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Basic opamp circuit

  1. Dec 22, 2011 #1
    Hi, i am using Pspice schematics to build up an inverting opamp with 10 gain using an input sinusoidal source. For other opamp model, i will need to connect Vp and Vn to the top and bottom of opamp. But for this opamp (shown in the fig. below). Where should i connect my Vp and Vn? And how can i use the simulation to find the gain? i try to use create netlist-->simulate
    and it said my some error on my circuit.


    I have try to search the example in google but cannot find any basic opamp with sin source. Most of them are using 741 or other model so they got the top and bottom connection in opamp.

    For other opamp, i have connected like this. The opamp pin 4 and 7 have been connceted to Vp and Vn. But i dunno how to connect those into my basic opamp?

    Thank you
    Last edited: Dec 22, 2011
  2. jcsd
  3. Dec 22, 2011 #2
    Try simulation first, maybe it does not require that as they assume it's there.
  4. Dec 22, 2011 #3
    Your second diagram looks correct and the output voltage -4.499 (4.5) is correct. (gain is 4.5)
    I must admit that I am not familiar with your simulation program but your circuit as drawn is OK
    Last edited: Dec 22, 2011
  5. Dec 22, 2011 #4
    Sorry, i have uploaded the wrong diagram for the second one. I have fixed it now
  6. Dec 22, 2011 #5
    Thanks for your reply

    I cannnot do the simulation. It said: Error in the circuit. But it doesnt say what kind of error is that
  7. Dec 22, 2011 #6
    Are you certain that you have the supply voltages connected the right way round .
    Your labels look correct but you have drawn your amplifier "upside down" .....double check
  8. Dec 22, 2011 #7
    Thanks for your reply

    I think that is correct. i have change the V+ and V- "upside down" as well.
  9. Dec 22, 2011 #8


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    What simulation program are you using? How did you place that part in your schematic (first diagram)?

    Can you do something like right-click on the opamp and edit its properties? The supply voltages may just be part of its properties for that simple opamp model...
  10. Dec 22, 2011 #9
    Hi, I am using PSpice Student V9.1 Schematics. I did not edit any setting in the opamp. I have try to connect it like that(fig. below) but it still not working. It said some problem with my voltage source

  11. Dec 22, 2011 #10


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    Science Advisor

    Remove the inverting input resistor from the positive supply. You just need the signal source here.

    Also reduce the output of the signal source to 0.05 volts so the output should be 0.5 volts peak, but inverted.

    It seems to be working as you would get an output equal to almost the voltage of the negative rail with your circuit, and that is what you are getting.

    Are you observing the waveform at the output of the opamp?
  12. Dec 22, 2011 #11
    How come there is a 15V label at Vp? If you have +/-15V supply and you have an offset at Vp and a gain of -10, you are not going to get any wave form at the output, all you get is -15V hung on the rail!!!!
  13. Dec 23, 2011 #12
    I think the problem is the Vp =15v at the input.
    The label for your voltage input says Vampl = 1 which sounds sensible.
    The gain is x10 so with 1v. In Vout would be 10V
  14. Dec 23, 2011 #13
    I have try to what u suggest. The circuit seems like not working. If i reduce the output source to 0.05v the output will be 0.05v(there is no gain?) I cannot do the simulation to get the waveform because it said there is an error in my circuit but it doesnt say what kind of error is that
  15. Dec 23, 2011 #14
    Yes:(. i cannot get any gain. What i get from the waveform is just the my voltage source(-/+ 15V). Do you think i put my source at the right place?
  16. Dec 23, 2011 #15
    If i remove the voltage source (Vp and Vn) from the circuit. i will get 0V in the circuit.
  17. Dec 23, 2011 #16
    Is it possible for you to remove the Vp from the input signal (V1)
    The other circuits you have drawn do not have Vp connected to the input
  18. Dec 23, 2011 #17
    If you cannot remove 15V at Vp, then put the supply voltage at +/-180V!! You should see the wave form riding on top of -150V offset.

    Just hope the simulation will not give you a puff of smoke on the screen to tell you that you burn you amp!!!! Hehe!!

    Merry Christmas

  19. Dec 23, 2011 #18


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    You should be able to connect wires from the power supply to the opamp. This will get around any problems with labels.

    The only input should be the 1 KHz signal generator. This should be checked to see if it has any DC offset. Look in the properties of the signal generator to see if there is any.

    I specified a small signal of 0.05 volts AC to see if there was any DC offset in the signal generator.

    With a gain of 10 the output should be a 0.5 volts (peak voltage) sinewave centered on 0 volts
  20. Dec 26, 2011 #19


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    Staff: Mentor

    That looks to be the key point. You are trying to use the symbol "Vp" as both a test point (on top of the signal generator) and as the label for a power supply.
  21. Dec 26, 2011 #20


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    The original circuit looks ok and it should have a voltage gain of -10. The feedback resistor is connected to the correct input if you wanted an Inverting amplifier.
    As long as the supply voltage connections are correct, the circuit should work fine.
    You should definitely not be connecting an input to Vp as that will force the output to limit at -Vn.
    If you change R1 to 100k, the ac gain should be -1. The input (-) pin should always measure at very nearly 0V because that point is held at a 'virtual earth' potential because of the action of the negative feedback.

    I think there must be some problem with how you are specifying the input data to your simulation.
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