how to calculate the amplitude for this sine circuit?

Discussion in 'Homework Help' started by danny2, Jan 4, 2010.

  1. danny2

    Thread Starter New Member

    Oct 28, 2009
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    how to calculate the output at U3 op-amp?
    let said my input is Vpeak=5.5V,frequency is 50Hz.
    by the way,i cannot get a pure sinewave from the circuit.how to enchant to get a smooth sine waveform?
    thank you.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    The diodes block the input to the opamp. Remove them or explain why they are there.
    The two 15V batteries, the two 50k resistors and the 200 ohm resistor also do not do anything and should be removed or explain why they are there.

    It should be like this:
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    In my circuit the input impedance is 5k ohms and since it has a lot of negative feedback the output impedance is about 0.075 ohms at 1kHz.
     
  4. danny2

    Thread Starter New Member

    Oct 28, 2009
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    The non-linear V/I characteristic of the diodes is being used to progressively compress the peaks of the triangle wave so it approximates to a sine wave.
    As the V10 triangular voltage increase,more current is shifted from D2 to D4 and from D3 to D1 until D2 and D3 cease conduction.Because the two 50k resistors ,the circuit is bipolar and functions similarly for positive and negative half cycle of the triangular wave(V10)


    as for the 15V batteries and 200ohm resistor.i dont know it.ah,u give me a tough question.i found this circuit from this forum.couldnt remember where it is now.
     
    Last edited: Jan 5, 2010
  5. Audioguru

    New Member

    Dec 20, 2007
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    How good is the sine-wave?
     
  6. danny2

    Thread Starter New Member

    Oct 28, 2009
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    what do you mean by how good is my sine wave?shape?harmonic?
    anyway.i attached my simulation result here and harmonic.

    TOTAL HARMONIC DISTORTION = 3.748306E+00 PERCENT

    so my sinewave consider good?
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    It is a pretty bad sine-wave but it is better than I thought it would be.
    The top and bottom still look like a triangle point.
     
  8. danny2

    Thread Starter New Member

    Oct 28, 2009
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    ya,i also thought like that too,abit sharp on top and bottom.so you know how to improve it?
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    If you add more diodes and resistors then the triangle wave will have more breakpoints.
    Look at the datasheet for the ICL8038 and XR2206 function grenerator ICs to see how many breakpoints they use.
    The ICL8038 uses transistors instead of diodes and the XR2206 uses transistors in a differential amplifier circuit to convert the triange wave into a sine wave.
     
  10. danny2

    Thread Starter New Member

    Oct 28, 2009
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    cannot use the IC,because not allow to used.
    in that case,by adding more diodes and resistors...i come out with this circuit,but i dont know how to put the op-amp to amplifier the voltage.
    and the THD= 5.388632E+00 PERCENT.

    This THD worst than the previous 1.nice shape?
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    You must plan and calculate the resistor values and input amplitude for low distortion.
    Yours appear to be random.
    Ask your teacher to explain how to do it.
     
  12. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Or read the full document I supplied to you in the other thread. It includes a rudimentary program to go stepwise through the approximations of the section slopes... Post #3 here http://forum.allaboutcircuits.com/showthread.php?t=32396
     
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