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Efficiency and maximum power transfer
"Antonio Vernucci" wrote in
: Someone may regard the following question a bit OT, but as it deals with impedances I have considered that the antenna newsgroup could be the most appropriate one where to post it. Let us regard a transmitter as an ideal RF generator with a resistance in series. It is well known that, for maximum power transfer, the load resistance must be equal to the generator resistance. Under such conditions efficiency is 50% (half power dissipated in the generator, half delivered to the load). To achieve a higher efficiency, the load resistance should be made higher than the generator resistance, although this would turn into a lower power delivered to the load (the maximum power transfer condition is now no longer met). This can be verified in practice: by decreasing the antenna coupling in a transmitter, one obtains a higher efficiency though with a lower output power. That said, now the question. But your statements are not true. The model you propose for a transmitter does not apply in general. Whilst it would be possible to build a transmitter like that, most transmitters that hams use are not built like that. So... it is a loaded question of a type, a question premised on a falsehood. Usually, when a transmitter is tuned for maximum output power, efficiency results to be higher than 50% (typically 60% for class-B, 70% for class-C). This would seem to contradict the above cited fact that, under maximum power transfer condition, efficiency is 50%. If the equivalent source impedance is not important, ie it does not need to be fixed by the design, there here is an analysis. If you take the case of a grounded cathode triode in class C with a steady signal, the conduction angle is usually somewhere around 120°. The anode current waveform is a little like a truncated sine wave, but even for the range of grid voltages where anode current is greater than zero, the transfer characteristic is not exactly linear, and the wave will be further distorted. If the nature of the anode load is that it is some equivalent R at the fundamental and zero impedance at all other frequencies, the power output can be determined by finding the fundamental component of the anode current waveform, squaring it, and multiplying it by R. The input power is the average anode current multiplied by the DC supply voltage. Efficiency is OutputPower/InputPower. By varying the grid bias, drive voltage, load impedance and supply voltage for a given triode, different efficiencies will be found, and the maximum could be well over 80%. Nothing in this approach to design attempts to fix the equivalent source impedance, the design is performed without regard to that characteristic. Nevertheless, some argue that the output network performs magic and achieves source matching naturally without designer intervention, and does this irrespective of parameters like the dynamic anode resistance, and the effects of feedback (such as cathode degeneration in grounded grid amplifiers which in turns depends on the source impedance of the exciter). Owen |
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