"Roy Lewallen" wrote in message
news:JrCdnR01Yp20tNHVnZ2dnUVZ_sednZ2d@easystreeton line...
There's a common misconception that, for a linear circuit, the maximum
efficiency and/or power available from a voltage source occurs when the
source resistance equals the load resistance (or, more generally, when
they're complex conjugates). But this isn't universally true, as I'll
show with a simple example.

Suppose we have a 100 volt perfect voltage source in series with a
variable source resistance, and a fixed load resistance of 100 ohms. If
we make the source resistance 100 ohms, the source delivers 50 watts, 25
of which are dissipated in the source resistance and 25 watts in the
load. The efficiency, if you consider the source resistance dissipation
as wasted, is 50%. But what happens if we reduce the source resistance
to 50 ohms? Now the source delivers 66.7 watts, of which 22.2 is
dissipated in the source resistance and 44.4 in the load resistance. The
power to the load has increased, and the efficiency has increased from
50 to 66.7%. The efficiency and load power continue to increase as the
source resistance is made smaller and smaller, reaching a maximum when
the source resistance is zero. At that point, the source will deliver
100 watts, all of which is dissipated in the load, for an efficiency of
100%.

The well known and often misapplied rule about maximizing power transfer
by matching the source and load impedances applies only when you're
stuck with a fixed source resistance and can only modify the load.

Roy Lewallen, W7EL

What seems to be overlooked here is that the source resistance at the output
terminals of the pi-nework in Class B and C amplifiers is non-dissipative, which
is the reason they can be loaded for delivering all available power for a given
grid drive, and still have efficiencies greater than 50 percent. One of the
myths circulated for years, and still prevelant, is that the reason for Class B
and C amps to have efficiencies greater than 50 percent is that the load
resistance must be greater than the source resistance. Tain't so.

I've proved the above to be true with extensive measurements using laboratory
grade instruments. Reports on those measurements are reported in Chapter 19 in
Reflections 2, and additional measurements taken after Reflections 2 was
published are reported in Chapter 19A, to be published soon in Reflections 3.
This additional chapter is listed here in the rraa for your information. If
anyone is interested in reading Chapter 19 in Reflections 2 it appears in my
website at www.w2du.com.

Walt, W2DU

"Walter Maxwell" wrote
What seems to be overlooked here is that the source resistance at the
output
terminals of the pi-nework in Class B and C amplifiers is non-dissipative,
which
is the reason they can be loaded for delivering all available power for a
given
grid drive, and still have efficiencies greater than 50 percent. One of
the
myths circulated for years, and still prevelant, is that the reason for
Class B
and C amps to have efficiencies greater than 50 percent is that the load
resistance must be greater than the source resistance. Tain't so.
____________

Walt - what is your thinking on the point that untuned, solid-state
amplifiers also can have PA DC-to-RF power conversion efficiencies of 70% or
more at the device level?

In fact the solid-state, analog FM broadcast transmitters supplied by Harris
Corporation and others need no tuning to produce their rated output power
into a 1.3:1 SWR or less, anywhere in the FM broadcast band 88-108 MHz.
Even the harmonic filter needs no changes, and maintains harmonics at -80
dBc or better. They are frequency agile, and can be reset from one carrier
frequency to another, anywhere in the FM band with a transition time of a
few seconds

The overall AC input to r-f output efficiency of these transmitters exceeds
60% (includes the exciter, control system, IPA, and cabinet fans).

RF

Richard Fry wrote:
"Walter Maxwell" wrote
What seems to be overlooked here is that the source resistance at the
output
terminals of the pi-nework in Class B and C amplifiers is
non-dissipative, which
is the reason they can be loaded for delivering all available power
for a given
grid drive, and still have efficiencies greater than 50 percent. One
of the
myths circulated for years, and still prevelant, is that the reason
for Class B
and C amps to have efficiencies greater than 50 percent is that the load
resistance must be greater than the source resistance. Tain't so.
____________

Walt - what is your thinking on the point that untuned, solid-state
amplifiers also can have PA DC-to-RF power conversion efficiencies of
70% or more at the device level?

In fact the solid-state, analog FM broadcast transmitters supplied by
Harris Corporation and others need no tuning to produce their rated
output power into a 1.3:1 SWR or less, anywhere in the FM broadcast band
88-108 MHz. Even the harmonic filter needs no changes, and maintains
harmonics at -80 dBc or better. They are frequency agile, and can be
reset from one carrier frequency to another, anywhere in the FM band
with a transition time of a few seconds

The overall AC input to r-f output efficiency of these transmitters
exceeds 60% (includes the exciter, control system, IPA, and cabinet fans).

RF

I know this question was directed to Walt, but I'd like to mention that
I've designed and built solid state class C amplifiers at the 5 - 10
watt level which have measured efficiencies of greater than 85%.

Roy Lewallen, W7EL

"Richard Fry" wrote in message ...
"Walter Maxwell" wrote
What seems to be overlooked here is that the source resistance at the
output
terminals of the pi-nework in Class B and C amplifiers is non-dissipative,
which
is the reason they can be loaded for delivering all available power for a
given
grid drive, and still have efficiencies greater than 50 percent. One of
the
myths circulated for years, and still prevelant, is that the reason for
Class B
and C amps to have efficiencies greater than 50 percent is that the load
resistance must be greater than the source resistance. Tain't so.
____________

Walt - what is your thinking on the point that untuned, solid-state
amplifiers also can have PA DC-to-RF power conversion efficiencies of 70% or
more at the device level?

In fact the solid-state, analog FM broadcast transmitters supplied by Harris
Corporation and others need no tuning to produce their rated output power
into a 1.3:1 SWR or less, anywhere in the FM broadcast band 88-108 MHz.
Even the harmonic filter needs no changes, and maintains harmonics at -80
dBc or better. They are frequency agile, and can be reset from one carrier
frequency to another, anywhere in the FM band with a transition time of a
few seconds

The overall AC input to r-f output efficiency of these transmitters exceeds
60% (includes the exciter, control system, IPA, and cabinet fans).

RF

Hello Richard,

Sorry, Richard, I have no knowledge of solid-state untuned amps, so my thinking
on them is zero, nada. As you'll note, all of my discussion on the subject
concerns only tube amps with a pi-network output, and I've specifically stated
these conditions.

If you've read Chapter 19 and its addition as Chapter 19A, do you agree with my
position that the output resistance at the output terminals of the pi-network is
non-dissipative?

Walt, W2DU