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#9
June 14th 08, 10:28 PM
posted to rec.radio.amateur.antenna
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Efficiency and maximum power transfer
"Walter Maxwell" wrote in
: "Owen Duffy" wrote in message ... (Richard Harrison) wrote in news:23000- : Jim Lux wrote: "in a linear system" It produces no significant harmonics, so the system is linear. That is a new / unconventional definition of 'linear'. The term is usually used in this context to mean a linear transfer characteristic, ie PowerOut vs PowerIn is linear. Considering a typical valve Class C RF amplifier with a resonant load: Conduction angle will typically be around 120°, and to achieve that, the grid bias would be around twice the cutoff voltage. If you attempted to pass a signal such as SSB though a Class C amplifier that was biased to twice the cutoff value, there would be no output signal when the peak input was less than about 50% max drive voltage, or about 25% power, and for greater drive voltage there would be output. How could such a transfer characteristic be argued to be linear? Owen Owen, 'linear transfer characteristic' isn't the only context for the use of the word 'linear'. Even though the input circuit of a Class C amplifier is non-linear, the output is linear due to the energy storage of the tank circuit that isolates the input from the output, therefore, the output is linear. Proof of this is that the output signal is a sine wave. In addition, the voltage and current at the output terminals of the pi-network are in phase. Furthermore, the ratio E/I = R appearing at the network output indicates that the output source resistance R is non-dissipative, because a ratio cannot dissipate power. This resistance R is not a resistor. Hi Walt, A few issues.... Yes, I understand the context in which you mean linear (though I have issues with your proposition)... but my comment was referring to the assertion that 'no harmonics' relates to linear operation which seems to me to refer to the transfer characteristic linearity context. I do have issue with your stated 'proof'. Firstly, I must qualify that we are talking steady state... the mention of resonant loads means we are in the frequency domain. Whilst it might seem that the tank circuit / pi coupler / whatever is just a network of passive parts and they are all linear, the energy that is supplied to that circuit in each cycle depends on the resonant load impedance and traditional PA design methods suggest that that Eout/Iout relationship is not linear for changes in load Z, although it might be approximately linear over a small range. I recognise a distinction between resistance (the ratio of E/I) and a resistor (one type of component that exhibits resistance)... but I would not claim that resistance is just a 'ratio' because it implies it is a dimensionless ratio. Owen |
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