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Re: Using Class C Amplifiers for SSB



The definition of a class "C" amplifier is one that conducts for less that
50%
of the input waveform. This fact makes it impossible to operate it in a
linear
fashion because it does not accurately transfer the portion of the signal
envelope that exists before conduction occurs. The addition a of bias signal
(carrier leak through) still does not restore this lost information. The
spastic switching
on and off of the amplifier will produce distortion which will be manifested
as
spurious signals (harmonics, splatter, etc.). This is OK in a class "C"
amplifier
that is used as it is intended because the drive is constant and relatively
large.
Some class "C" amplifiers conduct for less that 25 % of their input
waveform.

This is like a friction motor in a child's toy in that you "stroke" it to
get the inertia
wheel going faster and faster. When you stop "stroking" it, the wheel
retains
some of it's energy and operates the toy for a short time.

The circuit in the output of the amplifier "rings" at it's resonant
frequency just
as the friction motor in the child's toy thereby producing a clean signal. A
linear
amplifier depends less on resonance and more on "linearily" reproducing the
signal at a higher level.

This is a crude description at best. The ARRL handbook gives a very good
description of all classes of amplifiers and their characteristics.



I have thought of buying the units that were advertised an "L" band 20 watt
amplifiers and may well still get one. The fact is that if you can't get to
the
individual stages in the amplifier chain to make them "B" or "AB" by
changing their DC bias point it is a lost cause and will only cause splatter
and distortion. If it is a hybrid amplifier, getting to the individual
stages
is unlikely without destroying the unit. If it is a discrete component
amplifier,
there is a very strong possibility that the amplifier could be made to
operate
satisfactorily in a linear manner.

Any comments or suggestions will be welcome.

73

Tony
W4TAS


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