On Sat, 2007-09-01 at 20:02 -0400, John Huggins wrote:
> 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).
Linearity is not high on the priorities of CB makers, less so for those
making illicit amplifiers. If mostly linear for SSB/AM most PA's run
class AB or B. A single active device works for RF class AB or B
decently.
>
> 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 180Â 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.
Typically a tube tank circuit with a Q of 12 is adequate to complete the
sinewaves, and a low pass filter with adequate harmonic rejection rounds
off the half sines from the single ended PA. Class C is excellent for CW
and FM with very good amplifier efficiency though the threshold and
limiting can sometimes shorten signal rise and fall times make key
clicks.
>
> 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.
Class A has the worst power efficiency with generally the best
linearity. Its not often used for much power because of the poor power
efficiency. As noted above (and on other posts) AB or B need not be push
pull for RF applications.
>
> 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?
Yes, but the distortion products from carrierless schemes are very
strong. A CB "linear" for AM depends on the carrier for the linearizing
forward bias, but tends to still not meet the linearity state of the art
that is possible with proper biasing, linear devices, and harmonic
filtering.
> - In a good ol' Ten-Tec linear amplifier is the final Class A?
More like Class B.
> - Do most Ham Transmitters utilize pairs of power devices in a Class AB
> or B in a push-pull arrangement?
Yes, mostly because push-pull is good at reducing 2nd harmonic outputs
which makes for easier output harmonic filtering and because the devices
handily available come in sizes where two make 100 watts.
There are RF FETs that run several hundred watts each.
And at higher frequencies (a threshold dependent of device size) push-
pull is often a bit more convenient in that the drive and load circuit
impedances are higher and some of the circulating current rattles
between sources or emitters and bypassing or grounding of those elements
is made less critical. For that reason power devices for 900 MHz and
higher bands often come in push-pull pair packages, two devices in one
container.
>
> 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.
Years ago TI published a book on solid state RF power amplifier design
that is still a good reference.
>
> Thanks for your help.
>
> John
> KX4O
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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