A op Amp Question

Eric

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Jan 20, 2002, 6:31:02 AM1/20/02

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Now let's see if I have this right,

the voltage at the + input pin should be about the same as - input at the
pin
because the inputs are a current driven device

So if there was a big difference the Op Amp would be faulty

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Bruce Carter

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Jan 20, 2002, 8:54:18 AM1/20/02

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>So if there was a big difference the Op Amp would be faulty

Or you are connecting it wrong. The biggest mistakes I see are people
trying to operate one open loop, or hitting the voltage rails of the
op amp without realizing it.

Bruce Carter
Texas Instruments Applications

Ian Field

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Jan 20, 2002, 10:01:34 AM1/20/02

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Not necessarily - most OP-AMP's are designed to work with a small
differential between the 2 I/P's but can tolerate any voltage difference
provided neither I/P goes outside the confines of the supply rails, using
the OP-AMP as a comparator would be a typical example of the I/P's could
swing to any value between -V & +V, usually in a comparator the -ve I/P
might be connected to a voltage divider to set a threshold voltage and the
+ve I/P is fed via a resistor from the voltage source being monitored - to
avoid false switching due to signal noise, the circuit is given +ve feedback
via a high value resistor from O/P to +ve I/P - this alone can push one I/P
close to either rail voltage while the -ve I/P is held about 1/2 way by the
divider.

"Eric" <er...@home.co.nz> wrote in message
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Sam Goldwasser

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Jan 20, 2002, 10:10:53 AM1/20/02

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bru...@mindspring.com (Bruce Carter) writes:

> >So if there was a big difference the Op Amp would be faulty

If the op-amp is used as an amplifier (e.g., not a comparator), then the
voltage difference should be very close to 0. However, as others have noted,
op-amps may be used for other purposes.

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Robert Macy

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Jan 24, 2002, 2:41:38 PM1/24/02

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Better to think of it as the OpAmp is a high gain device. If it is in its
active range (where the output is not very large), then the inverting and
noninverting input had *better* be about the same.

However, if something is preventing the OpAmp's output from satisfying this
requirement, then the two voltages can be quite different. Could be bad
OpAmp, open feedback, shorted output. etc.

Don't know what your ckt is, so can't tell you where to look next.

- Robert -

Eric wrote in message <2Tx28.788$zy4....@news02.tsnz.net>...

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