Frank wrote:
If you remember, Motorola used to publish Smith charts of the output
impedance for their power amplifier devices. Talking to one of
Motorola's design engineers; I asked "How do you derive these Charts".
His answer was; "We use a matching network and adjust it for the
required output power, then measure the input impedance of the network.
The complex conjugate of this impedance is then defined as the source
Z". The fact is these data are not the actual source Z of the device,

I had heard that also. For a typical VHF/UHF device, the manufacturer's
application engineers use an infinitely adjustable stub tuner to explore
the whole range of possible load impedances presented TO the device.

As well as measuring output power, the application engineer also has to
think about maximum voltage and current ratings, chip and bond wire
temperatures, and also IMD performance if the device is going to be
specified for linear operation.

The application engineer adjusts the load impedance to give the optimum
balance of all these factors, at a series of test frequencies. No
problems whatever about that.

The only technical issue is the *assumption* that the conjugate of the
load impedance is equal to the output impedance of the device. Most
manufacturers now tend to avoid that assumption, because it is a totally
unnecessary distraction for the transmitter designer who has to use the
device.

All the designer has to do is create an output network that presents the
manufacturer's recommended load impedance TO the device. This network
replicates the impedance transformation of the original stub tuner
setup, but uses mostly fixed components for obvious practical reasons.

Apart from a very few special applications where reverse termination is
important to avoid ghosting and similar effects, the transmitter
designer doesn't have to think about the device's output impedance at
all.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White, G3SEK wrote:
"The only technical issue is the "assumption" that the conjugate of the
load impedance is equal to the output impedance of the device."

It`s true if maximum power is being transferred.

King, Mimno, and Wing sat so on page 43 of "Transmission Lines,
Antennas, and Wave Guides":

"If a dissipationless network is inserted betweeen a constant-voltage
generator of internal impedance Zg, and a load of impedance ZR such that
maximum power is delivered to the load, at every pair of terminals the
impedances looking in opposite directions are conjugates of each other."

The authors, Arnie, Larry, and Alex were all teaching at Harvard in 1945
when their book was published.

Walter Maxwell, W2DU has been saying the same thing yet has mistaken
nay-sayerrs.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Ian White, G3SEK wrote:
"The only technical issue is the "assumption" that the conjugate of the
load impedance is equal to the output impedance of the device."

It`s true if maximum power is being transferred.

King, Mimno, and Wing sat so on page 43 of "Transmission Lines,
Antennas, and Wave Guides":

"If a dissipationless network is inserted betweeen a constant-voltage
generator of internal impedance Zg, and a load of impedance ZR such that
maximum power is delivered to the load, at every pair of terminals the
impedances looking in opposite directions are conjugates of each other."

The authors, Arnie, Larry, and Alex were all teaching at Harvard in 1945
when their book was published.

Walter Maxwell, W2DU has been saying the same thing yet has mistaken
nay-sayerrs.

Best regards, Richard Harrison, KB5WZI

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.

73,

Frank

Frank wrote:
"Richard Harrison" wrote in message
...
Ian White, G3SEK wrote:
"The only technical issue is the "assumption" that the conjugate of the
load impedance is equal to the output impedance of the device."

It`s true if maximum power is being transferred.

King, Mimno, and Wing sat so on page 43 of "Transmission Lines,
Antennas, and Wave Guides":

"If a dissipationless network is inserted betweeen a constant-voltage
generator of internal impedance Zg, and a load of impedance ZR such that
maximum power is delivered to the load, at every pair of terminals the
impedances looking in opposite directions are conjugates of each other."

The authors, Arnie, Larry, and Alex were all teaching at Harvard in 1945
when their book was published.

Walter Maxwell, W2DU has been saying the same thing yet has mistaken
nay-sayerrs.

Best regards, Richard Harrison, KB5WZI

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.

Exactly.

Richard's assertion relies on at least three things being true:

1. That maximum power is being transferred - for most states of
transmitter tuning, loading and drive levels, that is obviously *not*
true.

2. That the transmitter can be accurately represented as a
"constant-voltage generator of internal impedance Zg", i.e. as a
Thevenin source.

