For background see
http://en.wikipedia.org/wiki/Electronic_amplifier
On Sep 1, 2007, at 5:02 PM, John Huggins wrote:
- What is the final amplifier class (A, AB, B or C) used in, say, the
Ten-Tec OMNI VII and/or Orion II?
I don't know for sure but I would guess AB (as that's what everyone
uses). About 50% efficiency at best.
- Is it actually possible to use Class C with an amplitude modulation
scheme like AM, DSB and/or SSB?
Yes :-)
But you have to work at it. And you won't find this in a CB box.
Any modulation scheme can be describe by a combination two time-
varying parameters: amplitude and phase modulation. Even SSB (which
is a combination of both). This allows a technique called Kahn
Envelope-Elimination-and-Restoration (EER) that was more popular
amongst military systems when SSB was new, to use big non-linear
amplifiers to amplify linear signals.
You need to extract envelope modulation information from the signal
(say an SSB signal). Then you limit the signal and feed it into a non-
linear (class C or higher) amplifier to amplify it. This non-linear
amplifier is then high-level modulated with the envelope information
(at audio rates) adding back the envelope and resulting in an
faithfully amplified signal.
In the old days this was big tranformers and class C amps. Good old
fashioned AM technology.
Today they use ERR for commercial large SSB transmitters with a
switching power supply that are easily modulated at audio rates and
solid-state PAs. The resulting aplifiers have *very* high efficiency.
E.g. 97% efficiency claimed for this:
<http://www.contelec.com/SSM.html>
Every OSCAR since Oscar 7 has used this technique (HELAPS) in its
transponders to maximize efficiency. About 80% IIRC.
<http://www.amsat.org/amsat/sats/n7hpr/ao7_2.html>
<http://www.amsat.org/amsat/archive/amsat-bb/200511/msg00025.html>
This technique has also become popular in the microwave field too. It
is used today in a most wireless cards and in GSM (at least) cell
phone systems. When you generate the ODFM signal in a DSP it's easy
to do the envelope extration digitally and pass the envelope and
phase-modulated signal to the class D or E PA resulting in 90%
efficiency.
Doherty amplifiers follow a similar idea -- mixing a class B and a
class C amplifier one to deal with peaks and the other for carrier.
These two are being adapted for linear use in UHF/SHF and microwave
applications.
- In a good ol' Ten-Tec linear amplifier is the final Class A?
No, almost certainly AB perhaps A. A class A amp is only about 25%
efficient. So that's 4kW in for 1kW out. No one does it that way
because there is no point.
- Do most Ham Transmitters utilize pairs of power devices in a
Class AB
or B in a push-pull arrangement?
AB. It's easier to bias the PA transistors on a little to avoid cross-
over distortion.
--
Kevin Purcell
kevinpurcell@pobox.com
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