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#30
June 16th 08, 04:32 PM
posted to rec.radio.amateur.antenna
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Efficiency and maximum power transfer
Owen Duffy wrote:
(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. Or, as I used it, that superposition holds. One can build an amplifier or other device where the Pout(Pin) =straight line, but is not linear in the formal sense. Say you built a widget that measured the input frequency and amplitude, then drove a synthesizer at that frequency and amplitude = 2*input amplitude. 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? It would not be.You're right The active device isn't linear. neither is the whole assembly. I think, though, that sometimes we take a more casual view of linear (e.g. people talk about the linearity of a log detector.. referring to the deviation from a Voltage out=dBm in straight line.) And, some confusion about nonlinear devices in a building block that is, by and large, linear (e.g. a power op amp with an AB2 output stage and a fair amount of negative feedback) with some constraints on frequency and amplitude. Owen |
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