Problem with TL081

Discussion in 'Homework Help' started by javi1991, Jul 21, 2014.

  1. javi1991

    Thread Starter New Member

    Jul 3, 2014
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    Hey there, I'm trying to test the TL081 opamp model I downloaded for LTspice IV, so I made a simple summer circuit as shown on the pic.
    The problem is also shown in the pic, Vin/off seems to be very large and therefore the circuit just doesn't work (I expect Vs to be 3+2=5 V but I'm getting -51 V). Any suggestions? Thanks!
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to the forum!
    You've got the inputs to the inverting and non-inverting inputs swapped over.
     
  3. javi1991

    Thread Starter New Member

    Jul 3, 2014
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    Well, thanks for the answer... but it's still happening the same thing, as you may see. Maybe I'm still having something wrong there?
     
  4. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Strictly speaking that's not really a summing circuit.

    To make it a summing circuit I think you need R1=2k, and another 1k from the non inverting terminal to ground.
     
  5. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi javi,
    It works for me, is your model OK.?
    E
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Am I being nit picky or is V- setup wrong?
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Ok, I am looking at Eric's V- setup and OP definitely has V- setup wrong.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Strange. It works ok in my LTS sim for various opamps (didn't trythe '081).
     
  9. ericgibbs

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    Jan 29, 2010
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    hi alec,
    I tried other OPA types, also no problems

    E
    TL801 model attached
     
  10. javi1991

    Thread Starter New Member

    Jul 3, 2014
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    Thank you very much, maybe there was something wrong with my model... I tried the same with a TL082 and worked!
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Check the pinout assignment in the model file. It may be different than what LTspice uses for the op amp symbol. Look at a different LTspice op amp model for comparison.
     
  12. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    In the original diagram the inverting and non inverting terminals are swapped. To get it to work at all you need to swap those as found in Eric's drawing. I had to look really close to see this as the (+) and (-) inputs show up very very tiny on my screen.
     
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