3. That as part of #2, Zg is a constant.

With all due respect to Richard - and above all, respect to Walt - it is
a tall order to prove that all three of those requirements for conjugate
matching are being met. I believe they can only be exactly met under a
few very special sets of operating conditions.

But it then follows that, for all *other* operating conditions, the
complete end-to-end conjugate matching referred to by King et al does
*not* exist.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White, G3SEK" wrote in message
...
Frank wrote:
If you remember, Motorola used to publish Smith charts of the output
impedance for their power amplifier devices. Talking to one of Motorola's
design engineers; I asked "How do you derive these Charts". His answer
was; "We use a matching network and adjust it for the required output
power, then measure the input impedance of the network. The complex
conjugate of this impedance is then defined as the source Z". The fact is
these data are not the actual source Z of the device,

I had heard that also. For a typical VHF/UHF device, the manufacturer's
application engineers use an infinitely adjustable stub tuner to explore
the whole range of possible load impedances presented TO the device.

As well as measuring output power, the application engineer also has to
think about maximum voltage and current ratings, chip and bond wire
temperatures, and also IMD performance if the device is going to be
specified for linear operation.

The application engineer adjusts the load impedance to give the optimum
balance of all these factors, at a series of test frequencies. No problems
whatever about that.

The only technical issue is the *assumption* that the conjugate of the
load impedance is equal to the output impedance of the device. Most
manufacturers now tend to avoid that assumption, because it is a totally
unnecessary distraction for the transmitter designer who has to use the
device.

All the designer has to do is create an output network that presents the
manufacturer's recommended load impedance TO the device. This network
replicates the impedance transformation of the original stub tuner setup,
but uses mostly fixed components for obvious practical reasons.

Apart from a very few special applications where reverse termination is
important to avoid ghosting and similar effects, the transmitter designer
doesn't have to think about the device's output impedance at all.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

It seems everybody is in agreement with the fact that you cannot match a
power source to the load. As Ian states: "nobody cares, what the device
output parameters are, only that it is capable of delivering the required
power to a desired load". This still leaves the question unanswered as to
"What is the actual device S22"? I have read that "With HF linear devices
the large signal S parameters are close enough to the small signal values".
For non-linear devices load-pull techniques are used. I have never seen
high power transistors characterized with S parameters, but have not worked
with such designs for a number of years, so am probably out of touch. A
tentative search of the web did not find any info. It seems TRW and
Motorola are pretty much out of the power semi-conductor industry.

I am tempted to synthesize a transmission line based on the per-unit length
parameters, and see how the load power varies as a function of source Z. It
seems to me the only important factor is the applied voltage. The input
impedance is only a complex number. The transmission line could be
considered a "Singly terminated network", the synthesis of which is trivial,
and performance independent of source Z. I have trouble with the concept of
"Reflection"; how can charges (electrons) flow in both directions
simultaneously. Charge flow results from the E field within the conductor.

73,

Frank

On Tue, 07 Dec 2004 18:57:25 GMT, "Frank"
wrote:

|Hi Richard,
|
|With solid state power amplifier design; the criteria was always that you
|must present an impedance, to the output devices, such that the desired
|output power is delivered to the load (while not exceeding device
|dissipation). Any attempt to optimally match the load to the source
|impedance will result in over-dissipation, and probable destruction of the
|source device -- probably by excess collector/drain current. If you
|remember, Motorola used to publish Smith charts of the output impedance for
|their power amplifier devices. Talking to one of Motorola's design
|engineers; I asked "How do you derive these Charts". His answer was; "We
|use a matching network and adjust it for the required output power, then
|measure the input impedance of the network. The complex conjugate of this
|impedance is then defined as the source Z". The fact is these data are not
|the actual source Z of the device, but are probably considerable higher.

The data were useful as presented. When using a Smith chart for
matching network design, the published data could be used as the
"starting point" for the network and the rotations were made toward
the load; the opposite from the usual case of matching a load to a 50
ohm source.

