"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