance than others. The point is that if you want to talk/write about one of these impedances, you need, to prevent misunderstanding, use a precise term, such as 'incremental output source impedance' and define it. -- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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