Owen Duffy wrote:
These are not necessarily the same value. In fact, the dynamic source
resistance is usually much higher than the required load resistance, and
the ratio is usually higher for a pentode or tetrode than for a triode
operating at the same voltage and current.

So, immediately, there is an apparent conflict with the proposition that
the dynamic source resistance and the load resistance are the same.

Does that take into account the step-down transformation?
The "source load" that results in the "source load line",
is not the physical load in the system. It is the physical
load in the system transformed by the transmission
line, the filters, the tank circuits, and the transformers.
In short, it is the transformed load seen directly
*by the source - at the source*.

For instance, a source may have a dynamic source
resistance of 1000 ohms. A 20:1 tank circuit
transformation takes it to 50 ohms. The load line
for that amp has a slope of 1000, not 50.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote in
t:

Owen Duffy wrote:
These are not necessarily the same value. In fact, the dynamic source
resistance is usually much higher than the required load resistance,
and the ratio is usually higher for a pentode or tetrode than for a
triode operating at the same voltage and current.

So, immediately, there is an apparent conflict with the proposition
that the dynamic source resistance and the load resistance are the
same.

Does that take into account the step-down transformation?

Cecil,

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.

The "source load" that results in the "source load line",

I don't really understand the concepts of a "source load" or "source load
line". Perhaps your meaning is the load in the anode circuit of the PA, I
will read on with that interpretation.

is not the physical load in the system. It is the physical
load in the system transformed by the transmission
line, the filters, the tank circuits, and the transformers.
In short, it is the transformed load seen directly
*by the source - at the source*.

Ok...


For instance, a source may have a dynamic source
resistance of 1000 ohms. A 20:1 tank circuit
transformation takes it to 50 ohms. The load line
for that amp has a slope of 1000, not 50.

I am not comparing apples with oranges, not comparing impedances on
different sides of the pi network.

To expand the first example with the details:
-if Ql is 10 and Ra is 1400 ohms, a 1% decrease in the extenal 50 ohm
load results in a 0.26% decrease in the anode load impedance

Rl=50, |Za|=1400.0;
Rl=49.5, |Za|=1396.4, a 0.26% decrease in |Za| for a 1% decrease in Rl.

You cannot think of a PI coupler (and the original post was discussing a
PI coupler) in this application as an idealised symmetric n:1
transformer, whilst this coupler has an apparent ratio of 28:1 (1400/50),
incremental impedance changes are in a quite different ratio.

A PI network is not in the general case symmetric, your example of a 20:1
"tank" circuit (and I would argue that "tank" is usually used to mean a
parallel tuned anode circuit, typically link coupled) is not symmetric
and the point of my post was to say that Zin/Zout is not a straight line,
and general analyses based on a fixed ratio are likely to be flawed.

Owen

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