[TenTec] Final Amplifier Class

[Kevin Purcell]

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 at pobox.com