Generating square wave from sinusoid using uA741

Discussion in 'General Electronics Chat' started by mghg13, Oct 4, 2014.

  1. mghg13

    Thread Starter Member

    Jul 17, 2013
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    Hello guys,

    Recently I was experimenting some basic op-amp circuits based on the common uA741 IC. In one of the circuits, I used the uA 741 as a comparator. Basically, I applied a 50Hz, 6V rms sinusoid between the inverting and non-inverting inputs. I expected that the output would be a perfect square wave, but the actual result was a bit suprising to me.

    During positive saturation of the op amp, the output voltage was not a constant. In fact the value DECREASED by approximately 0.1V by the time negative saturation was triggered. Similarly, during negative saturation, the output voltage was not constant. This time the value INCREASED by approximately 0.1V by the time the next positive saturation was triggered. See the attached picture for a sketch of the waveform I obtained

    This suprised me because the circuit is showing some sort of 'capacitor discharging' effects while NO capacitors were used in the circuit.

    So what practical reasons there might be for the op amp to exibit such a behaviour?

    thanks
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    All op-amps have a defined slew rate, i.e. how quickly the output can ramp up and down, specified in units of V/us.
    If you want faster slew, use a comparator such as LM311 or LM339.
     
  3. mghg13

    Thread Starter Member

    Jul 17, 2013
    63
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    Thank you very much for your prompt answer MrChips.

    Ok I understand what you mean. But why is the output sloping downwards and not upward during positive saturation.
    And similarly for negative saturation, why is it sloping upwards and not downwards. Hope you understand what I mean.
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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  5. crutschow

    Expert

    Mar 14, 2008
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    Is you oscilloscope set for DC on the input, not AC?
     
    mghg13 likes this.
  6. MrChips

    Moderator

    Oct 2, 2009
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    Sorry, I didn't notice the drawing you had posted. My response applies to the rising and falling edges, not the droop in the signal.
    Check your scope probe frequency adjustment and the DC/AC setting as crutschow suggested.
     
    mghg13, absf and ErnieM like this.
  7. mghg13

    Thread Starter Member

    Jul 17, 2013
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    Thank you very much studiot, crutschow and MrChips

    Now I understand :)
     
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