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Re: Using Class C Amplifiers for SSB

Jon Ogden wrote:

> Well, the only way I've seen it done (with a Class C amp) is in a Dougherty
> Amplifier configuration.  This type of amplifier uses a Class C amplifier as
> a peaking amp to amplify the signals at their peaks.  During other parts of
> the modulation cycle a linear amplifier is used.  The two are phased in such
> a way as to make the whole thing work.  It's a pretty fascinating way to
> develop a more efficient linear amplifier.  I've got a paper on it somewhere
> that I should scan in someday.  The biggest problem is that due to the
> matching/combining structure, it is inherently relatively narrow band
> (narrow band meaning less than 10% bandwidth).

FYI -- the linear amplifier design by DJ4ZC which has been used in most of our
satellites uses a scheme invented/patented by Karl similar to the Doherty
amplifiers. Karl calls his scheme HELAPS, standing for High Efficiency Linear
Amplification by Parametric Synthesis.

In essence, consider the signal you want to amplify as consisting of a phase
component and an amplitude component; assume that there is a 3-port black box that
does the vector analysis on the input signal and with two output ports --
A(mplitude) and P(hase).

The P component (as with any FM/PM signal) can be amplified in a non-linear, hard
limiting (i.e. Class C) amplifier to achieve high efficiency..

Now let's feed the A signal into a good old AM Plate Modulator (some of us still
remember that the plate is the "collector" in a thermionic glow-FET) to vary the
supply voltage feeding the hard-limiting P amplifier, and hence re-modulating the
P amplifier.

Voila -- we have a high efficiency amplifier nearly linear where the RF power is
developed by the hard limiting P amplifier and the signal amplitude information
comes from the AM modulator.

In Karl's designs, the "plate modulator" is actually a Class-D switching power
supply. Because the transponder bandwidths are several hundred kHz, the switching
power supply operates at frequencies up to a couple of MHz (i.e. several times the
bandwidth). Transponder bandwidths have been dictated in part by the speed of the
Schottky switching diodes and power MOSFETs  and the loses in the associated
"swinging" inductor in the Class-D modulator.

Because the P amplifier's "plate" voltage is changing, the amplifier does not see
a constant load impedance (a flaw in ordinary AM transmitters which caused the
Doherty design to happen) and power is lost. In Karl's design, the "Parametric
Synthesis" magic black box not only separates the A & P components, but it
intentionally pre-distorts the A & P signals so as to cancel the distortion
introduced by the amplifier. With this scheme, Karl's design ends up with intermod
distortion products that are ~30 dB down.

You certainly wouldn't want to operate next to a station with -30 dB IMD in the
CQWW contest. But if there are many nearly equal signals in a satellite
transponder passband, the level of IMD simply raises the noise floor a little bit
for everyone. The LEILA active notch filter in the AO-40 IF chain was designed to
try to kill off alligators that hog too much of the transponder's power and create
bad IMD for all other users.

73 de Tom, W3IWI

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