op amp problems, not using specified op amp or components

Discussion in 'The Projects Forum' started by ninjaman, Nov 28, 2014.

  1. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello

    I have a circuit
    upload_2014-11-28_16-21-14.png im not using the specified mcp6002(general op amp, single supply) or components, namely the diode or feedback resistors. I have an OA91 diode (germanium) 4760 in place of 5K and 678 in place of 648 resistors. the caps are polyester box type(I don't know too much about these) they are the correct value and non polarized so I thought they would be ok.
    I put in the correct supply and a signal generator from arduino, 1.1VPP at 4kHz. I used my little oscilloscope to measure the sinewave going in and the flat dc coming out. I got the correct values going in and about .38mV coming out. I measured this on a cheapo multimeter and got the same output. the gain I got was 8 using, 1+ R2/R1 * Vi. I used Vi as 0.5Vp as it was rectified through the diode. though I didn't remove the diode voltage drop which would put it down to 0.2Vp. the output at pin 1 should be 2.4vP
    I don't know if the caps are the problem, polyester supposedly not good at high frequency which is where I will be using them.
    not sure what else I may be doing wrong. any help would be great

    thanks
    getting closer to Christmas, WOOOOOOOOOOOOO

    simon
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,377
    494
    "Form follows function."

    Since you changed the form, it stands to reason that it changed the function.

    Do you want the function or not?
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    What opamp are you using? That is likely the most critical component in that circuit.
    It must be a modern low-voltage CMOS rail-to-rail type.

    What is the input referenced to? Ground? That means it swings 550mV above ground and 550mV below ground? Or is it riding on a DC level?
     
    Last edited: Nov 28, 2014
  4. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello

    the op amp is a lm324
    the output should be a dc level I think 2.4 volts
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    When powered from 5V, the LM324 cannot pull its output pin any higher than ~2.5V, so it will not work in this circuit. Try a LMV324, instead.

    I asked about the INPUT level.
     
  6. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    There was a reason for the choice of the op amp. The MCP6002 is a rail-to-rail single supply op amp that is perfect for this application. Use it.

    Bob
     
  7. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello MikeML

    the input is 1.1vpp from the signal generator, this goes through the diode which I imagine has a diode drop of 0.3 volts and rectifies the voltage.
    I did just order some of the mcp6002 from RS components they will arrive on Monday hopefully.

    thanks
    simon
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    It is not a great circuit. Do want the level of V(out) to change with different amplitude of the input signal?

    233a.gif
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    3,704
    While the op amp is an important feature, the feedback resistor (which you said you left out), is critical. Feedback is the only thing that prevents the op amp output from either pegging at low, high or (less likely in your case) oscillating wildly. Use the feedback resistor.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    His feedback resistor is 4760 in place of 5K.
     
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