On Jun 12, 9:44*am, Wimpie wrote:

A real class C amplifier with very small conduction angle has
efficiency 50% when optimally tuned.

Would that not be evidence that the amplifier source impedance
necessarily is lower than the load impedance?

When it operates at the
transition of current/voltage saturation is can show 50 Ohms output
impedance for very small change in load impedance. But as soon as the
load mismatch is above about VSWR = 1.05…1.1, output VSWR/impedance of
the amplifier changes rapidly.

For some additional input -- I have taken part in factory tests of
high-power Class C single-tube/tuned cavity FM broadcast transmitters
driving 50-ohm test loads measured to have 1.03VSWR, showing a DC
input to r-f output efficiency of the PA to be in excess of 80% --
including the loss in the harmonic filter. Load power was measured
using calorimetric methods. In fact, 80% PA efficiency is the
published spec for this transmitter line as long as load VSWR relative
to 50 ohms is 1.7:1 or less (any phase angle).

Those results don't appear to be fitting very well with the idea that
the tx source impedance can be ~50 ohms for loads with low VSWR.

RF