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#29
June 16th 08, 07:56 PM
posted to rec.radio.amateur.antenna
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Efficiency and maximum power transfer
"Jim Lux" wrote in message ... 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 Owen, I didn't realize that this thread was specific to 'linear transfer characteristic'. I thought the thread topic was sufficiently broad so as to include the subject of linearity of the output of the tank circuit that permits the use of theorems that require the output to be linear. Richard H's and my posts were simply reminders that the energy storage in the tank circuit is the reason for the linear relation between voltage and current at the output of both Class B and C amplifiers that results in a sine wave. From that perspective I believed our posts were legitimate to the thread topic. Apparently we were wrong. And Owen, I'm somewhat surprised that you don't agree with the flywheel analogy with respect to the smoothing effect of the energy storage in the tank circuit. This analogy has been around for decades--it's not my invention. IMHO, the periodic energy spurts from the pistons entering the flywheel is precisely an analog of the energy spurts of the periodic current pulses entering the tank citcuit. Why do you not agree? Even the pendulum swing is appropriate, because if you trace the position of the pendulum with respect to time you'll discover the trace is a perfect sine wave, while the short spurt of energy supplied by the spring at the beginning of each cycle is just sufficient to overcome the energy dissipated due to friction at the axis plus the aerodynamic resistance. How could this not be an appropriate analogy? Sorry to have forced you away from the thread topic with questions not pertaining to the thread. I am also curious as to why the subject of 'linear transfer characteristic' with respect to Class C amps was even considered, because the Class C amp has always been known to have a distorted output relative to its input. I would agree that the subject is appropriate when considering Class AB and B amplifiers, but not C. Walt, W2DU |
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