Richard Clark wrote in
:

On Tue, 27 Feb 2007 01:07:12 GMT, Owen Duffy wrote:

Richard Clark wrote in
Let's treat this like the Chinese Box problem.

If you didn't know what the load was, could you explain it any
differently? No. Apriori knowledge is not a proof.

Richard, I content that:

Contend or offer in contention.

Richard

Yes, my spelling mistake.


- the power output of the PA; and
- the efficiency of the PA may be (and usually are) sensitive to the
load impedance.

This is not contending nor contention and is content only for a non
sequitur. The line following a tuner exhibits considerable loss (poor
efficiency) that can only occur on the basis of power and mismatch.
You yourself offered in other correspondence that it exceeds cable
attenuation specifications found only in a matching condition. To

I am being picky, but "it *may* exceed cable attenuation specifications
found only in a matching condition, it may also be lower". If I said it
as you stated, I made an error. The common statement (and I have no doubt
made it) that VSWR exacerbates line loss is actually wrong in the general
case. (Having Googled my own web site I see one statement along those
lines which needs further qualification!)

suggest that a PA's sensitivity is somehow exhalted in the face of
identical, ordinary behavior of a passive component is hardly
seperable. Consider the simple substitution to your quote:
- the power output at the terminus of the line; and
- the efficiency at the terminus of the line may be (and usually are)
sensitive to the load impedance.

I meant the output at the PA terminals where an lumped constant load
would be attached for comparison.


....

Though it is often asserted that the PA will get hotter as a result of
"reflected power" being dissipated in the dynamic output impedance of
the PA, and that this may / will damage the PA, the power explanation
doesn't work numerically in the general case.

Heat is the outward proof of power and is always demonstrable in both
specific and general cases. Occurrences of other, significant
radiation from the source (as long as that source physically occupies
a substantially minor region of wavelength) is exceedingly difficult
to achieve.

You don't offer a numerical proof of a general case, and given that
the general case must allow for the specific cases already allowed in
your discussion above - that may be an untenable assertion for you.
Those specific cases are demonstrably caloric and must follow the same
math you suggest.

I suspect you are trying to argue differences by degree (no pun
intended as to heat); but I seriously doubt you can produce the math
to do that. The arguments that flow from that involve what is called
source resistance, and those arguments are legion in this forum (where
naysayers embrace a refusal to accept or name ANY value - a curious
paradox and an engineering nihilism I enjoy to watch).

PAs can be designed to behave as an equivalent fixed voltage or current
source with fixed source impedance of Zo, but HF PAs are not usually
designed in that way.

I know that there is a vein of thought that the process of adjusting a PA
for maximum output always, somewhat magically, creates a match condition
where the source impedance is the conjugate of the load at the PA
terminals, but it is contentious. What of broadband PA designs with no
such adjustment, are they source matched over a broad range of
frequencies? Observations are that experiments to discover the source
impedance by incrementally changing load current can produce a range of
values for the same PA on different frequencies, and at different power
levels. Why do amplifiers with say tetrodes and triodes which exhibit
such different dynamic plate resistance but requiring the same load
impedance deliver the same equivalent source impedance?

I am also aware that supporters of the inherent source match position
assert that you must be selective in choosing tests for source impedance.
It is all rather unconvincing when only some of the implications of a
particular source impedance are effective.

It is my view that modelling the PA as a fixed voltage or current source
with fixed source impedance of Zo, and where reflected waves on a
transmission line are absorbed by the matched source is not a good
general model for HF PAs.

The application of small signal analysis to amplifiers that sweep from
near cutoff to near saturation is suspect.

I believe that it is sound (in the steady state) to resolve the forward
and reflected wave voltages and currents at the source end of the
transmission line, calculate the complex impedance, and predict the
effects of that impedance as a PA load using the same techniques that
were used to design the PA.

Owen