The problem with class E, D or sometimes class C amps comes from a
behavior they exhibit often called "ramp-up". With no input signal, the
amplifier draws no current and has no output. As you drive a signal
into the amplifier, you will get no output at all until you reach a
threshhold level which finally provides the amplifying device with
enough voltage to turn it on. At that point, you'll get an output from
it, and often this output is not far from saturation. It's common to
see a class C amplifier whose minimum non-zero power out is only 6dB
less than saturated.
Now, if we wrap a feedback loop around it, the feedback loop has to
contend with extraordinary variations in device gain, and that loop
becomes a monstrous challenge to design. If the feedback loop has a
time delay of a few milliseconds, then one cannot correct for envelope
power variations any faster than that. SSB signals vary considerably in
fractions of a millisecond, so it would have to be a faster loop than
that. However, the large variations in gain will probably produce an
unstable loop.
There is, as a mathematician might say, a solution, however, and it's
been implemented commercially. Rather than varying the drive level, you
vary the power supply voltage. You sample the power of your driving
signal (from your exciter, for instance) and convert that to a
time-varying signal that follows the original amplitude. Now you strip
the modulation from the original signal, leaving only the RF carrier
with intact phase information. Feed that RF carrier into the Class E
amplifier at a fixed level. Now take the varying DC level and use it to
control the output voltage of a switching power supply which supplies
the voltage to the Class E amplifier. This was done (class D, actually)
by the Continental Corporation in a series of SSB shortwave transmitters
sold overseas. They achieved an efficiency of over 80% at all power
levels (note that the so-called 65% efficiency we often achieve with our
class AB amplifiers is onlhy at saturation - it drops rapidly as our
input power decreases).
For ham use, I don't think efficiency is a big worry. Unless extreme
efficiency can allow us to use smaller devices for a given output power,
this may be effort best spent elsewhere.
By the way, the technique is called "envelope removal and restoration"
and has been fairly well-documented and prototypes described, by Dr.
Frederick Raab of Green Mountain Radio. If you have access to some of
the RF Design magazine trade show proceedings from about 1989 you can
find many of his papers. It makes for fascinating reading, and I must
admit temptation to build such a beast for ham use.
Arlen
>>SAW A HIGH POWER, INEXPENSIVE SOLID STATE AMP DESIGN IN QST A COUPLE
>>MONTHS AGO. IT WORKS FB ON CW, BUT CANNOT BE USED ON SSB DUE TO
>>NON-LINEARITY.
>> MY IDEA IS THIS: BRIDGE A COMPARATOR/ CORRECTOR CKT TO SAMPLE
>>OUTPUT, THEN MODIFY INPUT, TO MAKE THE THING NEAR-LINEAR RESPONDING,
>>USING SEVERAL MILLISECOND DELAY.
>> THERE IS A ROMANCE TO THE OLD TUBE AMPS, BUT ALSO BIG NEED FOR
CHEAP,
>>SOLID STATE POWER.
>> PLEASE LET ME KNOW YOUR THOUGHTS ON THIS. CUD SAME
COMPARE/CORRECT
>>CKT NOT BE USED FOR TUBE AMPS, RUNNING NON-LINEAR, HIGH EFFICIENCY
>>CONFIGS??
>> DOUG WALLER - NX4D
>
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