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> B. To use a cheap two-or-three transistor amplifier.  I get transistors
> three for a nickel, and resistors four for a penny, so any concievable
> combination of bipolar  transistors and resistors would be plenty cheap.
> FETs (the other obvious choice) are probably out of my price range.  So
far,
> I've not been able to get anything to work so well.

If you're so far just looking at the voltage from a photo transistor with 2M
collector resistor, you obviously don't care about speed or a few 10s of mV
offset.  In that case you can make a "do it yourself" op amp with a few
transistors and resistors.  The attached schematic shows what the op amp
front end looks like.  This is a comparator between the IN and FEEDBACK
signals.  You will still have to take one of the OUT signals, and amplify it
by a large voltage gain, then feed the result back into FEEDBACK.  The
result will then be a reasonably good replica of the IN signal at much lower
impedence.

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********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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