help me please

Discussion in 'General Electronics Chat' started by ksalatawi, Nov 29, 2004.

  1. ksalatawi

    Thread Starter Member

    Nov 29, 2004
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    I have project about op-amp and i want you to help me please i but the question in attachment with the message
     
  2. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I may have missed something, but I think the circuit cannot work.

    The amp needs a gain of 2 to amplify a 5v swing on the input to a 10v swing at the output. This gives a value of 10k for Rf. With a bias of 2.5v on the +input, the amp output would invert the input change and swing between -2.5v and +7.5v With the 2.5v offset provided by the battery at the output, Vo would then swing from -5v to +5v. So far, so good.

    The trouble is the supply+ rail is only +6v and so the output cannot possibly swing to +7.5v.

    It would work with a +-9v supply, though.
     
  3. ksalatawi

    Thread Starter Member

    Nov 29, 2004
    21
    0
    thank you
    but still the graph how ican draw it
     
  4. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Graph for Vg/Vo

    Let x be the input voltage and y the output voltage.

    Scale the graph as x = 0 to 5, and y = -5 to +5

    Draw a straight line from 0x/+5y to 5x/-5y

    That is how the graph *should* look except for the supply rail problem.

    With the limitation of the 6v positive supply rail, the slope cuts off at approximately +3.5y, so draw a line from 0x/+3.5y to 0.75x/+3.5y
     
  5. ksalatawi

    Thread Starter Member

    Nov 29, 2004
    21
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    thank you very much
     
  6. ksalatawi

    Thread Starter Member

    Nov 29, 2004
    21
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    what is the equation of the diagram
     
  7. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Vo=5-2(Vg)
     
  8. ksalatawi

    Thread Starter Member

    Nov 29, 2004
    21
    0
    thank you very much
     
  9. dragan733

    Senior Member

    Dec 12, 2004
    152
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    The schematic is Ok. On the output of the opamp is 7,5V as a result of the sum of signals Vg on the - , and the voltage 2,5V of the + of the opamp.
    Then:
    Vo= - (Rf/5)Vg + (1+Rf/5)2,5 - 2,5
    Vo=(Rf/10)(5-2Vg)
    For: Vg=5V, Vo=-5V, then: Rf=10K
    or, for: Vg=0V, Vo=5V, then: Rf=10K,
    then:
    Vo=5-2Vg
     
  10. dragan733

    Senior Member

    Dec 12, 2004
    152
    0
    Yes, the power supply is not good
    Or to explain better the schematic.
    The Output voltage is:
    Vo=5-2Vg
    The Output voltage of the op amp is:
    Vop=7.5-2Vg
    For the range of the voltage: Vg=(0-0.75V) the Output voltage of the op amp is alvays 6V, then Vo=4.5V

    In the attachment one can see the results graphic
     
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