The output of Op-Amp[LM324] has error as non-inverting Amp

Discussion in 'General Electronics Chat' started by booboo, Apr 25, 2015.

  1. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    Hi everybody
    I'm a newbie and working on LM324 to learn Op-Amp. this is my circuit:

    [​IMG]

    One of the resistors is 217ohm and another 978ohm. then the gain should be 4.50691 as you can see. so far so good. I know this is for ideal Op-Amp. I applied 0.15v to the input and got 0.94v in the output. then the gain is 6.26667. again I applied 0.35v to the input and got 2.03v then gain should be 5.8!?o_O
    again I applied 1.86v and got 10.31 then the gain is 5.54301!:confused: holy crap! why isn't the gain stable? because of input offset voltage? How can I stabilize it?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    DickCappels and booboo like this.
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As said the gain formula is wrong.
    Also you are loading the opamp quite a bit.
    Try to make the circuit with resistors that are 10 times bigger.

    Bertus
     
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  4. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    Woops! yes. Thanks
    Thanks
    What do you mean "you are loading the opamp quite a bit"?
    I have already searched whole of the internet but couldn't find a mathematical approach to choose the best resistor for op-amp.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    You have 978 + 217 = 1195 from the output to ground.
    Normaly you would see values from 10K to 1M as feedback resistors in stead of the 978 Ohms that you are using.
    The resistor to ground must be adapted to get the correct gain.

    Bertus
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,140
    1,789
    If the output of the opamp is at +12V, then by Ohm's Law the current from the opamp output to GND is 12V / 1195 Ω ≈ 10 mA.
    Most opamps can source this much current, but are much happier with less.
    Without mental gymnastics you could use 9.78 KΩ and 2.17 KΩ and the opamp would now source about 1 mA.
     
  7. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    Thanks guys
    I used 145.6kohm as Rf and 2.14kohm as Rin and got an acceptable output. for example 70.1mv in the input, I got 4.93v in the output.
     
  8. cornishlad

    Member

    Jul 31, 2013
    196
    25
    Single ended supply...I admit to being quite rusty with opamp stuff but thought that in this arrangement the dc conditions should be established at the + input..usually a rail splitting pair of resistors...then the input thru a capacitor. The bottom end of the 217ohm should be via a capacitor..please shoot me down if I'm wrong :)
     
  9. #12

    Expert

    Nov 30, 2010
    16,278
    6,790
    LM324 "Common mode input range includes ground."
    This is a DC amplifier circuit. No need to capacitively couple DC to the input or the ground of the feedback loop.
     
  10. cornishlad

    Member

    Jul 31, 2013
    196
    25
    Should've read the question properly..nicely shot down !!
     
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