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Inverting Op Amp

Discussion in 'General Electronics Discussion' started by bluesteel, May 4, 2010.

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  1. bluesteel

    bluesteel

    3
    0
    May 4, 2010
    Hi All

    ive had a look around but cant seem to find any answers for what im trying to do

    i have a signal that basically goes from 5v to 0v and i need to invert it to 0v to 5v

    i have used a simple inverting op amp circuit based on an lm741

    the actual input voltages are 4.8v to 0.9v and i cant quite get the output to reflect that properly.
    i have changed the input resistor to change the high output side, and that is now up to 4.5v and that is good.
    i cannot however get the low output side to drop under 1v

    attatched is a diagram with what i have

    any help would be awesome

    cheers
     
  2. bluesteel

    bluesteel

    3
    0
    May 4, 2010
    sorry heres the image!

    [​IMG]
     
  3. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    The first rule of operational amps is that they work to make the voltage difference between the inputs equal zero. So if one input is grounded, and a positive voltage is fed in through the input resistor, then a negative voltage would need to be fed in through the feedback resistor. But there are no negative voltages in your circuit. So how is it supposed to work?
     
  4. bluesteel

    bluesteel

    3
    0
    May 4, 2010
    sorry, im a little bit lost with your reply (i dont understand the theory very well)

    i need to feed a negative voltage through the feedback (100k) resistor?

    it is currently working as its meant to in the diagram, but the voltage on the output needs to drop down below 1v, instead of the 1.9v which its currently doing

    cheers
     
  5. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    Here's a thought experiment. Make the input resistor and feedback resistor an equal value. Make the input signal +5V, and assume the output is 0V. Then pin 2 will be at +2.5V. Now make the input signal 0V and assume the output is +5V. Then pin 2 will be at +2.5V. So put a constant +2.5V on pin 3. How does it work when the input voltage goes through the range of 0V to +5V?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,305
    2,738
    Jan 21, 2010
    The 741 may not be able to get as close to the supply rail as you wish.

    are the 2 ground connections really the same, or is the power supply ground really the negative supply rail?

    I suspect that for what you want, you need to bias the non-inverting input to around 2.5V and then have an inverting amplifier with a gain of 1 (not 3 that you have now. You will also need an op-amp that can swing the output all the way to the supply rail.

    Alternatively, you could have a negative supply rail and the 741 should be able to do it.
     
  7. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    It is "working" because it has gone into a condition called "phase reversal" Calling this circuit "working" is like calling a dead clock "working" because it is right twice every day.

    The circuit violates just about every specification for that particular op-amp.

    To do what you want to do, you need a "rail-to-rail input/output" op-amp, apply 2.5V on the + input, make both resistors be 10K and run from 5V.

    Edited to add: Since you have +12V available, you can get an op-amp whose input includes the "negative rail" and whose output goes all the way to the negative rail. The input and the output do not have to go to the positive rail (12V), since you are working with only 5V.

    ---55p
     
    Last edited: May 6, 2010
  8. NetDevil

    NetDevil

    5
    0
    May 6, 2010
    sum up

    just to sum up:

    - You need another opamp for this job i would suggest TLC082 (i usally use this)

    - you need 2 power sources both positive and negative, othervise it will not be able to go to zero.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,305
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    Jan 21, 2010
    In NetDevil's reply, those 2 options are either/or -- you need to take one of those steps, you don't need to do both.
     
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