This is long, wordy! Delete NOW if of no interest.
Note: apologies for my errant post of an incomplete
draft of the following note, here is what I had intended
to post when complete. The miss-post also retained the
subject about low vswr which I also intended to change
before posting, Jim Reid, KH7M
Now my thoughts about the topic:
Dr. Jerry wrote, in part:
> There is no conjugate match to the output of any active
> source.....
> All those that demand conjugate matching through to the
> active device are wrong.
> Its given the more maddening appearance of there being
> a conjugate match because of the way RF solid state devices
> are measured. Commonly they are installed in a test jig, and
> the universal matching network is adjusted for maximum power
> output (a side effect on conjugate matching of no time varying
> components) and then the test jig is split and the Z shown to
> the active device is measured and its conjugate declared the
> output Z of the device. It is NOT. The impedance measured is
> the load the allows the active device to produce maximum
> power (within distortion and efficiency and gain limits). For circuit
> designers, it is convenient to say the output impedance of
> the device is r + jx and the load then must r - jx to be a
> conjugate match but that's an improper statement of fact.
> Its plain wrong, but it makes circuit designers produce output
> networks that supply the appropriate load for the device.
>
> During any one cycle of RF the output Z of the active device
> varies from open to nearly shorted and that's nothing that
> can be matched by a constant impedance. Any time the
> active device is a constant impedance, its not producing any
> output, its output Z MUST VARY to produce output.
>
> But if the load Z is set for the available voltage swing to produce
> the desired output power, the power supply is saved and the
> power output is controlled.
Now, if only Bruene had been able to state what Dr. Jerry has
given us in the above so clearly....... Instead, Bruene set about
to annihilate Walt Maxwell's work back in 1991, in his QST article:
"The RF Power Amplifier and the Conjugate Match",
W. Bruene, QST, November, 1991.
So, Dr. Jerry has expressed his and Bruene's view. Now here
is the explanation by Walt Maxwell:
" I realized that much more measured data was needed to prove
that output resistance of the pi-network of the amplifier is exactly
equal to the load impedance after the network is adjusted to
deliver all the available power. Using a standard IEEE procedure
I took many more measurements that indeed prove conclusively
that the network output resistance and the load resistance are
equal when the network is adjusted as stated above"
Sounds very similar to what Dr. Jerry describes above. I believe
the problem between these men is one of the use of words and
language to describe what they are doing/talking about. However,
Maxwell has attempted to again describe his non-dissipative
resistance in his claim of an actual conjugate match:
"We now return to clarify the misconception concerning output
and load resistances. The term source resistance RS of an RF
power amplifier, as is often misused (and confused with RP) in
referring to the source of RF power delivered by Class B and C
amplifiers, reveals still another prevalent misconception. This
misconception is that the entire source of power in these
classes of tube-type amplifiers is a dissipative resistance.
In clarifying this misconception, we will use the example by
Terman to demonstrate that the source of RF output power
in a Class C amplifier is the combination of two resistances;
a non-dissipative resistance related to the characteristics of
the effective load line, and a dissipative plate resistance RPD.
Resistance RPD is not plate resistance RP, as determined
from the well-known expression RP = DEP ¸ DIP. From this
expression it is evident that RP is the result of a small change
in plate current due only to a change in plate voltage, which is
not the source of power in RF power amplifiers as is claimed
by many who have misinterpreted the expression. The source
of power is actually derived by a large change in plate current
resulting from a change in grid voltage.
One portion of the non-dissipative resistance is the reciprocal
of the total conductance from both plate and power supply to the
input of the pi-network tank circuit. At that point in the typical
amateur pi-net Class B and C amplifier, the load is the tank input.
The source is the combination of two parallel conductive paths to
the tank: 1) the blocking capacitor in series with the active device,
the tube(s)3, and 2) the same blocking capacitor in series with the
RF choke and the voltage of the power supply. These two
conductance paths are paralleled at the input of the tank,
operating at different, but overlapping times throughout the
cycle. The other portion of the non-dissipative resistance is
related to the operating load line, which will be discussed in
Sec 19.3a.
Plate resistance RPD is dissipative, whose value is determined
by the power PD dissipated as heat by the plate divided by the
square of the average DC plate current IDC, the current measured
by the DC plate ammeter. Note in Terman's Statement 3 below,
that dissipated power PD is the product of the instantaneous
plate-to-cathode voltage and the instantaneous plate current.
We know that energy is transferred from the plate circuit of the
amplifier to the pi-network by periodic pulses of plate current that
flow during the conducting portion of the RF cycle. Knowledge
of the non-dissipative portion of the source resistance will allow
you to understand why Class B and C amplifiers can deliver all
of their available power into a conjugately matched load with
efficiencies greater than 50 percent. This concept is important,
because the ability of these amplifiers to be conjugately matched
has been incorrectly disputed due to three erroneous assumptions
that have caused many amateurs and engineers alike to be misled."
So, are these two differing "explanations" antagonistic or
complementary? To me, both make sense.
73, Jim, KH7M
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