Simple op amp question!

Discussion in 'General Electronics Chat' started by rougie, Aug 11, 2013.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello,

    Can someone kindly look at the attachment which shows a simple non-inverting op camp configuration.

    To do the exact circuit as shown in OpNotes attachment, I would need three power supplies. One for positive voltage, one for negative voltage and one as the input voltage. So I did the circuit below but with the (VEE/-15VDC)
    connected to ground instead. So I am just using a positive 15VDC and ground as the op amp rails.

    Then, I have 1 volt at the non-inverting input. I am supposed to get 11 Volts but I get 14.51? I have omitted RL in my circuit.

    Does someone see why I am not getting 11VDC?

    thanks all!
    r
     
    Last edited: Aug 11, 2013
  2. sjgallagher2

    Member

    Feb 6, 2013
    111
    7
    Well ground is not the same as -15V, so that might be the issue. In order for the supply to be accurate you would if anything need +30V on one end and you reference at +15V, meaning I think you would have to bias everything +15V. Let me see if I can't figure out some exact calculations to explain what's happening, maybe someone will come along and help in the mean time. Be back soon.
     
  3. tindel

    Active Member

    Sep 16, 2012
    568
    193
    Which amplifier are you using? It could be that the common mode input voltage that you are feeding it (1V) is too close to ground for it to regulate properly.
     
  4. sjgallagher2

    Member

    Feb 6, 2013
    111
    7
    True, I just realized that I don't think the -V is important at all here, dumb mistake on my part :p just disregard whatever I said haha
     
  5. tindel

    Active Member

    Sep 16, 2012
    568
    193
    My sjgallagher2, you had the right idea - the amplifier doesn't really care what your rail voltages are as long as your input voltages are properly within the common mode input voltage requirements it should work.

    :D
     
    Last edited: Aug 11, 2013
  6. WBahn

    Moderator

    Mar 31, 2012
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    I second tindel. Check the specs for the common-mode input range.
     
  7. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello fellows,

    But that's what the example is showing me to do!

    "Common input mode"? I am looking in the spec of the LM741 and I can't find such a parameter. I see the "Common mode rejection ratio" but I don't think that that's what we are looking for.

    I see however that it's "supply voltage" is +/-18 VDC and that the "differential input voltage" (VDIFF) is 30V.

    So then could this mean I definitely need +15VDC at VCC and -15VDC at VEE?

    Thanks
     
    Last edited: Aug 12, 2013
  8. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    The parameter you are looking for the LM741 datasheet is input voltage range. This is stated as min +-12V typical +-13V. This with +-15V supplies.

    What you can read into this is that the inputs must be at least 3V higher than the V-. With V- connected to ground, this means the input cannot go below 3V. This is why the LM741 is not considered a single supply op amp.

    You need to use a more modern op amp designed for single supply use. These typically allow the inputs to go to ground or even a little below, and the outputs to near ground. An LM324 would work.

    Bob
     
  9. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    ah, I see!

    So even if I would put the input at 3volts according to formula:

    Vout = (1+Rf/Rin) x (+3) = 33VDC

    and that would not work either since my rails are 0-15VDC. So if I am to do any examples with this op amp I need +/- 15VDC as the rails.

    I think I will try an LM324 instead.

    Thanks BobTPH
     
  10. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    Yep. An LM324 can put the output anywhere from a few millivolts to a few volts below the upper rail. You can also get rail-to-rail input and output op amps if needed.

    Bob
     
  11. WBahn

    Moderator

    Mar 31, 2012
    17,715
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    Then it's a bad example, especially if it is telling you to do this with a 741.

    Get used to it -- lot's of bad examples out there. Sometimes they started out as decent examples and then got tweaked without actually being checked and the new conditions don't work because of some limitation of the part.

    Another common name for it is the "input voltage range".

    This only gives a bit of information and nothing definitive. But it says that VCC can be as much as 36V higher than VEE but that, under those conditions, you can still only have 30V difference between the inputs. These are likely absolute maximum specs and not maximum operating specs. The difference is that if you exceed the absolute max specs, you can't come back to them and complain if you let the magic smoke out, where as the max operating specs say that, as long as you adhere to them, the device will operate within some specified level of performance.

    No, it doesn't mean that. It only paints a very partial picture of the contraints.

    If you can, link to the specific data sheet you are using (if you have a hardcopy, you can either scan it or you can search for a comparable datasheet available on the web).
     
  12. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Great I will try this as soon as I have a chance!

    thanks
     
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