Non-Inverting amplifier trouble

Discussion in 'General Electronics Chat' started by odm4286, Dec 22, 2015.

  1. odm4286

    Thread Starter Active Member

    Sep 20, 2009
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    So I'm having a bit of trouble with a simple Non-Inverting Amp. A screenshot of the multisim and breadboard versions are below. The circuit works as expected in multisim, but I can't seem to get the real life version to work. My output does not change regardless of changes to my input. When I change VCC the output suddenly changes. Right now VCC is 5VDC and my output is a constant 1.33 volts. Any idea what I am doing wrong?

    The op amp I'm using is a 741, and my gain is 2

    [​IMG]

    [​IMG]
     
  2. #12

    Expert

    Nov 30, 2010
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    Wrong chip. The ua741 I just looked up is not guaranteed to output within 5 volts of the positive supply with a 7.5 ma load. That is not a very good example, but it is the concept you are facing. You want to get within 2 volts of the positive supply rail with a 1 ma load. That is why you get results by giving it more Vcc.
     
    JohnInTX likes this.
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The 741 is a very old op amp. You would do well to throw them away and buy some more modern ones.
    The simulator doesn't know about things like common mode input range. I may tell you you have an amplifier, but you need to read the data sheet carefully to make sure it will work in real life. don't get me wrong I love simulators - wouldn't build anything without running it first, but it will let you break your parts.
    A good alternative for general projects might be the LM358.
     
    PeterCoxSmith likes this.
  4. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Ditch the 741- it's an obsolete, 40 year old dog of an opamp.

    You will feel far less frustration using a modern rail-to-rail opamp.
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
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    Excellent idea. Please suggest a RRIO op amp in DIP that runs from + and - 15 V like the LM741 does, and doesn't cost a bundle?
     
  6. #12

    Expert

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    That's a different Thread. This one wants to use +5V for the supply.
     
  7. hp1729

    Well-Known Member

    Nov 23, 2015
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    Okay, how about a specific suggestion? No argument the LM741 is ancient.
    Maybe AD8031? Only about $5.00 a piece. Compared to $0.50.
     
    Last edited: Dec 23, 2015
  8. Sensacell

    Well-Known Member

    Jun 19, 2012
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    The MCP6024 is a personal favorite.
     
  9. odm4286

    Thread Starter Active Member

    Sep 20, 2009
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    Thanks everyone, so its the chip holding me back. Good to know! Maybe this is a bit outside the scope of this post but how would you get the 741 to work as a non-inverting amplifier then? I mean my circuit is correct, but the chip does not respond to my input AT ALL(aside from VCC of course).

    Even if I disconnect my input to the non-inverting term, my output remains the same :(
     
  10. Sensacell

    Well-Known Member

    Jun 19, 2012
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    The 741 performs well if its run from a +15V and -15V split supply.
    Then you have headroom that accommodates the limitations of the part.
     
  11. hp1729

    Well-Known Member

    Nov 23, 2015
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    Not a bad price either. About $1.00.

    Thanks,
     
  12. dl324

    Distinguished Member

    Mar 30, 2015
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    Did you check the maximum operating voltage? Not very good as a general purpose opamp.
     
  13. crutschow

    Expert

    Mar 14, 2008
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    The LM324 quad or the LM358 dual are cheap op amps (usually less the a half buck $US) that will operate off a single supply down to 3V (the input and output will go to zero but only to within a volt or so of the positive rail, so they're not rail-rail).
    There performance is not great (similar to the 741) but they're usually fine for non-critical/hobbyist use and would work in the OP's circuit.
     
    #12 likes this.
  14. dl324

    Distinguished Member

    Mar 30, 2015
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    And not bad for a 40 year old design. LM358 works for most of my applications. Using more opamp, or anything for that matter, than you need to get the job done shows a lack of craftsmanship.

    LM741 would work for the OP if he took the time to read the datasheet and understand it's supply requirements.
     
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    There is nothing wrong with your circuit, and with the correct opamp part it will work as intended. As you can see, you've poked a hornet's nest regarding the 741. It is from another era when there was a much more clear line between analog and digital parts, and analog and digital designs. Now that people want analog circuits that run on a single power supply, and a low voltage one at that, many older parts just won't work. Even the LM358 recommended above (and a personal favorite) will struggle in your circuit with any input greater than about 1.5 V. The 358 has an input common mode range that includes the negative supply rail and a much better output stage, which cover 1/2 of the 741 problems. But neither its input nor output ranges get very close to the positive rail, so the output probably won't be greater than 3-4 V no matter what the input is.

    Since you don't need any speed for what you are doing, consider one of the newer CMOS opamps (no, that is not an oxymoron). Many of them have rail-to-rail input and output ranges.

    ak
     
    #12 likes this.
  16. dannyf

    Well-Known Member

    Sep 13, 2015
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    The input signal could be out of range for the opamp. Raise it a little bit more.
     
  17. #12

    Expert

    Nov 30, 2010
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    Aww, I don't think it's all THAT bad.:D
    AG doesn't live here any more (inside joke) and I still have a drawer full of LM741 chips that I got for free because they were of the 15% that wouldn't oscillate (as they should have) in a commercial product. Fourty years ago we replaced dozens per day and I put the slow ones in my pocket.:rolleyes:

    The only problem I have with the LM741 is that I have never designed a circuit pathetic enough to use one. Now that I've had my 65th birthday, maybe I'll get senile and start thinking on a level which is compatible with the LM741.:p

    Happy Holiday!
     
  18. AnalogKid

    Distinguished Member

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    Dude, look behind you...
     
  19. crutschow

    Expert

    Mar 14, 2008
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    I follow Satchel Paige's advice "Don't look back. Something might be gaining on you." ;)
     
  20. #12

    Expert

    Nov 30, 2010
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    Are you referring to the trail of posts behind me? Those which indicate that I am already senile?
    Not to worry. I'm enjoying it.:p

    When I passed 50, I quit designing things to last more than 50 years. Now, my design goals are more like 10 years...and I'm still not as forward thinking as most commercial designers who plan on a 3 year (or less) life cycle. Give me ten more years and I'll be down to playing with batteries and LEDs.:D
     
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