Simple non-inverting op-amp circuit not amplifying

Discussion in 'Homework Help' started by jameselder232, Oct 31, 2010.

  1. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    Hi, as in the title i have simulated the circuit below, and the gain should be equal to 10 right? Well i am powering the 741 op-amp with +2.25v and -2.25v, and the input signal is a sine wave of 400mv pk-pk. You can see a snapshot of my output graph as well in the screenshot below, i am using a program called Yenka for simulation.

    [​IMG]

    This is really bugging me as its a simple circuit, trust me i have played for ages trying to get this to simulate properly, changing from voltage rails to batteries etc.

    Thanks,
    James
     
    Last edited: Oct 31, 2010
  2. hgmjr

    Moderator

    Jan 28, 2005
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    You have drawn the non-inverting opamp incorrectly. Take a closer look at your hookup.

    hgmjr
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    The 1k in your circuit should be from the negative input terminal to ground.

    hgmjr
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    It is worth noting that a 741 is specified only for +/- 15 volt rails. The op amp may not be able to swing the output to within 2 1/2 volts of either rail. A more modern micropower op amp would likely work a lot better.
     
  5. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    0
    I've tried the same circuit with +/-15 volt rails and the output doesn't change. And i don't understand how the circuit is wrong? are the resistors the wrong way round?

    Thanks,
    James
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    You have the hookup of the components incorrectly connected.

    If you move the end of the 1k from the output of the opamp to the negative input you should get a much closer result.

    hgmjr
     
  7. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    Ah yeah i can see it now, its such a stupid mistake but i just couldn't see it :). Thank you.
     
  8. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    And now the voltage rail values also have an effect, i get clipping when using +/-2.25v rails, even though the gain only amplifies the output upto +/-2v. I found that actually any rail voltages below +/-4v give me clipping with an input of 400mv pk-pk. And at 200mv pk-pk in, any rails below 3v give me clipping.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    If you think about it. The reason that you got a gain of +1 before was that you had the opamp connected at a voltage follower. The 1K resistor simply acted as a load resistor having little or no effect on the gain of the opamp.

    hgmjr
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    I need you to draw your circuit hookup and post it here.

    hgmjr
     
  11. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    Yeah i understand it just ensured the output was the same as the input. Usually i would be able to draw the circuit my own way, but as my simulation program does not let me switch the order of the input terminals, i got a little confused and ended up connecting the feedback wrong. I'm just hoping my labs have some op-amps available which can function fully with rails of +/-2.25v as my specification says to use 3 AA batteries.
     
  12. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    Here you go:

    [​IMG]
    Input is 400mv pk-pk, as you can see rails are +/-2.25v and clipping occurs on the output. Gain is 10, thus theoretical output is 4v pk-pk.
     
  13. hgmjr

    Moderator

    Jan 28, 2005
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    The circuit acts as though the opamp is open loop.

    What happens if you change the value of the 9K resistor to 1K. This equates to a gain of +2.

    hgmjr
     
  14. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    I still get clipping:

    [​IMG]
     
  15. hgmjr

    Moderator

    Jan 28, 2005
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    Something must be wrong with the power rails then.

    What happens if you increase them?

    hgmjr
     
  16. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    If i increase them to +/-2.5v or above for an input of 400mv pk-pk there is no clipping at a gain of +2. For a gain of +10 this threshold is +/-4v.
     
    Last edited: Oct 31, 2010
  17. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    I simulated the same circuit with voltage rails as opposed to the batteries and i get the same situation. One thing i noted earlier is that for a gain of +10, an input of 200mv pk-pk, i require only +/-3v or above to get no clipping.
     
  18. hgmjr

    Moderator

    Jan 28, 2005
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    It is just possible that the opamp model is set to mimic the behavior of the 741 for input supplies less than 5Volts.

    hgmjr
     
  19. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    Sorry i don't understand. I am using a 741 op-amp in the simulation. Also when the input is 200mv pk-pk then i require voltage rails of at least +/-2.2v for a gain of +2. So it appears the minimum required voltage rails to get no clipping, reduces as the input signal reduces in magnitude.
     
  20. jameselder232

    Thread Starter Member

    Oct 31, 2010
    40
    0
    Just wanted to say i started reading about Millman's theorem after seeing your profile and it does look alot quicker than the alternatives from the little that i've read, i'm definitely going to read up more about the theroem. :)
     
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