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Op amp help

  1. Feb 20, 2010 #1
    I am play with and op amp right now but getting unexpected results. I am trying to create a non-inverting amp. Here is how I have it hooked up:

    opamp.jpg

    According to my calculations, I should be getting about 5.3 volts between the output and ground, but my voltage meter is showing 0.22 volts. Any ideas what I am doing wrong?
     
  2. jcsd
  3. Feb 20, 2010 #2
    Ground the negative side of the 15 volts.

    Bob S
     
  4. Feb 20, 2010 #3
    I tried that, but now I get a constant 7.8 volts no matter what the input.
     
  5. Feb 20, 2010 #4
    Last edited by a moderator: Apr 24, 2017
  6. Feb 21, 2010 #5
    Use your original circuit and hook up the 15V power supply to 2 series connected 1k resistors forming a voltage divider. Use this divider to bias the signal ground at 1/2 of the supply voltage. This "virtual ground" should be used as a reference for all other measurements. For example your power supply measurement at pin 7 (V+) should measure +7.5V and pin 4 (V-)should read -7.5V. Be sure to connect the negative terminal of your 1.3V input signal to this virtual ground. This is a crude method of providing a balanced supply for your circuit but it should work for your purpose. In the future you may want to use a split power supply that provides both positive and negative voltages.
     
    Last edited: Feb 21, 2010
  7. Feb 21, 2010 #6

    vk6kro

    User Avatar
    Science Advisor

    Welcome, Live2Learn.

    Yes, something like this:

    http://dl.dropbox.com/u/4222062/opamp.PNG [Broken]

    Both inputs are referenced back to the virtual ground at the junction of the two 1 K resistors.
    V1 is a 1000 Hz 1 v p-p signal shown as the white trace and the output is shown as the green trace. Gain is about 4.
     
    Last edited by a moderator: May 4, 2017
  8. Feb 21, 2010 #7
    You might choose a better op amp that allows input signals that included ground.

    In the data sheet you will likely see VSS+0.0V or VSS-0.1V for values named something like VIN+ and VIN-.
     
    Last edited by a moderator: Apr 24, 2017
  9. Feb 21, 2010 #8
    OK, I tried this method and I am getting +7.5 v at V+ and -7.5 at V-, but now I am just getting a constant 6.5v from the output. The input voltage does not affect the output voltage.
     
  10. Feb 21, 2010 #9
    JerryG, your new configuration schematic should work.

    Check hookup. Something may be connected incorrectly.

    Measure voltages on operational amplifier.

    Replace operational amplifier.

    Replace resistors.

    When I have a problem like this it is usually because I'm doing something really stupid.
    Check everything.
    Could there be a problem with your voltmeter?
    Could the OA be oscillating?
    Are you using a unity gain OA?
    7.8 V is about 1/2 the power supply voltage. Are you measuring 1/2 power supply voltage?
    Measure all voltages with OA removed.
    Check data sheet for OA. Read the small print.
    Change OA types.

    Ah the fun of electronics.
    Let us know what the problem is.
     
  11. Feb 21, 2010 #10

    I have checked the connections many times. Voltage meter works properly. I even tried something simple like a voltage follower (buffer amplifier) and it did not work. I guess I could go buy another op amp. What are the chances that a brand new op amp from frys came DOA?
     
  12. Feb 22, 2010 #11
    It is possible that your circuit layout is such that there is excessive inductive coupling from output to input at high frequencies, leading to an oscillation. I suggest you use your corrected ckt diagram in post #4, with a single 1nF to 10nF capacitor in parallel with the 10k feedback resistor to add a little stability. What is your circuit layout, and is the IC in a socket? You have the same pinout as an 8-pin 741 available at Radio Shack and elsewhere, so that is a reasonable substitution.

    Bob S
     
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