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?
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote in
t:

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

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

K7ITM wrote:
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?

That's the first amplifier that is taught in EE 202.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote in news:IUfQh.24090$uo3.16335
@newssvr14.news.prodigy.net:

That's the first amplifier that is taught in EE 202.

I missed the relevance of the class A amplifier.

The example that I worked up in the original post was for a design anode
load of 1400 ohms, using a practical PI coupler to a 50 ohm external load.
It is theoretical treatment of the kind of coupler circuit that you would
expect to be in the transmitter for which Walt reported his detailed
measurements.

Owen

On Apr 2, 3:58 pm, Cecil Moore wrote:
K7ITM wrote:
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?

That's the first amplifier that is taught in EE 202.
--
73, Cecil http://www.w5dxp.com


You haven't moved beyond that "first amplifier that is taught in EE
202"?

If your 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

K7ITM wrote:
You haven't moved beyond that "first amplifier that is taught in EE
202"?

If one can understand the simple amplifier then one
can move on to a more complicated amplifier.

If your 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?

My goal is not to deliver as much power as possible.
My goal is to understand the nature of the source
starting with the simplest one.
--
73, Cecil http://www.w5dxp.com

On Apr 2, 8:52 pm, Cecil Moore wrote:
My goal is to understand the nature of the source
starting with the simplest one.

Are you sure? In other threads you consistently refuse
to analyse the simplest of sources on the basis (as far
as I can tell) that it is not the 'real world'.

In light of your new approach, which I wholeheartedly
endorse, perhaps you will reconsider your response in the
other threads and try to "understand the nature of the
source starting with the simplest one".

....Keith

On Apr 2, 4:52 pm, Cecil Moore wrote:
K7ITM wrote:
You haven't moved beyond that "first amplifier that is taught in EE
202"?

If one can understand the simple amplifier then one
can move on to a more complicated amplifier.

If your 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?

My goal is not to deliver as much power as possible.
My goal is to understand the nature of the source
starting with the simplest one.
--
73, Cecil http://www.w5dxp.com

If your goal "is to understand the nature of the source starting with
the simplest one," why would you add resistors you don't need and make
it more complicated than the simplest one? What if "the simplest one"
turns out to lead you into believing generalities that are not true,
when considering a more general one will avoid that?

My goal, in the context of Owen's basenote, remains to deliver as much
clean RF power to the external load as I can. Unnecessary resistors
need not apply. Matching networks better pass muster with respect to
their performance not only at fundamental frequencies, but also at
others, especially at harmonics. Not all the networks I've posted
about in this thread do pass muster, but are enlightening with respect
to Owen's observations, I believe. Simplest doesn't remain
interesting for very long.

FWIW, I don't see anything in Owen's postings in this thread that
_precludes_ a source impedance that's equal to some particular load
impedance, or to its conjugate. Rather, I see a suggestion that the
source impedance does not necessarily have to be equal to any
particular value, and in the general case does not have to be equal to
the conjugate of the design load impedance. With that I agree. I've
seen a great many examples of it. I gave a few of them earlier. I've
also worked on the design of broadband RF amplifiers which are
designed specifically to be 50 ohm resistive sources, through the use
of feedback to set that impedance. You don't need brute-force
resistors to do it; most of the time, you don't need to do it anyway,
but in the case of instruments used for measurement, it can be
important. In the case of video amplifiers where ghost-causing
reflections are to be kept to a minimum, it can be important. In the
case of a ham narrow-band SSB, CW, FSK, FM or AM transmitter, I
question whether the source impedance is ever important, or is ever
accurately known. Perhaps someone can convince me otherwise, though a
well-thought-out, well-presented example.

Cheers,
Tom