On Wed, 08 Dec 2004 15:51:07 GMT, "Frank"
wrote:
Maximum power transfer with conjugate matching is undisputed. The problem
with semi-conductor devices is that you cannot necessarily conjugate match
because the device operating parameters may be exceeded.
Hi Frank,
If we return this from the ethereal landscape of sub meter
wavelengths, back to the point of Bob's measurements at HF, and lately
the MF; then matching and issues of the final are trivial with a
million examples in the market today.
I know you would probably like to get to the nut of this, and it will
return you to Motorola's AN1526. This work contains much of the
language offered by correspondents here (in times past), but through
the rather gauzy filter of their memory. Usually they couch the
disassociation of Source Z to a transistor through poor context (in
other words, not reading the entire subject, but just a phrase).
There is the presumption these posters embrace:
"They consider the best match is achieved by a simultaneous
conjugate match of the input and output. However, power amplifiers
provide higher power gain and better efficiency at the rated
output power if the output is purposely mismatched. An added
benefit of doing this is potentially unstable devices, conjugately
matched, can be operated stably under these more optimum
mismatched conditions."
This is the usual mistake of misattribution between the distinctions
of a Conjugate Match, and a Z Match. However, you will note that
here, and elsewhere in the reference, that no one denies the Source
has a Z, and it is significant (all within values I've offered) and
that it is still closely held to the expected load (later I will show
exactly held).
Another dismissal offered by posters is the supposed invalidity or
inaccuracy of the load-pull method (which I find curious after having
calibrated active loads suited for just this purpose). I will turn
again to this same reference:
"Although the technique has been known for some time, the
widespread availability of desktop computers and automatic tuning
systems is just now making this method more attractive,
particularly for higher power devices. The characterization
process is conceptually quite simple."
Then there is the subject of S parameters, which is introduced early
by Motorola with this admonition:
"Many first time RF power designers, brought up on a diet
of small–signal s–parameters, previously used for solving
small signal text book problems, assume these same
techniques are applicable to bipolar class–C and class–AB
power amplifier design."
This selection actually introduces the presumption above. We have one
poster here that violates this admonition with abandon - but with
regard to transmission lines.
When the poster pines further for a Large Signal S parameters:
"However, the authors are not aware of these parameters
being used successfully above a few watts of output power."
When Motorola actually gets down to design:
"The load line resistance is the optimum load impedance
for the internal collector node of the transistor, neglecting
the junction and parasitic device capacitance."
What a concept! Same as before, Same then, Same now.
The ONLY contretemps revealed by this tempest in a teapot is the
forced conclusion that a conjugate match was required (a common
mistake of not knowing the difference between Conjugate Matching and Z
Matching) which was in turn driven by higher frequency operation (much
of this is couched in the 900 MHz band) and parasitics already noted
above. The final and most compelling admission from Motorola is found
with their statement:
"It is up to the device designer to choose which
impedance gets published. One is just as valid as the other.
However, quite frankly, gain is what sells devices."
And of course this discussion will do nothing with those who utter
"you are not going to change my mind." ;-)
To be continued - no doubt.
73's
Richard Clark, KB7QHC
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