Hello Richard,

You asked for what it IS, several times, so you virtually left me no
choice.

Especially the first one (Zout = 9 ohms where the load is 16 Ohms,
RCout = -0.28) is very well documented as it was designed to have this
value. Without the extra measures, the value was below 9 Ohms. The
amplifier is time limited fully short circuit proof, so it was not
difficult to determine Zout at various loads. I put myself into
problems when publishing the documentation over here.

When we go back to my first posting in this thread (the first reply)
where I stated that amplifiers do not show 50 Ohms in general, is not
that strange and doesn't violate Walt's conclusions.

When I understand it well, Walt assumes tuning for maximum output at a
given drive (not necessarily the maximum drive). As mentioned earlier,
I support his findings.

When you don't touch the plate and load capacitor and change from 50
Ohms to a load with VSWR = 2, you violate Walt's conditions and the
amplifier will no long behave as a 50 Ohms source, hence what happens
with the 50W reflected power from JC's posting becomes more
complicated then just a transmission line problem.

Walt made a statement in this thread that an amplifier that obeys ZL =
Zout can have higher then 50% efficiency. I fully support this
statement also (Example Class C amplifier with narrow conduction angle
at the edge of current/voltage saturation, with efficiency over
75%). However, change the load and you get a complete other
situation. You have to retune to get back to Walt's condition.

From my experience, many devices with conventional amplifiers (like
the valve with pi-filter, or single transistor stages) are now being
replaced by other topologies where ZL Zout under specified load (as
these types of amplifiers are not optimized for maximum gain, but for
maximum power added efficiency). I think Tom, KT7ITM also have this
experience.

The example given by JC (huge reflection) and my own experience with
conventional and high efficiency topologies resulted in my statement
that a PA is not a 50 Ohms source. Maybe I had to be more precise to
mention the exceptions: forward power control loop as mentioned by
Roy, -tuning to maximum output after each change of load, -specially
designed negative feedback, -adding an attenuator. The reader can now
determine the category for his PA.

In case of doubt use the forward power measurement technique, when it
changes under varying load, your amplifier doesn’t behave as a 50 Ohms
source.

Best Regards,


Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me