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Effect on the output using a TL 072 op-amp

  1. Mar 24, 2016 #1
    1. The problem statement, all variables and given/known data
    Replace the ideal op-amp by the TL 072 [the default op-amp] and note the effect upon the output. Can you explain it?

    2. Relevant equations
    upload_2016-3-24_21-4-0.png

    3. The attempt at a solution
    i can see there is a large spike on the output volts of the trans-impedance op-amp using the TL 072 op-amp. however im struggling to explain why.
    here is the what the waveform looked like using an ideal op-amp
    upload_2016-3-24_21-10-55.png
     

    Attached Files:

  2. jcsd
  3. Mar 24, 2016 #2

    berkeman

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    What analysis are you running there? You just want to get the DC setpoints, right? And the results for the ideal opamp look wrong -- how can -1nA through the 10M resistor and the 10:1 divider give you something * 10^-30? The result for the TL070 opamp looks more reasonable...
     
  4. Mar 25, 2016 #3
    the output for the ideal op-amp is 100mv
    the output for the TL072 is *10^-30.
    im running simetrix.
     
  5. Mar 25, 2016 #4

    berkeman

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    Ah, sorry, I got them backwards. There must be some hookup problem with the TL072 schematic. Why are there blue X's on the input & output connections for the ideal opamp schematic, but not on the TL072 version?
     
  6. Mar 25, 2016 #5

    berkeman

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    Also, can you probe all of the DC voltages in the circuit for each circuit?
     
  7. Mar 25, 2016 #6
    the blue is because the op-amp was highlighted at the time it makes no difference to the output.
    each circuit has a dc voltage probe on it next to the marker.
     
  8. Mar 25, 2016 #7

    berkeman

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    I only see the output node probe. What about all of the other nodes?
     
  9. Mar 25, 2016 #8

    berkeman

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    Also, are the power supplies hooked up correctly? In Micro-Cap SPICE, I have to make explicit connections to device terminals. See how your +/- power terminal inputs still have open squares on them? Doesn't that mean they have nothing connected to them yet?
     
  10. Mar 25, 2016 #9
    upload_2016-3-25_17-17-41.png
    this is how i connect up the power supply and ive put makers on each probe
     

    Attached Files:

  11. Mar 25, 2016 #10

    berkeman

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    Can you probe the power supply terminals of the opamp? I still think they are not connected...
     
  12. Mar 25, 2016 #11
  13. Mar 25, 2016 #12

    berkeman

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    Why are they only +/-10mV?
     
  14. Mar 25, 2016 #13
    because that is the input we were given for the circuit
     
  15. Mar 25, 2016 #14

    berkeman

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    No way. The power supply rails for an opamp need to be in the +/-12V range. Not 10mV. Set them to +/-12V for both simulations and see what happens.
     
  16. Mar 25, 2016 #15
    i see what what i ahev done now the original question stated "Calculate the current I to give an output voltage of 0.1 V if Rf = 10 MΩ, R1 = 90 kΩ, R2 = 10 kΩ." ive taken 0.1v as the input voltage.
     
  17. Mar 25, 2016 #16

    berkeman

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    So what are you going to set as your power supply values now? Do both simulations work now with +/-12V rails?
     
  18. Mar 25, 2016 #17
    upload_2016-3-25_17-44-8.png
    changing the voltage to +/-12 has no difference to the output?
     
  19. Mar 25, 2016 #18

    berkeman

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    It looks like you have the + & - voltage rails swapped?
     
  20. Mar 25, 2016 #19
    ive restructured it now
    upload_2016-3-25_18-30-29.png
     
  21. Mar 25, 2016 #20

    berkeman

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    Looks to be working better! Now you can look into the small errors that the TL072 has versus the ideal opamp. What numbers do you get for the ideal opamp circuit now?
     
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