RadioBanter - View Single Post

August 18th 03, 03:13 PM

(Richard Harrison) wrote:
Floyd Davidson wrote:
"Unless of course that shuts down ,or even destroys the source devices
in the process (adjustable load gets hottest for least power to the
source as proposed by H. Adam Stevens)."

If you are going to quote me, the least you could do is quote
what *I* said, without editing it to suit your needs.

Regardless, whatever it is that the rest of this nonsense is
supposed to mean, it certainly doesn't relate to what I posted.

H.`s variable load can determine a match because maximum power transfers
if the load impedance is the conjugate of the source. But, least power
input may not coincide with maximum power output. Low internal source
impedance as compared with the load impedance usually is more efficient,
but not necessarily so. With impedance produced by switched-off time as
in the Class C amplifier, impedance is not only a function of internal
loss.

A Class A amplifier has constant power input so that the more power
delivered to a load, the less power is dissipated in the amplifier.

Simple determination of source impedance is to divide the open-circuit
voltage by the short-circuit current out of the device. That requires a
source impedance that is the same at matched loading as with a shorted
load. As Floyd notes, the extremes, open and shorts, can shut-down or
destroy from too much voltage or current.

Near the region of a conjugate match, less severe voltage and current
are likely. Maximum power transfer to the load occurs at the conjugate
match point. Heat rise would be the likely cause of damage to a
transmitter at maximum output. Power output measurements can be made
quickly to limit transmitter and load temperature rise in and around the
load impedance which gives maximum power output. A versatile load can be
used to determine the conjugate match and thus the source impedance.

The transmitter may not be designed to deliver maximum power output even
momentarily and may have built-in protection to prevent it.

Feedback may not increase maximum power output without regard to
distortion. In an audio amplifier, the output impedance may be made to
look like a short to the speaker and thereby put the brakes on its
movement as it tends to ring on after excitation. The same feedback
lowers distortion at a given power level, or said another way increases
power output for a given level of distortion, but maximum power output
without regard to distortion is unaffected by the feedback. R-F
amplifier negative feedback likewise has no effect on the all-out
maximum power.

Best regards, Richard Harrison, KB5WZI

--
Floyd L. Davidson http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)