>> In one of my electronics classes, the professor proved
>> conclusively
>> that maximum power transfer occurred at 50% efficiency,
>> but I see
>> amplifiers that exhibit 60% efficiency. Please explain
>> why, Tom. tnx
I can only guess. Most likely the professor either never
read the conjugate match theorem, didn't understand it, or
thinks that anything that the only thing that every limits
energy is a dissipative resistance.
> Although I have not done the mathematics, since power
> transfer
> occurs at less than 360 degrees when the amplifier is
> operated
> in other than class A service, why don't you try doing an
> integration
> of the power/efficiency while the tube is in conduction
> (actually
> delivering power to the tank circuit)? I suspect you
> might find
> a roughly 50% efficiency during the "power pulse" and what
> is
> measured by amateur methods is an average over time
> further modified
> by failure to account for the "feed through power" in
> common grid
> circuits.
A class C amplifier, when tuned for maximum power with a
fixed an mount of drive into a 50 ohm load, looks like 50
ohms in a load pull and can have an efficiency of 90% or
more.
A class AB amplifier (assuming it can make very low anode
voltage on the current peak compared to operating voltage)
can go over 70%, and also looks like 50 ohms when tuned for
maximum energy transfer.
So does a automotive alternator. We did load pulls on
alternators using fixed armature current (feedback loop from
regulator open). Maximum energy transfer was very close to
when the alternator source impedance looked like the load
resistance, and efficiency was well over 80% at higher power
levels.
The conjugate match theorem never tells anyone anything
about source efficiency. Efficiency is 50% is the available
source power is very large compared to power delivered to
the load and maximum power is limited only by a series
resistor (that dissipates power), but that analogy doesn't
apply to non-linear resistances or energy limited sources of
any type, including our power amplifiers.
73 Tom
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