Can't get simple op amp circuit to work
Can't get simple op amp circuit to work
(OP)
I'm using an MC34071 op amp with a "FlexiForce" load sensor connected per the drawing supplied by the maker of the load sensor (Tekscan):
http: //www.teks can.com/fl exiforce/s pecs_flexi force.html.
The output of the op amp goes to about +8.5 volts and won't change much (changes about .2 volts) as I change the load on the sensor. I can get the voltage at the inverting input to change from positive to negative by varying the load. It seems like I have something not connected correctly but I can't find any errors. I've tried 3 different op amps, 2 breadboards, and different feedback resistances but still no luck. Any suggestions?
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The output of the op amp goes to about +8.5 volts and won't change much (changes about .2 volts) as I change the load on the sensor. I can get the voltage at the inverting input to change from positive to negative by varying the load. It seems like I have something not connected correctly but I can't find any errors. I've tried 3 different op amps, 2 breadboards, and different feedback resistances but still no luck. Any suggestions?





RE: Can't get simple op amp circuit to work
How do you generate the -5v? If you put this voltage at ground, do you get 0 volts at the output? It should be.
RE: Can't get simple op amp circuit to work
An op-amp should never have any significant voltage differential between its inverting and non-inverting inputs. Consider the gain of the op-amp: typically 10^6 or greater, open loop. For any non-saturated output voltage, the differential between the inputs must be very small indeed. You can measure it, but you need laboratory class instruments. The op-amp gain remains the same when the loop is closed, but the circuit gain is modified by the feedback network. In a negative FB circuit, the feedback acts to reduce the differential input voltage to almost zero. In the special case where the non-inverting leg is grounded, the op-amp creates a 'virtual earth' at the inverting input because the ungrounded leg is 'virtually' at earth potential as a result of feedback reducing the input differential. If you are measuring a significant input differential, there is something wrong with the feedback, or the circuit gain is far too high for the sensor (or open loop).
A last daft thought - you did spot that the upper power connection (pin 4) on the example circuit is -9V and the lower connection (pin 7) is the positive? The drawing is correct, it just breaks with convention.
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RE: Can't get simple op amp circuit to work
http://www.onsemi.com/pub/Collateral/MC34071-D.PDF
TTFN
RE: Can't get simple op amp circuit to work