First project, help required!

Discussion in 'The Projects Forum' started by mchl_hemingway, Mar 16, 2013.

  1. mchl_hemingway

    Thread Starter New Member

    Mar 16, 2013
    7
    0
    Hi,
    I was hoping someone could outline the dimensions of my ignorance. I'm a physics student hoping to start a PhD this year and need to get myself to a reasonable standard with analogue electronics. I've set myself a project which has shown me how little practical electronics I know. Class D amp, what a mistake. Always biting off more than I can chew. At the minute I'm just scraping together the low side using op-amps. I understand the theory of class D operation and perhaps the schematic will be obvious to the more experienced. It’s just square-wave > triangular-wave > PWM signal. I’m just using a test sinoidal signal on the non-inverting input of the final op-amp. This is in ISIS Proteus btw.

    Though I have had some nice looking signals, I think I’m missing some of the basics. I’m constantly dancing a fine line with non-convergence, eg the triangular signal out of the 2nd op-amp was non-symmetric (slightly lifted w.r.t 0v) so I added R7 to pull it down slightly. This works fine until the resistor reaches 33K then bang, non-convergence error. I have the feeling that I’m missing the basics. I’ve spent days tinkering and don’t seem to be getting anywhere. I understand all the components, it’s the artistry I’m lacking. For now my hope is to produce a reasonable low end stage (input signal to PWM signal for driver stage).


    If someone could cast their eyes over this schematic and list the mistakes it would be greatly appreciated. That is if you can stop yourself laughing. Any contributions will be appreciated. I know there are ungrounded pins on the op-amps but this makes no difference in my case. I’m going to keep working on this over the next few months and try to build this for a cheap subwoofer I have. I really want to order an oscilloscope and bubble etcher, etc but I think for now It would be jumping the gun. Many thanks
     
    Last edited: Mar 16, 2013
  2. patricktoday

    Member

    Feb 12, 2013
    157
    42
    You could replace the input to the second op amp (+) with a pot between the two rails; this would allow you to fine-tune the offset just slightly below center. If you apply a voltage divider to the output you're applying a scaling multiplier to the result where this would perform a subtraction.
     
  3. mchl_hemingway

    Thread Starter New Member

    Mar 16, 2013
    7
    0
    Thanks a lot, seems obvious now I think about it. It's gonna be a case of solving one problem at a time. Thanks for helping with the first.
     
  4. patricktoday

    Member

    Feb 12, 2013
    157
    42
    Sounds good! And between me and you and the dog's ear, I don't think your problem specification is quite clear on the other issue.
     
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