The Efficiency of the AM Linear Amplifier versus Plate Modulated Transmitter

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The efficiency of a linear amplifier is exactly the same whether it is amplifying SSB or DSB AM with carrier.  Efficiency is a function of the amplitude of the signal up to the point of saturation.  Theoretically, the efficiency should be more than 60% when the signal level is just under the saturation point.

At zero signal level, the efficiency is zero, since the tube draws some static plate current but the output is zero.  At intermediate points between zero signal and the saturation point, efficiency varies in direct proportion to the amplitude of the signal.

The reason that the AM linear gets a bad reputation for "inefficiency" is because with the unmodulated carrier, which should reside midway between zero signal and the saturation point, the efficiency is approximately half the maximum peak efficiency.  If the peak efficiency is 60% as mentioned above, the unmodulated carrier efficiency is about 30%.  This is most obvious with full carrier AM because of the 100% duty cycle of the carrier, whereas with SSB, voice peaks that hit the midway point are of short duration and cause less heating of the final.

With SSB, the average signal level with the typical human voice (without a lot of processing or overdriving) is about 30% of the amplitude of the maximum peaks.  This means that the efficiency of a SSB linear running a clean signal averages something on the order of 18% most of the time even though it may run 60% or more on the maximum peaks. By the same token, the AM linear peaks close to 60% efficient on positive modulation peaks.  

With a sine wave tone modulating the carrier 100%, the tube will run cooler than when there is no modulation of the carrier. A properly operating AM  linear draws steady plate current regardless of modulation, therefore the DC input is invariable.  With 100% sine wave modulation, we see a 50% increase in total rf output, accounting for the upper and lower sideband energy in addition to the carrier. Since the DC input is the same regardless of modulation, that extra 50% has to come from somewhere, so that means the final runs at higher efficiency to generate that extra power.

High level plate modulation is usually thought to be much more efficient than linear amplification, for running AM.  However, the efficiency advantage of plate modulation is less than might be expected.  With plate modulation, extra power is consumed by the modulator tube filaments, modulator tube plate dissipation, and the plate and filament supplies for the substantial sized driver stage required to properly run a class-B modulator. Remember, a high level modulator is nothing more than a linear amplifier operating at audio frequencies, subject to the same inefficiencies as those suffered by the RF linear amplifier.  The sideband power from a plate modulated transmitter is generated through a series of two marginally efficient amplifier stages: the high level class B or class AB modulator, followed by the class C RF amplifier. On top of that, the modulation transformer itself is less than 100% efficient, so some of the audio power is wasted before it is delivered to the class C final.

When considering the total efficiency of the transmitter, i.e., the ratio of energy delivered to the antenna compared to the energy consumed from the a.c. power mains, linear amplification has very close to the same efficiency as plate modulation. The same is true with control grid and screen grid modulation, which operate very similarly to linear amplification.

When total costs are taken into account, the extra expenditure required for larger tubes with adequate plate dissipation for linear operation is easily offset by the added costs of the modulator tubes, modulation transformer, driver transformer and driver tubes, so the linear amplifier or grid modulated final may actually be cheaper than a plate modulated rig with the same rf output.

The greatest advantage of the plate modulated class-C final is ease of tune up.  Linear amplifiers and grid modulated finals require very critical adjustment of rf grid drive level and antenna loading, to assure linear modulation capability up to 100%.  With a plate modulated amplifier, if the modulator is adequate for the final, a wide range of grid drive and plate loading will work, with a great degree of tolerance for variations away from nominal adjustments.