In a message dated 2/26/03 10:01:38 AM Central Standard Time,
MorgusMagnificen writes:
> Subj:Re: [Amps] 2879 biasing
To: <amps@contesting.com>
> Date:2/26/03 10:01:38 AM Central Standard Time
> From:<A HREF="mailto:MorgusMagnificen";>MorgusMagnificen</A>
> To:<A HREF="mailto:Dennis12Amplify";>Dennis12Amplify</A>
>
>
>
> I am having a little trouble keeping track of who is who in this
> bias/idling current debate.
I was using a different color and that didn't work, so I'll try using italics
with an underline this time.
However, whoever posted this last block of suggestions. has made an
excellent
> one.
That was me, Dennis O, and I thank you for your support.
The 'ideal' way (in a very pragmatic sense) would be to build a little
processor that
> detects gaps in the speech (a reasonable task) and runs a gated controlled
> current loop to set the bias voltage or current (it doesn't matter which,
> since the loop would be closed on the collector idle current.)
You don't need a processor to do that, just a zero crossing detector and a
gated sample and hold to the feedback circuit in your bias regulator.
>
> An important requirement for this scheme is the actual presence of gaps of
> at least a few milliseconds in the audio.
Not necessarily true.
The important requirement would be the sample time.
If the sample could be done quick enough, it could even be done at an RF
rate, which would be the perfect overall solution.
It's quite easy to do that on 160 meters but becomes a considerably more
challenging task on 10 meters and above.
How about if everybody interested in this subject gets out his scope and
takes a
> look at the RF envelope during speech. A convenient pickoff point would be
> either end of a directional wattmeter BEFORE any averaging circuits.
Not necessary.
Everyone needs to breathe sooner or later!
>
> Although a bit involved for a simple goal, this could all be accomplished
> with circuitry which I use regularly, in a simple little module. I would be
> willing to collaborate with someone in producing one for some trials.
Already done, but thank you anyway.
Before jumping in, however, how important is this really?
Ask Skipp and some of the other HAM purists.
I really don't think it is worth doing when an extremely simple circuit can
accomplish almost the same thing, which is why I never went public with the
alternative.
In my experience with
> audio power amplifiers, where distortion specs where far more demanding (<<
> 1%), it was not difficult to get moderately good bias tracking using
> traditional systems (which I think have been the topic of discussion
> recently here . A few semiconductors and some passives were all that was
> needed to keep idle current within a reasonable range.
The difference is primarily in the duty cycle and the associated thermal
variations between transmissions that are present in an RF amplifier which is
used for ICAS service which do not exist, or at least to that great of an
extent, in any CS audio amplifier that I have ever worked on.
I have seen similar periodic, (ICAS or Pulsed), requirements in some medical
amplifier applications, and I'm sure there could be many others; but not
necessary for audio; or requiring the same spectral purity.
Regards,
Dennis O.
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