Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Inverting Op-amp Always outputting .6V

  1. Jan 20, 2010 #1
    Ok So I am pretty baffled at the moment. I am trying to get my inverting op-amp to work LM324. I have 15V supply going in. The + is going to ground with 5.7k resistor. The - has Rin and Rf both 10k. So this should output -1 * Vin. Vin is supplied by +2.8V from 2 AA batteries. But all I can get out is .6V. Thanks for any input.
     
  2. jcsd
  3. Jan 20, 2010 #2

    berkeman

    User Avatar

    Staff: Mentor

    You are powering the opamp between 15V and ground, and want to get a negative output voltage out of it?
     
  4. Jan 20, 2010 #3

    berkeman

    User Avatar

    Staff: Mentor

    If you change your supplies to +/-9V (like use two 9V batteries), it should work a bit better.

    Welcome to the PF, BTW!
     
  5. Jan 20, 2010 #4

    vk6kro

    User Avatar
    Science Advisor

    If you connect the LM324 like this:
    LM324.PNG

    with the + input connected to a reference point that is half the supply voltage, the output should move between 0 and the + supply voltage whe the input moves from near the + supply voltage to zero.

    The 324 has to have a single supply and it cannot accept input very close to the + supply voltage.
     
  6. Jan 20, 2010 #5

    berkeman

    User Avatar

    Staff: Mentor

    Oopsies. Thanks for catching that vk6kro!
     
  7. Jan 20, 2010 #6

    vk6kro

    User Avatar
    Science Advisor

    Actually you were right, Mike. Checked the data sheet. It doesn't have to have a single supply, but it is usually used like that.

    Incidentally in the sticky above,
    https://www.physicsforums.com/showthread.php?t=315139

    the link
    More simple schematics :http://www.uoguelph.ca/~antoon/gadgets/gadgets.htm [Broken]

    seems to have gone faulty or been shifted. Been like that for a while.
     
    Last edited by a moderator: May 4, 2017
  8. Jan 20, 2010 #7

    berkeman

    User Avatar

    Staff: Mentor

    Got it, thanks. I was able to find where the "gadgets" poster moved to after university, and updated the link. Man, what a lot of schematics!
     
  9. Jan 20, 2010 #8

    uart

    User Avatar
    Science Advisor

    FYI guys. When an OpAmp is said to feature "single power supply operation" it usually just means that the input common mode range extends all the way down to (and perhaps even very slightly below) the negative rail.

    As long as you respect both the maximum supply range (Vcc-Vee) and the input common mode range then it matters not whether you use a +/- supply or just a single supply. For example +/- 6V is no different to single supply 12V as far as the opamp is concerned (for any opamp, and not just for a single supply type), provided that the input common mode range restrictions are obeyed. The only difference is that with a single 12V supply you are more likely to want to reference your input signal to the -ive rail (0 volts) instead of the center. Hence the input common mode range is normally the only real consideration as to whether and opamp is described as "single power supply" or otherwise.
     
    Last edited: Jan 20, 2010
  10. Jan 20, 2010 #9

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    I was wondering about all this. I've never heard of an op-amp that is 'single supply only'. The inputs and output are designed to work within the supply rails. I don't think the op-amp typically cares where what we call 'ground' is at relative to these supply rails.
     
  11. Jan 21, 2010 #10

    dlgoff

    User Avatar
    Science Advisor
    Gold Member

    I attended a seminar that National put on when they first introduced the LM124/LM224/LM324. I still have their little application booklet, Quadzilla. Here's what the have to say, which has already been said here:
    Boy does this bring back memories.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Inverting Op-amp Always outputting .6V
  1. Inverting op amp circuit (Replies: 12)

Loading...