About op amps...

Discussion in 'The Projects Forum' started by J6Str, Jan 28, 2012.

  1. J6Str

    Thread Starter New Member

    Jan 7, 2012
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    Hi everyone...quick question:

    I built this little circuit that's supposed to amplify a small voltage unto a higher voltage to run a few devices...on Multisim, as I have the diagram setup (see attached) works fine, but when I put it on my circuit board....no amplification whatsoever. So my question is...

    Am I supposed to do something with the unused pins? That's really the only problem I can see with the circuit.

    Thanks!
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,645
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    Hello,

    How is the circuit powered?
    Is it single supply or is it dual supply?

    Bertus
     
  3. J6Str

    Thread Starter New Member

    Jan 7, 2012
    5
    0
    I believe it's just a single supply in the diagram I made, however the input voltage is the output of a digital circuit which gives off +/- .3V at it's output. I honestly have no idea whether it is supposed to be Single or Dual Supply...

    See what I'm trying to do is amplify that small .3V to around 20V. As I mentioned before, in Multisim it seems to work as I have it laid out in the circuit...but I can't get an output when I actually build the circuit in my board.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In the shown diagram there are NO powerconnections given.
    If the powersupply is single, the input voltage is to low.

    Bertus
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    896
    A lousy old 741 opamp has input pins that do not work properly if they are less than 3V more positive than its negative power supply pin 4 and more than 3V more negative than its positive power supply pin 7.
    Multisim does not know this.
     
  6. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    Are you under the impression that you can get more voltage out of an op amp then the power supply voltage going to the op amp? I ask this since you seem to have no idea about the power requirements of an op amp. :confused:
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    896
    In most threads from students we see that Multisim simulation software always shows an output swing that is higher than the supply voltage.
    Multisim shows an output swing even when the opamp is not powered.
     
  8. J6Str

    Thread Starter New Member

    Jan 7, 2012
    5
    0
    Yes I am a student, and Yes I don't know much about Op-Amps.

    This idea came about from some random moment I had after I went through a bunch of random components and decided to try and make something simple. An Instructor advised me to use an Op-Amp. And to clarify, THIS IS NOT AN ASSIGNMENT FOR SCHOOL!!! Just for my own personal enjoyment...

    My idea seemed simple at first: Have a few spare LED's from a previous project, an IR Sensor/Receiver that was omitted from another project, and decided simply to emulate The Force (Yes, the Star Wars Force). By this I mean break the IR's beam, and light up an LED. (I know a little about the IR, about the power required for the LED) My problem is amplifying the Vout of the IR Sensor to run the LED(s) since the Vout on the Sensor is shown in the datasheet as +/- .3V.

    Just a project to mess around and branch off from there. Teacher said use Op-Amps, use Multisim to see if it works, and go from there. And now I'm here, asking for direction. So any help would be appreciated...

    All I'm asking really is for direction...simple "Try this or research that..."

    Thanks.
     
  9. #12

    Expert

    Nov 30, 2010
    16,257
    6,761
    You'd have to give that chip about +25 volts and minus 4 volts for its power. The spec sheet says it will survive as much as 44 volts total supply.

    Try lowering the gain to keep the calculated output voltage lower and you can use more reasonable supply voltages.
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    If you are interested in learning more about opamps, you might take a look at the attached PDF's.

    Bertus
     
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