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Thank you, Phil, for the link to the article.
The maximum power transfer theorem is often misapplied without understanding that, though maximum power TRANSFER occurs, the efficiency is 50%. In this case the author acknowledges this and the addition of the resistor does stabilize the circuit since this tends to approach a constant current source for the final. The negative feedback mentioned is the real advantage at the price of a drop in overall efficiency.
A question remains for me though and this is how does one measure the “transistor efficiency”? My assumption is that this is done by measuring the DC drain current (average) and the DC drain to source voltage (average) with the product being the DC input power and then dividing this into the output power. Next question is where in such a circuit does one measure the output power? It seems the author does this by measuring the RMS current through the loading coil and calculating the “load” power using the Rloss + Rrad numbers. Since the loading coil R is included in the Rloss this method places the Rcoil in the load rather than in the source. Since the desired performance measure is the power delivered to the antenna system including the Rcoil as part of the transmitter rather than the load would give a more useful indication of the amplifier efficiency than just the “transistor efficiency”.
For example, one transmitter with a high loss coil and another with a low loss coil could have the same efficiency (that is, the same output power) if the coil loss is counted as part of the load yet the power delivered to the radiator will differ (assuming the same ground loss for both).
By convention the output power would be measured at the load which would be after the loading coil and the coil loss would reduce the efficiency quite a bit but the result would be better in terms of comparing different amplifiers with internal antenna loading coils.
Those familiar with farming know of the Nebraska Test for tractors which resulted from the disparity between engine horsepower and the power available to do the work. The Test measures the PTO and drawbar horsepower which is the power which does the work and which is always less than the engine horsepower. I see a similarity here.