This method was really an early example of load pull characterization.
Maury Microwave app. note 5C-041 is one reference for this. Another
is "A New Load Pull Measurement Technique Eases GaAs
Characterization", Microwave Journal, Nov, 1980, pp. 63-67.

|I don't remember anybody actually trying to measure the large signal S
|parameters of solid state devices.

|I seem to remember that tube amplifiers were designed based on the source
|impedance calculated as 2Vp/Ip, (Where Vp is the plate voltage, and Ip the
|plate current), and have no idea how, or if, it relates to the actual source
|Z of the device.
|Anyway, I am not convinced that source Z is important.
|Where I think some confusion may have come from is Hewlett Packard's 12 term
|error correction analysis derived for vector network analyzers. Here source
|Z is important because measurements are made in both directions.

It's not the error correction that is confusing. The error correction
simply removes systematic errors from the measurement(s). The
parameters usually measured when 12-term correction is called for are
the small-signal S-parameters. If I understand you correctly, "source
Z" is actually the output reflection coefficient (s22), the signal
exiting port 2 due to an input to port 2.

The amplifier output reflection coefficient can be very important,
even in high power amplifiers, when non-dissipative filters are used
for harmonic or spurious rejection. Such filters are commonly
specified and measured in 50 ohm systems and function by reflecting,
not dissipating, the out-of-band energy. When driven by other than a
50 ohm source the rejection will be other than what is measured in a
matched condition.

Frank wrote:
I have trouble with the concept of
"Reflection"; how can charges (electrons) flow in both directions
simultaneously.

Have you ever stood on a cliff overlooking the ocean and seen
ocean waves rolling in and smaller waves rolling back out? The
smaller waves rolling back out to sea are reflections of the
large waves incident upon the shore. The small outflowing wave
meets a large incoming wave and seems to disappear, only to emerge
on the ocean side of the large wave with its identity still intact.

If ocean waves can flow both directions using the same H2O carriers,
why would anyone have difficulty in accepting EM waves flowing
in both directions using the same electron carriers? The energy in
the ocean waves travels much faster than the water molecules. The
energy in an EM wave travels much faster than the electrons.

Ever played with a long rope fastened at one end? You can send a
wave down the rope and receive a reflected wave. If you time it
just right, you can have a forward wave and a reflected wave
meet in the middle of the rope and be unaffected by each other
as long as things remain linear. A forward EM wave has no effect
on a reflected EM wave and vice versa as long as things remain
linear.
--
73, Cecil http://www.qsl.net/w5dxp

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

Frank, WE6CB wrote:
"Maximum power transfer with conjugate matching is undisputed."

Great! W2DU has made progress.

Frank also wrote:
"The problem with semi-conductor devices is that you cannot necessarily
conjugate match because the device operating parameters may be
exceeded."

True for certain voltages, currents, and drive. The maxima may not all
be acheived simultaneously. It`s true for vacuum tube amplifiers too.
The total device dissipation can`t be exceeded without reduced life
expectancy.

Another caution is the difinition of "maximum available power". In an RF
amplifier this is specific to a certain set of operating conditions.
Maxium available power is with fixed B+ (or minus) voltage and drive.
Current, too, is limited to non-destructive values.

Terman says on page 76 of his 1955 edition:
"Alternatively, a load impedance may be matched to a source of power in
such a way as to make the power delivered to the load a maximum. (The
power delivered to the load under these conditions is termed the
avalable power of the power source.)"

Best regards, Richard Harrison, KB5WZI

Frank wrote:
"I have trouble with the concept of "Reflection", how can charges
(electrons) flow in both directions simultaneously."

The wave, or signal flowing in one direction is distinctly different
from that flowing in the opposite direction. Upon reflection, the phase
between current and voiltage prodiuced by the wave is inverted. Only
phase of the volts or amps is inverted by reflection, not both. The
phase relation between volts and amps is the key to the direction the
wave is traveling on the line. It`s the wave which travels. The volts
and amps are generated by the wave traveling on the line. The line is
just guiding the wave.

The traveling forward and reflected waves, traveling in opposite
directions on the same line, produce the familiar standing wave patterns
through superposition of volts and amps. Transmission lines and the
appurtenances used with them have no problem keeping values associated
with the waves straight with proper design. A directional wattmeter can
separate the two directions of travel wery well indeed. It knows one
direction from the other by whether the volts and amps are in-phase or
out-of-phase.

Best regards, Richard Harrison, KB5WZI