"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