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Old 20th May 2009, 03:03 PM   #71
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Quote:
Originally posted by kenpeter
Yet another glorious distraction! Like you didn't see one coming.

Quasi Complimentary Opto-Alephs?
I did not see that one coming.

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Old 20th May 2009, 10:30 PM   #72
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Join Date: Nov 2007
Location: Dallas
Not the fastest way to abuse an optocoupler...
Been reading up on how to stretch opto bandwidth.

We can cascode the collectors to hide Miller, but there
ain't really enough voltage between gate and source
in this application to squeeze such a cascode.

We can use just collector-base junction as photodiode.
Thats very fast, but then you still have to amplify it...

Easiest cheat might be 47K ~ 470K bleeder from base
to emitter. Lose some gain, but shuts down quicker...
Letting bases float as I did, the sure ticket to slowville.

And obviously the whole thing needs some diamond
style buffering to get the input impedance up. Easy.
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Old 10th June 2009, 11:15 PM   #73
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Another screwy idea: Did not exactly improve the thing I was
shooting to fix. (Drive bandwidth can't make 20KHz rail to rail),
but still might be of lame slackademic interessence or whatnot...
I'm still working on the original problem.

JFETs didn't work better than bipolars as comparators. But in
this case, matched sums (from matched pairs) make voltage
spanned across the input buffers equal. Toshiba 2sk389/2sk
109 matched pairs. I abuse the models for 2sk170/2sj74 as
JFET singles. It seemed plausible...

Equal, but not necessarily right. Got to jigger with resistors
to equalize thresholds at the MOSFET gates. Inequality is
almost assured. And mind you, the thresholds of the JFET
comparators have to be significantly less than thresholds
of the MOSFETs to be controlled.

Whatever else, the input impedance increase seems useful.
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