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#191
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I quoted Terman, saying:
"The power delivered to the load under these conditions (a conjugate match) is termed the AVAILABLE POWER of the power source." The match between source and load is the best it can be and can`t be improved when you have a conjugate match. A conjugate match is an empowerment but does not cause you to put out any particular power. That`s up to you. Suppose you have a conjugate output match to a Class-B power amplifier you are driving with your SSB transmitter. Instantaneously, average power from the amplifier is following the modulation. A single steady tone ideally produces a particular output at at one radio frequency. Want more output? Increase drive to the amplifier. Want less? Reduce drive to the amplifier. You may have a conjugate match under only one condition, some conditions, or under all conditions, but given life`s usuall imperfections, I would place no bets, except against all conditions. Best regards, Richard Harrison, KB5WZI |
#192
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On Wed, 8 Dec 2004 20:55:18 +0000 (UTC), "Reg Edwards"
wrote: and if that isn't enough, to further complicate matters, the internal impedance of the transmitter changes as the load impedance is varied Hi Reggie, Such arguments are as juvenile as the claim no one can travel a straight line because the earth is rotating under them. A quadrillion miles of experience would suggest this too is trivial to accomplish. Aren't you the one who is so charmed with the legacy of Kelvinator who chimed that such chimera without calculation are the chatter of chimps in the forest canopy? 73's Richard Clark, KB7QHC |
#193
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Just a comment -
The design, from start to finish, of a linear power amplifier is based solely on a device's ratings - volts, amps, watts, etc. Its RF internal impedance plays no part in it. At HF it is never specified by the manufacturer. Even ARRL bibles don't mention the subject of Rint. It's superfluous. Does anybody know what it is? Give us some numbers. As for conjugate matching - don't make me laugh. --- Reg. |
#194
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Richard, you've slipped into one of your incoherent phases. Try again in a few days time when you are feeling better. ;o) ---- Yours, Punchinello. |
#195
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Reg, G4FGQ wrote:
"The active device generally behaves as a current source." As Reg also wrote: "I can`t imagine why this conversation has continued for so many years by more or less the same group of experts." Agreed! Reg seems to have answered his own question.The same people recite the same arguments in hopes their view of reality will be accepted. Fat chance! Time has inured them. Reg has faithfully proposed constant-current behaviour from all vacuum valves and transistors as I recall. I agree that most of these devices have extremely high plate ond collector resistances as linear amplifiers. Current through them is almost constant regardless of anode voltage. As most transmitter power amplifiers exceed 50% efficiency by a good margin, these devices are not operating as Class-A linear amplifiers. They instead operate as HF switches. These are turned-off most of every cycle and are only on for short pulses. Harmonics and other noise is cleaned up by output filters. It`s the only thing which makes the output linear. During the output device`s conduction, its saturation volts are very low and its current is very high, giving the device a very low impedance while switched-on. You may not infer a low impedance from the d-c volts and amps feeding the final amplifier. These are the averages, almost, of the device amps. The device saturation volts sre what counts toward its dissipation and loss. The transmitter usually has no built-in indicator of saturation voltage. It wouldn`t read much anyway.Device impedance depends mostly on its ratio of off to on times. This is a form of lossless resistance. Dissipation is zero in a sewitched-off device. The d-c volts and amps are related to the output device(s) internal impedances used as a switch when the transmitter output is considered. A high voltage and a low current accompany a high internal impedance but they won`t be nearly so high as the spec sheet plate or collector resistances. We have d-c power input to the amplifier. We can measure HF power output. The difference is dissipation, but loss resistance does not represent the total source resistance because we have non-dissipative resistance in the device off-times. There have been measurements of transmitter internal output impedances which indicated that they did indeed match their loads. I have not done it myself but have no reason to doubt the reports. Best regards, Richard Harrison, KB5WZI |
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