Negative feedback voltage follower Op amp confusion

Discussion in 'General Electronics Chat' started by Art Mazur, Dec 26, 2015.

  1. Art Mazur

    Thread Starter New Member

    Dec 26, 2015
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    0
    I'm trying to build a typical voltage follower circuit. I would like to input an analog signal that ranges from 0-5v and have that same 0-5v repeated out of the output of an op amp.

    I'm using an LM741 from radioshack, and the V+ and V- is connected to 12v power.
    https://www.radioshack.com/products/operational-amplifier?variant=5717537733

    Now here is the problem I have. For testing purposes I am using a pot so that I can sweep 0 to 5v and verified that this is working correctly, however when I test the output (with negative feedback connected), the lowest voltage I am seeing is around 1.9v. For some reason I cannot go below this voltage. The 1.9 to 5v range works correctly.

    Am I doing something wrong? How do I get the full 0-5v range of the signal?
     
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,811
    Wrong chip. The LM741 chip can not output a voltage within a couple of volts of the power supply...in either direction. Give it a +12v supply and a -12V supply so it's working in the middle of its range and it will start working right.
     
  3. Art Mazur

    Thread Starter New Member

    Dec 26, 2015
    3
    0
    Do you have a recommendation as to what would work in my application? Would an LM324 be a better choice?
     
  4. #12

    Expert

    Nov 30, 2010
    16,298
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    I'm not up to speed on that right now. Somebody will be along in a minute and recite some oldies but goodies and maybe a modern rail-to-rail chip.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    LM324/LM358 can pull within mV of the voltage that its Vss pin is tied to. It can only pull up to within ~1.5V of the voltage that its Vdd pin is tied to. Running on +12V and 0V, you could expect the output swing to go from ~10mV to about 10.5V

    Good news about the input common-mode range of the LM324/LM358: the inputs work all the way to -0.5V when Vss is 0V.
     
  6. #12

    Expert

    Nov 30, 2010
    16,298
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    Lucky guess. It was exactly one minute until a helper with the right answer showed up.:p

    Darn! Now the web clock says two minutes.:(
     
  7. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    If you still want to using 741 to try, you need to using a 10K pot to do the offset job for the op amp, it can draw down the voltage to -V, pin 1,3 of 10K pot connecting to pin 1, 5(offset) of 741 and pin 2 of 10K pot connecting to -12V, if you wiring correctly then you just adjust the 10k pot and measuring the output(pin 6) to check the voltage changing, you can adjust the output voltage to match what you want via adjust the 10K pot .
     
  8. Art Mazur

    Thread Starter New Member

    Dec 26, 2015
    3
    0
    Thank you for clearing this up for me. I picked up an LM324 and it seems to be working perfectly.
     
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