Repeatedly some of you have mentioned the supposed advantage of running a device
at very much lower output power than its saturation level, to improve linearity.
This is true only to a point, not all the way down to zero! To get good
linearity, an amplifier should of course not be driven into hard saturation,
that much should be pretty obvious. But running it at a very low level, such as
1/4 the maximum non-sat output, brings the distortion up again, due to crossover
distortion, unless it's class A.
Another very important point is efficiency. The dissipation of an amplifier does
NOT vary in direct proportion to the output level it's run at! An amplifier gets
more efficient, the higher it is driven. A decent class AB amplifier might be
60% efficient when driven just to the brink of clipping. A theoretically perfect
class B amplifier would be 78% efficient at this level. But if the same
amplifiers are driven to only one quarter that power level, the perfect class B
one becomes only 39% efficient, and the practical class AB one just 30%. A
consequence is that a practical 1500W output amplifier, designed to operate
close to saturation, will dissipate 1000W (plus filaments, fans, bleeders, etc),
while an amplifier designed to saturate at 6000W and run at 1500W will disipate
3500W, plus the four times larger heater, bleeder, fan etc! That is if it uses a
relatively low quiescent current. If instead that current is higher, the
situation is even worse!
This makes clear that improving the IMD in the brute force way, by using an
oversized amplifier, results in an extremely large, heavy and expensive
amplifier, that's also expensive to operate. It's a very poor method for
improving the IMD!
With more clever circuit design, such as negative feedback, bias modulation, and
in extreme cases predistortion, a lot more IMD improvement can be obtained at
far lower cost, and no added weight nor power consumption.
Keep in mind that most tube type ham linear amps aren't using any negative
feedback at all, let alone any other linearizing techniques, and that even those
solid state amplifiers that have a large gain excess rarely use it to provide
more linearity by negative feedback, but instead burn off the excess drive power
in an attenuator! Now that's a real waste. Something as simple as increasing the
negative feedback to consume the excess gain of modern MOSFETs goes a long way
in reducing IMD.
Manfred
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