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Re: Using Class C Amplifiers for SSB

on 1/2/01 4:35 PM, Tony Langdon at tlangdon@atctraining.com.au wrote:

> I very much doubt it.  I recall a friend who did research on using class C
> amps as linear amps (i.e. for cleanly amplifying signals which varied in
> amplitude), and he was using feedback to "linearise" the amp.  I think the
> general idea was to compare the output amplitude with the amplitude of the
> original signal, and adjust the operating conditions of the amplifier to
> keep the gain of the system effectively constant (can't remember if it was
> drive, supply voltage or some other parameter that was used for gain control
> by the beedback system).

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).

The concept was invented during the 1930s actually when radio receivers had
pretty poor sensitivity.  The broadcasters used to use megawatt or greater
transmitters in order to be heard.  However, the power consumption for the
class A or AB amplifiers was enormous.  So a fellow named Dougherty invented
it.  When crystal sets went away and the modern super-het came about,
broadcasters didn't need to use such high power levels so the Dougherty fell
into relative obscurity (it may have been used by the Int'l SW BCs).  Not
until today's digital world where clean, efficient amplification of digital
signals is needed did the amp gain in popularity again.

As far as using feedback to adjust the amplifier and linearize it, several
different techniques can be used.  Feed-forward and Cartesian Feedback are
two widely in use today (Feed-forward is by far the most common).  However,
in all these cases (at least what I have seen), the amplifier stages are
either Class A or Class AB.  Relatively good linearity is still needed in
order to achieve the necessary specs of IMD products < 60 dBc.



Jon Ogden
NA9D (ex: KE9NA)



"A life lived in fear is a life half lived."

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