Why does this circuit invert it's output?

Discussion in 'The Projects Forum' started by GRNDPNDR, Apr 1, 2012.

  1. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
    435
    7
    I don't know what you would call this circuit, but it's designed to turn a piezo signal into a ON/OFF signal. This is for a kick drumpad.

    I'm fairly sure I have it built correctly according to the diagram and real world tests confirm it's functionality as perfect, but since I don't fully understand it I'm not sure if it's supposed to be inverting the output or not. I would tend to think it shouldn't.

    Best I can do to replicate the piezo source is use an AC source.

    R1 -used to control main pad sensitivity.
    R4 - used to adjust kick pulse duration. Also used to control with rebound hits, so multiple hits aren't registered from a single kick.

    Attached is my circuit with scope, and the original circuit.

    Can anyone see why this would invert the output? and why the op-amp seems to turn the AC signal into a square wave? (I think it's supposed to)
     
  2. rfredel

    New Member

    Jan 23, 2011
    25
    3
    Hello,

    in the first diagram you use the op-amp as a non-inverting amplifier with open loop amplification. So you get a square wave output.

    The second diagram shows the op-amp as inverting amp with close loop amplification. The amplification can adjust with R1.

    I think that's the mainreason of the invert output.

    Best regards

    Rainer

    PS: Sorry, if my English is not so good.
     
    GRNDPNDR likes this.
  3. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The LM386 is an an audio amp, NOT an operational amplifier.

    You need to use an opamp whos' common mode input range includes ground.

    As noted, you have swapped the inverting and noninverting inputs - but you can't simply use an audio power amp instead of an opamp. They are different critters.

    Also, you cannot use a TL07x, TL08x nor LM35x family opamp, as they cannot "see" within 3v of the negative rail.
     
    GRNDPNDR likes this.
  5. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
    435
    7
    I'm not sure what you mean by open/close loop. What's different on my circuit from the diagram?

    I see I had the wrong pins on the op-amp connected.
     
  6. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
    435
    7
    Thanks for pointing that out. I was luckily able to find a SPICE model and create the component in multisim for simulation.

    I think it's working the way it's supposed to. I get convergence errors in simulations, but that could be any number of ridiculous things.
     
Loading...