Hi Dennis,
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We don't need you to defend Bill,
I just thought you were being overly harsh on him when he was just trying to
provide some additional useful input to the forum.
I know he had personally spent a lot of time making those measurements
because I had asked him to do it as a personal favor to me.
I requested he do that for me because I wanted to see if his measurements
tracked with the ones I was getting.
I was just looking for a second source of input to see if the parametrics I
had obtained could be duplicated in another lab.
As far as him 'sharing' that data with the rest of the group, I have
requested him not to do that at this point in time.
So blame me for his lack of backup data.
> I'm sure he's old
> and smart enough to post his own information.
> If he actually has the data based on a working
> design, I can't understand why would he not
> want to provide that information upon a polite
> request.
I answered that in the paragraph above.
>
> I didn't like his amplifier operation from a little
> dirty to clean upon one (or even two) key downs. I
> consider it poor practice.
I think you misread his, or my, statement.
Keying the mike and saying CQDX a few times and then identifying your
station before you unkey is only one keydown.
The junctions heat up internally before you can say the 'X' in the initial
verbalization of the 'CQDX' call.
Remember that the 2SC2879 is really a large number (over 20) seperate well
matched small signal transistors all wired in parallel inside a flange mount
that uses 40 ga. stitchbond wires to make the connections to the individual
transistor elements. Those tiny junctions heat up very quickly!
>
> Thermal characteristics of a pn junction are often
> taught in BE101, but RF amplifier linearity, setup
> and operation is probably not part of a basic
> class.
That is correct, but when I taught that class I made it part of it.
That's because I wanted the class to understand that solid state device
biasing, and class of operation, was basically the same for both the push
pull audio and push pull RF amplifier circuits.
I know I oversimplified it, but I explained they were both biased AC
amplifiers with the only difference being the frequency of operation.
I did that so the term RF amplifier would not intimidate them later.>
>
> Don't patronize me with "basically a big tube
> man" statements.
OK.
> I also don't need or your second "hard to make
> the bias and class of operation transitions
> between tube and solid state comment.
OK. >
>
> Please post your information separate from
> anything Bill might eventually throw out. I
> don't need you to describe RF amplifier bias
> circuits for me.
What would be your idea of a perfect amplifier bias circuit and how would you
implement it in an inexpensive and reliable configuration?
Maybe your thoughts would reflect mine, and them maybe they wouldn't, and in
that second case I would have to disagree with the last sentence in your
previous paragraph.
I need Bill to provide the >
> proof to his previous statements. I'll be
> happy to comment on some of your
> information contained in your last Email.
> That was all I asked from Bill.
And I jumped in here so he wouldn't have to.
I'm not ready to share the summation of 2 years worth of our data collection
activities with the public at this point in time.
>
> In a separate thread or direct Email, we can
> go over various ss bias networks with their
> related pros and cons. I agree with the later
> part of your post that temp tracking bias
> supplies are better.
Thank you.
> Precision tracking bias circuits have been
> available, described for years that prevent
> significant shifts in bias, they are not
> really hard to find.
Then I must be missing something somewhere, because I have not seen a really
good one yet!
Those circuits that use a diode heatsunk to the flange of the hottest
running power output device comes close, but it can't track the instantaneous
heating effects of the power output device's junctions inside the case; so
there is always a 'significant' time delay between the application of drive
and when the regulator begins to adjust the bias; which is often longer than
the circuit Bill described.
Often, in a serious overload condition, the junctions overheat long before
any thermal bias tracking circuit or any thermal breakers attached to the
heatsink can protect the devices from self destruction.
My solution to the problems listed above would be a circuit that would need
to do at least three things, and do it relatively cheaply:
1) Monitor the quiescent current during the 'off' intervals between syllables
of a SSB transmission, (zero signal current detector and a sample and hold
circuit), and then use that measurement as feedback to dynamically adjust the
bias currents to the devices.
2) Monitor the peak DC currents through the devices and limit it to a
'sustainable' safe area of operation, (to include thermal 'foldback' for a
long duration overload), if an overload occurs.
3) Monitor those currents in such a way as to not limit the maximum output
power of the amplifier.
Please note that I never mentioned VOLTAGE anywhere in my example...
Regards,
Dennis O.
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