Electronics > Beginners
TL081 Op-amp Experiment
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David Hess:
Modern operational amplifiers tend to have extra circuits to prevent phase inversion.  It is not only a problem with JFET input designs; bipolar and MOSFET input designs can suffer from phase inversion as well although not always because the input stage gets cut off.
Wolfgang:
... the good old Horowitz and Hill Art of Electronics book has some useful info about OpAmps and maybe also treats this problem.
Just use a newer OpAmp or even better an RRIO one and it will be gone.
wuiven64:
Hi,

Thanks everyone for the suggestions and ideas. I have repeated the experiment by replacing the supply of the TL081 op-amp from +5 V at V+ and 0 V at V- to +5 V at V+ and -5 V at V-. The experimental result is attached at below. As shown in the results, the TL081 op-amp operates properly.

wuiven64:
However, why does the output voltage without R1 is high (4.73 V) compared to output voltage with R1 (1.35 V) when the input voltage is 0.00 V as the supply of the TL081 is +5 V at V+ and 0 V at V-?

Can anyone explain why this happen?
Audioguru:
When an opamp that has the Phase Inversion problem has its input voltage too close to its negative supply voltage then its input transistor or Jfet becomes reverse-biased and draws a lot more current than normally. A series resistor reduces that abnormal input current so the opamp acts differently.
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