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Problems with using LM324 as an inverting amplifier

Discussion in 'General Electronics Discussion' started by ZanShini, Mar 2, 2018.

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  1. ZanShini

    ZanShini

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    Mar 2, 2018
    As part of my project I need to invert a pulse train without changing it's amplitude. Hence I want to use an Op-Amp as an inverting amplifier with unity gain. I'm using the circuit given in the following tutorial.

    https://www.electronics-tutorials.ws/opamp/opamp_2.html

    And also the schematic I'm using.

    https://imgur.com/a/pu8L3

    I chose LM324 to implement the circuit. I've used a single Op-Amp i.e. pins 1, 2 & 3, two 1K resistors for input and feedback resistances and connected pin 4 to battery's +ve terminal and pin 11 to the
    -ve terminal. I tested it with a 9v battery and as far as I understand, the output should be -9V. But my multimeter shows the output fluctuating around 0.98V.

    I want to know if I'm doing something wrong or if LM324 can't be used as an inverting amplifier?
     
  2. kellys_eye

    kellys_eye

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    Jun 25, 2010
    You can't invert the output without having a split-rail power supply.
     
    ZanShini likes this.
  3. duke37

    duke37

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    772
    Jan 9, 2011
    You cannot get a negative output if you do not supply a negative source.

    What do you mean by inverting?
    If the signal is 0 to 5V, do you want 5V to 0 or 0 to -5V?
     
  4. AnalogKid

    AnalogKid

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    Jun 10, 2015
    That is a non-inverting amplifier, the opposite of what you want.

    ak
     
  5. ZanShini

    ZanShini

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    Mar 2, 2018
    You mean I need the supply to be greater than the output I desire right? That is if I desire to get -5v output with 5v input, I need to supply the opamp with +Xv and -Xv where X>5 right? Please excuse my lack of basic knowledge in Electronics.
     
  6. ZanShini

    ZanShini

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    Mar 2, 2018
    Actually in some video I heard that in Psipce schematic the + terminal is the inverting one. As you can see the output is -9v. It confused me too how the voltage input at + can give output in reversed polarity.
     
  7. ZanShini

    ZanShini

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    Mar 2, 2018
    Yes I want the output to be -5V if I give 5V input. And in my project I want to invert the polarity of a pulse train.
     
  8. BobK

    BobK

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    Jan 5, 2010
    Assuming you mean a pulse that goes from 0 to V.

    You can invert a positive going pulse by using a transistor and two resistors. Same circuit can also change the level of the high portion of the signal.

    You can also do it with a single supply opamp. You will need to bias the + input to half the voltage you want out.

    This circuit will invert a 0 to 5V pulse train. By changing the biasing point and the gain you can also make the output have any level you want.

    upload_2018-3-2_10-19-3.png


    Bob
     
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  9. AnalogKid

    AnalogKid

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    Jun 10, 2015
    That seems pretty clear. To do this, you will need a rail-to-rail opamp powered by at least +5 V and -5 V, or a conventional opamp powered by +8 V and -8 V or more.
    That is much less clear. The problem is that in electronics the word "invert" can mean two different things. Inverting a voltage is what you describe above. But inverting the polarity of a pulse can mean inverting its *logic* polarity. In this case the signal still goes from 0 V to +5 V, but the high and low parts of the pulse wave are reversed, or "inverted". The times when the input pulse is at a logic 1, the output is at a logic 0, and vice versa.

    To help clear up things, tell us what is generating the input signal, and what the output signal goes to.

    ak
     
    ZanShini likes this.
  10. BobK

    BobK

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    Jan 5, 2010
    Okay, so my post is not what you wanted. Use an opamp with dual supplies, as AK said. Change the + input to go to ground, instead of the voltage divider in my circuit, and you are done.

    Bob
     
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  11. ZanShini

    ZanShini

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    Mar 2, 2018
    Thanks a lot to everyone who responded. It helped clear my basics about an Op-Amp and I learnt a lot from you guys. The circuit is working now. I hope for your help in the future too.
     
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