Help with Op Amp

Discussion in 'General Electronics Chat' started by car104, Mar 4, 2013.

  1. car104

    Thread Starter New Member

    Mar 4, 2013
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    Hey guys. I'm brand new to this forum. I'm starting to begin working with electronics as a hobby, and I'm attempting to tinker with op amps. I have an LM324 I got from RadioShack.

    The question I'm having is, as I think I understand, the op amp amplifies the difference in voltage between the inverting and non-inverting inputs. Right? So I've got, say +5v on the Vcc+ pin and 0v on the Vcc- pin. My problem is this: the output pin is outputting +5v. but since there is nothing going into the inverting nor the non-inverting input, shouldn't the output be 0v?

    Could this be because I'm supplying the op amp with 5v+ and 0v, instead of 5v+ and 5v-? Ideally, I'm trying to connect an iPod audio out to a 3.5mm jack. then connect this to the non-inverting input of the op amp. Then provide negative feedback to the inverting input, which is connected to ground.

    When I do this, though, my output is always high (5v) with their is power travelling to either input or not.

    I guess I'm just lost, and as much as I've researched it, I still can't figure out how to get one to function. Is there anyone that could explain this to me?
     
  2. #12

    Expert

    Nov 30, 2010
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    Op-amps do not amplify the difference between their input pins. The try to make their input pins be the same as each other by means of negative feedback.

    Your statement might be right if you include the word, "circuit". An op-amp circuit amplifies the difference of the inputs.

    I'm attaching some sample circuits so you can see how it's done. Ask again if it isn't clear in a few minutes.
     
  3. car104

    Thread Starter New Member

    Mar 4, 2013
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    I meant to put circuits. I couldn't find a way to edit the title. I've seen these circuits before. But as a general question. if I connect Vcc+ to 5v, and Vcc- to 0v(gnd), and nothing is connected to the inputs, why is the output still 5v? I was expecting 0v.
     
  4. #12

    Expert

    Nov 30, 2010
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    Because the input pins aren't connected to anything.
     
  5. w2aew

    Member

    Jan 3, 2012
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    64
    Floating inputs will have some offset, and they won't be properly biased, so the output will be unpredictable.

    Take a look for the appnote from Texas Instruments called "Op Amps for Everyone" - a very good basic reference for op amps.

    You might also be interested in watching this video on did on the basics of op amps, once you understand the fundamentals:
    https://www.youtube.com/watch?v=K03Rom3Cs28

    You might also be interested in this video that discusses power supply considerations for op amps in general:
    https://www.youtube.com/watch?v=MtccB9K09ck
     
    Last edited: Mar 4, 2013
  6. crutschow

    Expert

    Mar 14, 2008
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    Contrary to #12's statement, an op amp does amplify the voltage difference between its inputs but it does so with such a very high gain (typically upwards of 100,000) that the op amp is not useful without a negative feedback loop to establish the gain at a much lower value (unless you are using it as an open-loop comparator). The high gain means that the voltage difference between the two inputs under normal closed-loop operation is very small and thus the feedback circuit values determines the gain to a good (near ideal) approximation.

    This all means that you need to properly bias the op amp inputs with a feedback circuit to perform useful operations with it as the others in this thread have stated.
     
    Brownout likes this.
  7. #12

    Expert

    Nov 30, 2010
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    I just find it much easier to assume the inputs are very close to zero volts difference and work the math for the feedback like that instead of using less than a millivolt of difference and trying to multiply by the voltage gain of the chip while guessing how much more gain the chip has than the guaranteed minimum on the spec sheet.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    I understand that completely as that is the typical way to calculate feedback components and derive the circuit operation for op amps. I just wanted to make the point that the op amp does indeed amplify the voltage difference between the two inputs. But with the extreme high gain of an op amp the voltage difference so so close to zero for normal feedback operation of an op amp that the difference can be ignored. That generates the "virtual ground" that's often referred to with inverting op amp circuits when the plus input is connected to circuit ground.
     
    Brownout likes this.
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