In most all cases of "multi mode" amplifiers, Class AB2 is used.
Class A is nearly perfectly linear, but is never more than 50% efficient,
which is unacceptable at most moderate to high power levels. It may be OK
at low (legal) CB power, but since there doesn't seem to be any demand for
intelligible audio or non-splattering signals on CB, it seems to be a
complete waste of a heat sink! There doesn't seem to be much demand for
legal power, either, judging by the hand-painted sign I saw at a truck stop
recently that said "6- Pill Amps" with an arrow pointing to a nearby
shipping container.
Class C is quite efficient, but as you noted, not very linear. Great for
CW, FM, and AM where there is a separate modulator circuit such as plate or
PWM. SSB and amplified AM would be distorted and splatter would be
noticeable.
AB2 is "linear enough" for SSB and amplified AM and nearly as efficient as
Class C, making it the best choice overall. Almost certainly what the
Orion's and just about every other 50-200 Watt transceiver uses. All
high-power HF amps use that too.
Ham radio transceivers with Class A modes run at significantly reduced
output and become real 'room heaters'. In fact one of them has had some
serious failure problems when run in Class A mode for any amount of time and
many went back to the factory with burned up finals.
Ron N6IE
www.N6IE.com
----- Original Message -----
From: "John Huggins" <john@johnhuggins.com>
To: <TenTec@contesting.com>
Sent: Saturday, September 01, 2007 5:02 PM
Subject: [TenTec] Final Amplifier Class
> Ladies and Gentlemen,
>
> I frequently contribute technical knowhow to a CB trucker's forum
> (before you press delete please read on). The question came up
> concerning my assertion the single final transistor amplifier, typical
> in CB transmitter output circuits, operates as a Class A amplifier since
> the modulation required is AM and sometimes SSB, both being amplitude
> style modulations, and both desiring of good linearity in the
> amplification (at least I think so).
>
> I know for sure CB output circuits are not push pull since they have
> only one power amplifying transistor.
>
> Some are suggesting the less than 180° Class C amplification method is
> what CBs use. I understand this requires a good tank circuit to
> complete the sine waves, but is supposedly inappropriate for amplitude
> style modulations like AM, DSB and SSB. Various resources suggest Class
> C is fine for CW and FM.
>
> I am sure by now you are saying "That's interesting John, really, but
> what does this have to do with Ten-Tec?"
>
> Well, I suddenly remembered some advertisement for an HF Ham Transceiver
> that offered Class A mode in its output circuit for some improvement in
> some parameter. Funny thing is I guess I thought Ham Transceivers
> always used either Class A with a single device or, perhaps, Class AB or
> B in a push-pull arrangement.
>
> What I need to know from Tec-Tec and/or any of you is:
>
> - What is the final amplifier class (A, AB, B or C) used in, say, the
> Ten-Tec OMNI VII and/or Orion II?
> - Is it actually possible to use Class C with an amplitude modulation
> scheme like AM, DSB and/or SSB?
> - In a good ol' Ten-Tec linear amplifier is the final Class A?
> - Do most Ham Transmitters utilize pairs of power devices in a Class AB
> or B in a push-pull arrangement?
>
> Hopefully this will fine tune my EE knowledge of circuits and make sure
> I have not missed something I should know so I can continue to represent
> the ham community well when offering advice to the other radio groups.
>
> Thanks for your help.
>
> John
> KX4O
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
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