On 12 jun, 15:13, Richard Fry wrote:
It has been theorized that a circuit consisting of a Class C vacuum-
tube r-f amplifier using a tuned tank circuit in its output network
provides an operational “non-dissipative source resistance” of 50 ohms
for energy present at the output connector of the transmitter.

However the information and measured data provided in the text
excerpts below is not very supportive of that theory.

[deleted]

Hello Richard Fry,

First, I use your last name also to avoid confusion with Richard
Clark.

As you may know, I have also doubts about the conjugated match issue.
So I am happy with this posting. It is not that I dispute the
conjugated match condition itself, but whether or not the conditions
are met. In my opinion, required conditions aren't met in many cases,
so output impedance of many amplifiers is not equal to the load
resistor.

To avoid discussions about measurements and traceable measurement
instruments/procedures, I did some simulations on a valve amplifier
and a class C mosfet circuit (that actually isn't operating in class C
mode). Everyone can do these simulations at their own PC en get an
opinion. I also described the difference frequency method that is
easily implemented in simulation and gives the output VSWR almost
directly without complex calculus. The link is: www.tetech.nl/divers/PA_impedance.pdf.

Except for the condition of matching to maximum power output given
certain drive, all output VSWR of the amplifiers is far from expected
load impedance. Just a little voltage saturation (for example to get
better efficiency) did drop the plate impedance below the required
load impedance (voltage source behavior). When reducing the drive, the
plate impedance rises rapidly above the load impedance (current source
behavior).

So power entering the output of the amplifier will mostly be reflected
back into the cable, unless you specially design for output impedance.
I know, this is disputed by some members of this group, but therefore
I did the simulations and presented the results.

I specially took "amateur" examples to avoid comment that I only use
"exotic" examples. I did not copy results from professional
activities, so I will not run into problems. The circuits are simple
to ease reproduction in any spice based simulator.

I dispute:
"If the source impedance were equal to
the fifty ohm transmission line impedance, half of the transmitter's
output power would be dissipated in its internal output source
impedance."

A real class C amplifier with very small conduction angle has
efficiency 50% when optimally tuned. When it operates at the
transition of current/voltage saturation is can show 50 Ohms output
impedance for very small change in load impedance. But as soon as the
load mismatch is above about VSWR = 1.05…1.1, output VSWR/impedance of
the amplifier changes rapidly. So practically spoken, most high
efficient amplifiers have high output VSWR (or bad Return Loss), but
theoretically it can be 50 Ohms (for example). If there are doubts I
will add a special simulation example for this.

Best regards,


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