On Apr 2, 3:03 pm, Owen Duffy wrote:
Cecil Moore wrote . net:

Owen Duffy wrote:
Cecil Moore wrote:
Does that take into account the step-down transformation?

The two previous paragraphs that you have omitted in your quote
provide the context for the paragraphs that you did quote. The
context is in the anode circuit of the PA being discussed.

I'm in the process of moving and am having a hard time
keeping up.

If the amplifier were a class-A amp with a 50 ohm
load resistor driving a 50 ohm load, would what
you say still be true?

I don't understand "a 50 ohm load resistor driving a 50 ohm load".

The transformation issue pertains to the PI coupler, you cannot treat a
PI coupler in the general case as an idealised symmetric n:1 transformer.
It certainly isn't in a typical single ended RF linear amplifier.

Owen


A class A RF amplifier can certainly be fed its DC through an RF
choke, just as is done with other classes. There's no need to limit
the discussion to class A.

If you put a resistance Rshunt in parallel with the plates (or
collectors or drains), at the plates, such that the plate resistance,
Rplate, in parallel with Rshunt equals the load presented by the
output network to the plate circuit, then the source impedance seen at
the output terminals will be the same as the load impedance. That may
be a little confusing...let me put it differently. Consider an output
passive, linear network with two ports, the Plate port and the Load
port. When the Load port is loaded with Zload, the rated load
impedance, the Plate port presents an impedance to the plates, call it
Zpnetwork. If you put an additional load at the plates such that the
Plate port of the network "sees" an impedance equal to Zpnetwork
looking toward the plates, then when the network is connected to the
plates and that additional load, you will "see" a source impedance
equal to the conjugate of Zload looking back into the network's Load
port.

For example, let's say that we have a 6000 ohm plate resistance, and a
4000 ohm resistor we put in parallel with the plates (put it shunt
across the plate DC feed RF choke which is considered to be
essentially infinite impedance). The net resistance looking into that
is 2400 ohms. Assume a load of 50+j50 ohms. Assume an output network
that, when loaded with 50+j50 ohms, transforms that to 2400 ohms,
resistive. Then the impedance looking back into the output port of
the output network will be 50-j50 ohms. It doesn't matter if it's a
pi network, a filter, or a 81.52 degree long piece of 342.73 ohm
"lossless" transmission line.

But if the goal is to deliver as much clean RF power to the external
load as you can, why would you put an RF-dissipating resistor into
your amplifier?

Cheers,
Tom