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5v single supply opamp AC Gain

Discussion in 'General Electronics Discussion' started by calck, Apr 6, 2011.

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  1. calck

    calck

    3
    0
    Apr 6, 2011
    Hello,

    I have been looking around and cannot find the answer to my problem no matter what I do. I am still learning about Op-Amps and I have the task of taking an audio input from an MP3 player, typically at .2v peak to peak and I want to increase its gain as much as I can before it clips say 10x - the plan is to have the signal go into a VCF I have which is working fine but for testing purposes I have the output of the opamp only connected to my oscilloscope and nothing else.

    First for a test I tried a 741 and a 071 but then moved to a LM358. Below is my circuit it is a simple non-inverting amplifier. The top op-amp is my half supply reference and the bottom 2 is my 2 attempts. First I tried bottom left (a unity gain buffer) - It works fine and outputs the signal exactly the same as the input on my oscilloscope but the one on the right doesn't! it just flattens - I was trying to make 10x gain.

    when I remove the 1K resistor to Vref in the feedback so the feedback is only a 10K resistor it outputs the signal with a lot of noise!! why is this? is it oscillation? or thermal noise?

    I also want to ask about input impedance as I plan to use a similar circuit on the output of the VCF is it right to assume because of the very high input impedance of the amp that it will be roughly speaking the 1M ?
     

    Attached Files:

  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Can't think of anything helpful specifics here I'm afraid, the theory & diagrams are correct (apart from the input cap polarity, but that shouldn't throw it off that bad).
    There are some subtle differences to the circuit found in the LM158 datasheet (page 17) from National though, have a look at it and see if it helps.
    The answer to the last question is yes (even if I don't know what a VCF is).
     
  3. calck

    calck

    3
    0
    Apr 6, 2011
    Voltage controlled Filter - Its just a state variable filter, but its output load affects its resonance.


    Hmm Ill keep going at it in that case, just wanted to check if it was anything obvious.
     
  4. MattyMatt

    MattyMatt

    161
    0
    Mar 24, 2011
    Just a quick little link for ya, I am actually going to be doing a similar amp.... only for my Bass guitar... so a little Capacitor modification is necessary, but this site you may find very helpful.

    http://tangentsoft.net/audio/cmoy-tutorial/

    Matty-
     
  5. calck

    calck

    3
    0
    Apr 6, 2011
    well I worked it out, Resqueline thanks for pointing me to the one on the data-sheet (always what you have to do but I never do it.) I built it exactly like the data sheet and I get 10x gain then I gradually converted it to my layout and it seems to be something to do with the 100n capacitor to 0v on the voltage divider in feedback, without it the AC disappears and it just hangs at about 1/2 voltage.

    I need to find out what it does!! having a look through a lot of amplifiers in commercial schematics such as guitar pedals have this too, I imagine it creates a low pass filter? maybe there is HF oscillation at low voltages???
     
  6. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    It keeps the input offset voltage/current from upsetting the output. With the cap there you only get 1x the input offset (on the output), without it you'd output 10x the offset.
    Together with the op-anp & the feedback resistors it creates a 2-pole (is that the correct term?) high-pass filter where gain = 1 @ DC & gain = 10 @ HF.
    Observe that with your resistor value of 1k instead of the datasheet's 100k you'll need a 10µF cap instead of the 0.1µF to keep a 16Hz cutoff frequency.
    But I'm surprised to learn it utterly fails to work and just hangs at 1/2 the supply however, can't explain that w/o setting it up & delving deeper into the matter.
    I have observed different peculiarities in different op-amps previously.
     
    Last edited: Apr 8, 2011
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