I have a fair understanding of how voltage type op amps like
the 741 and scores of other similar differential op amps work in a
circuit. They have an almost infinite input impedance and a very low
output impedance as long as one doesn't load the output more than its
design specifications allow. The gain is set by the ratio of the
resistor between the output and inverting input and another resistor
between the inverting input and ground or virtual ground as the case
may be.
I now may need to build a circuit with a LM3900 Norton op amp.
While I have played around with them in the past, I never quite
understood much about them. Is the gain limited the same way? Do
they basically have what looks like a short circuit at the input which
would be the ultimate current sink which is why one externally limits
input current on one of these devices?
I remember destroying a couple accidentally by connecting the
inverting input to one of the supply rails or maybe I was using it in
single-ended mode and grounded it, but it went poof very quickly.
Mainly, do I set the negative feedback gain on a NOrton amp
the same way I would set it on a voltage op amp?
I have appreciated the help I have received on this list in
the past. As an electronic experimenter who is blind, I have run in
to data sheets that I couldn't read. This time, I could read most of
the TI data sheet but as usual, the important charts and graphs don't
come through. Some of this information simply can't be automatically
translated in to text.
The text part of the sheet, however, was quite clear and makes
me think that the inputs to a Norton amplifier are very low impedance
like a short.
If that is true, then the only thing I don't quite get is what
the circuit looks like to set closed-loop gain.
My present interest in the LM3900 is as the current amplifier
in a photo diode circuit. Photo diodes are run reverse-biased and the
microamp minority charge current increases with light exposure as I
understand it.
Thanks.
Martin McCormick WB5AGZ Stillwater, OK
OSU Information Technology Division Network Operations Group
--
Author: Martin McCormick
INET: [EMAIL PROTECTED]
Fat City Hosting, San Diego, California -- http://www.fatcity.com
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