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Multisim 8 question - negative voltages?

Discussion in 'Electronic Design' started by Marcin Szczepaniak, Feb 10, 2007.

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  1. Hi!

    I am trying to simulate some kind of regulated power supply using Multisim 8.

    Look at the circuit (It amplifies voltage at 200 miliohm serial resistor)

    It is minimal circuit which shows my problem.

    As you can see at picture - LM324 produces negative voltage relating to ground!
    But it is impossible - opamp is powered with +12 and 0V voltages.

    When i change LM324 with 'virtual opamp' it works as expected - there is no
    negative voltage at output.

    I know that LM324 is far from ideal but i think it doeesn't have ability to
    produce negative voltages, especially when there is no capacitor or inductor in
    this circuit.

    My question is - what am i doing wrong? What should i set in Multisim to have
    correct simulation ?

  2. Fred Bloggs

    Fred Bloggs Guest

    Who knows- are you doing DC analysis with an unrealistic limit on the
    maximum number or iterations? IIRC, the LM324 is problematic for SPICE
    in general, and the real part has a tough time getting Vo below 0.7V
    while sinking currents in the 3mA range anyway, and your circuit
    requires it to go to 0.24V- not going to happen.
  3. How much negative voltage? Spice models like to include input offset
    voltage, and often they use the part's worst-case spec'd value. As
    for the output range, most opamp models fail to accurately limit near
    the part's voltage extremes, which is ground in the case of the
    If you study the model, you'll see why it goes negative. But if the
    model has any integrity at all, it should be setup so it can't go far.
  4. Fred Bloggs

    Fred Bloggs Guest

    Almost -4V at a gain of 1....

    Spice models like to include input offset
    Input right at Vcc/2...
    Not a viable suggestion....
  5. John  Larkin

    John Larkin Guest

    You have a bad simulator *and* a bad opamp model. A real-life circuit
    can't do this.

  6. So which simulator should i use to have proper results?
    Could you recomend some software to me ?
  7. Damir

    Damir Guest

  8. I don't agree with John's remark that your results
    are due in part to a poor spice simulator. It's the
    opamp model you're using that's at fault. The best
    simulator package (most use the same Berkeley spice
    engine, with minor enhancements) can have a few poor
    models in its library, and even the worst simulator
    surely has a few excellent models in its library.

    Most simulators let you add models to their library,
    and most serious spice users spend plenty of time
    seeking better models or making our own. Here, you
    need a better LM324 macromodel.

    One suggestion: make a schematic of your '324 model
    by translating its netlist to a drawing. This will
    let you see for example how its output stage works.

    One other suggestion: test your models first with
    some simple test circuits. Then you'll know in
    advance if you can place much faith in the results.
  9. John  Larkin

    John Larkin Guest

    What's bad about Multisim (aside from the price) is bait-and-switch
    business practices, rotten support, and buggy device models.

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