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