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#21
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Cecil Moore wrote:
What is the steady-state impedance of the source at the fundamental frequency? Exactly Cecil. That's exactly why I said the concept of impedance is dubious for large signal devices and thus provides a second argument, albeit weaker, against the "impedance matching" idea for power amps. "The concept of 'output impedance' breaks down for large signal devices. For example, what is the output impedance of a class C or D amp taken when the transistor is on or off?" 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. |
#22
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In article , Cecil Moore wrote:
Ken Smith wrote: RF transmitter power amps are certainly "impedance matched" to the intended load. Take a look in the ARRL "The radio amateur's handbook". If you have the 1944 addition, you will need to start reading at page 96 in the lower right column. If you don't have that, try Motorola's AN-721. A CMOS Class-E amp is in full saturation (0.5v at 2a) for 10% of a cycle and off (12v at 0a) for the other 90% of a cycle. The tank circuit changes the digital energy to analog energy by filtering out everything except the fundamental frequency component. How in the world does one determine the steady-state impedance of the CMOS source? Isn't the best one can do with a digital switch is to keep it within specified parameters? The CMOS device dissipates 2 watts for 10% of the time - therefore 0.2 watts steady-state. For what you say here really to be true the transistors must switch very fast. About 25pS switching speed is needed at about 400KHz. If we take that to be the case however, I think you will see why matching still applies. Lets take the reactive component first. If there is a reactive component to the loading, the current in the switch will have a higher RMS value without that increase in RMS increasing the radiated power of the system. So the reactive component of the matching is fairly obvious. Imagine that you have a well designed Class-E circuit loaded with the load the designer optimized it for. Now imagine that you slightly increase the resistance slightly. When you do so, the current into the load will decrease but the voltage will not increase enough to compensate for this. Now lets assume that you slightly decrease the resistance. Since we are assuming that this is a well designed case, we can assume that the designer took steps to ensure that the output devices would be protected from excess currents. This could be done by reducing the operating voltage of the output section, for example. In any case, the voltage on the load will decrease by a larger factor than the current will increase. So it is obvious that the reactive part is matched and the resistive part is matched just as it would be in a non-class-E output section. -- -- forging knowledge |
#23
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In article , Cecil Moore wrote:
Ken Smith wrote: RF transmitter power amps are certainly "impedance matched" to the intended load. Take a look in the ARRL "The radio amateur's handbook". If you have the 1944 addition, you will need to start reading at page 96 in the lower right column. If you don't have that, try Motorola's AN-721. It appears that I may have canceled an earlier posting by accident so will repeat it. I answered your other copy of the posting. -- -- forging knowledge |
#24
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In article ,
Jim Thompson wrote: On Fri, 25 Feb 2005 09:47:16 -0600, Cecil Moore wrote: [...] A certain Class-E CMOS amp is in full saturation for 10% of a cycle, 0.5v at 2a. For the rest of the time it is off. The supply voltage is 12v. What is the steady-state impedance of the source at the fundamental frequency? Now, now, Cecil! Don't sully the thread with facts !-) This needs a 12V transistor that switches in about 25pS. Would that be likely on a current chip? -- -- forging knowledge |
#25
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In article , gwhite wrote:
Cecil Moore wrote: [...] "The concept of 'output impedance' breaks down for large signal devices. Motorola seems to disagree with you. Perhaps more importantly, I disagree with you. The large signal impedance of a transistor working into a tuned load still applies quite well. For example, what is the output impedance of a class C or D amp taken when the transistor is on or off?" When dealing with an RF output section you don't deal with just one part of the cycle. You can tell if the real part of the impedance is matched if: (1) If you increase the resistance of the load does the power decrease? AND (2) If you decrease the resistance of the load does the power decrease? 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. What do you mean by "maximum signal swing" in this context. I can get a bigger swing by leaving the output completely unloaded and hence causing the actual efficiency to be zero. The reactive component issue is still there too. Reactive loads cause increased currents in the output stage without delivering any power to the load so they still need to be reduced as much as practical. -- -- forging knowledge |
#26
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In article , gwhite wrote:
Ken Smith wrote: RF transmitter power amps are certainly "impedance matched" to the intended load. I'm sorry, but they are not. Nor are any power amps that I know of. Efficiency (and thus necessarily output swing) is what matters for power amps. To maximize swing requires load line matching, not impedance matching. I still say they are. Motorola AN-721 takes on the theory. AN-758 does a practical example matching 12.5 Ohms into 50 Ohms -- -- forging knowledge |
#27
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"Tam/WB2TT" wrote in message ... "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. Tam Tam An audio amplifier can have an improved capability of minimizing a speaker's deviation from its desired displacement if it has a low output impedance. That is, the speaker's cone tends to swing past its desired displacement when its terminals are loaded to a high impedance. Jerry |
#28
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Ken Smith wrote:
In article , gwhite wrote: Ken Smith wrote: RF transmitter power amps are certainly "impedance matched" to the intended load. I'm sorry, but they are not. Nor are any power amps that I know of. Efficiency (and thus necessarily output swing) is what matters for power amps. To maximize swing requires load line matching, not impedance matching. I still say they are. Motorola AN-721 takes on the theory. AN-758 does a practical example matching 12.5 Ohms into 50 Ohms Sure are. A 30 year ago 500 level course I took called Non-Linear Transistor Design said so. The way you handle class C etc. is by handling each harmonic separately in your analysis of the transistor plus tank circuit. You match to the harmonic you want. It may be the fundamental, or the third for a tripler, etc. tom K0TAR |
#29
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Jerry Martes wrote:
"Tam/WB2TT" wrote in message ... "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. Tam Tam An audio amplifier can have an improved capability of minimizing a speaker's deviation from its desired displacement if it has a low output impedance. That is, the speaker's cone tends to swing past its desired displacement when its terminals are loaded to a high impedance. Jerry Correct - in the case of an audio amplifier damping factor is critical to accurate reproduction of transients. In an RF amp, it is normally not required or desired. tom K0TAR |
#30
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On Sat, 26 Feb 2005 00:40:09 +0000, Ken Smith wrote:
In article , gwhite wrote: Ken Smith wrote: RF transmitter power amps are certainly "impedance matched" to the intended load. I'm sorry, but they are not. Nor are any power amps that I know of. Efficiency (and thus necessarily output swing) is what matters for power amps. To maximize swing requires load line matching, not impedance matching. I still say they are. Motorola AN-721 takes on the theory. AN-758 does a practical example matching 12.5 Ohms into 50 Ohms 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!) Cheers! Rich |
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