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LM741 OpAmp behaviour

Lakshmi Poola
Prodigy 20 points
Other Parts Discussed in Thread: LM741, UA741

Hello,

While working with LM741, I happen to apply input more than the supply voltage to the Op amp. And for all the circuits, like for a simple buffer, Schmitt Trigger circuit I noticed the same output. The circuit  and the simulation results are attached. What is the reason for this behavior, and why does the Op amp behave so for only one half cycle?

Thanks and Regards,

Lakshmi

  • 0
    TI__Guru* 93105 points

    Hello Lakshmi,

    The uA741 is a legacy op amp and I suspect its simulation model was developed a long time ago. Since you selected the "semiconductor" version of the uA741 model it is not possible for us to see what kind of model basis it uses internally. Many companies produced the uA741 and I am sure there are different simulation models for them out there. TINA has another uA741 op amp model listed under the "SPICE MACROS" tab and you may find your results will be different with it.

    On the other hand, it looks like the model is exhibiting output phase reversal which is a real characteristic of some op amps and may be a characteristic of the ua741. This is what can happen when the input signal amplitude exceeds the specified input common-mode voltage (VCM) range of the op amp. Modern op amps often have anti-phase reversal circuitry built in and don't exhibit the behavior when the VCM is exceeded.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    Hello Thomas,

    Thanks for addressing the issue.

    Actually I have tried with TI's ua741, in the 'Spice Macros' and also LM741. With all the Op amps the result is same. I tried looking for some information on the protection circuitry, but couldn't find much.

    The behavior is  observed only for the positive cycle of the input. Input signal kind of gets loaded, due to  which we see the results in the picture. Why this behavior only for one half cycle is puzzling.

    In the uA741, datasheet in section 9.2.2.2, it is mentioned the difference between the input and the supply voltage must be 0.5V, for proper operation. What happens if it is not met is not addressed anywhere, so just wondering. And  is there any method where we can counter this behavior or make a test circuit to find the proper working voltage range?

    The attachment is of  the  lab results, supply was close to 16V as input approaches the supply, the result is shown in the picture.

    Thanks and Regards,

    Lakshmi

  • 0 in reply to Lakshmi Poola
    TI__Guru* 93105 points

    Hi Lakshmi,

    Is the blue trace the input signal, and the brown trace the output signal? Once you exceed the specified common-mode input voltage range the input transistors no longer operate in their linear operating range. Unepected current paths can be activated and excessive input current my be drawn.

    Regards, Thomas

    PA - Linear Applications Engineering