741 inverting op amp circuit

Discussion in 'Homework Help' started by Random3s, Oct 19, 2014.

  1. Random3s

    Thread Starter Member

    Jul 30, 2014
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    Ive been stuck for a couple of days now trying to build an inverting op amp circuit using a 741 opamp. I don’t understand why I am not getting the results I expect, which according to theory should be a gain of -2.2.


    I am currently a student in electronics engineering; and I have read up on op amp operation which all seems fairly simple, in theory. I understand that the inverting op amp circuit feeds back a large portion of voltage back to the input, putting a muzzle on the large gain of an op amp, and that the inv and non inv inputs will do anything they can to equalize the voltage between the inouts, i.e make it 0 potential difference.


    I think I also suffer from a poor understanding of ground in circuit diagrams; as I understand ground (in a circuit), it means a point which is at a 0 potential with ref to another specific point…. In the case of my circuit then, then non inv input is grounded at 9v. I read that a floating input can cause issues; and that outputs can be unpredictable, I have also tried tying the non inv input together at 0v and 9v, but this puts 9v + POT across the inputs.


    From the circuit attached here are a sample of results:


    Vin Vout

    0.0v 8.75v

    1.94v 4.34v


    and @ 3.5v I presume the opamp saturates, as the voltage between the inv and non inv inputs starts to stray away from 0 and the out remains at 1.5v from this point, until the input reaches max.


    The above readings were taken with a supply of 16.98v


    Ive attached a circuit diagram and a picture of my effort, Id really appreciate a shove in the right direction here, if only to save my sanity.


    A quick run down of the circuit photo:


    The lower ground rail on the bread board is where ive grounded the non inv input, pot is also grounded here. Yellow cable is the input, via a voltage divider to the output. The red cable reaching over to the upper ground rail is the output.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    The very old and primitive (by modern standards) 741 opamp is about the worst one you could choose to use in this circuit. You need what in today's technology is a "rail-to-rail" amplifier.

    When powered as you have shown (+-9V), the crummy old 741 can only pull its output pin no higher than about 6V, and no lower than about -6V. Read the Voh and Vol spec on its data sheet.

    As to your question about "ground" in this circuit, it would be the tap between the two batteries. All other voltages would be measured with respect to this point. Put the black lead of your DMM there, and don't move it. Use the red lead to probe the other circuit nodes. Now report all eight pin voltages, as well as the pot wiper voltage...

    Looking at your breadboard, you may be missing some connections. The four red horizontal stripes may not be connected together intrinsically inside the breadboard; nor will the black ones.
     
    Last edited: Oct 19, 2014
  3. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    The impedance of your pot is ten times greater than the input impedance of your circuit. You need to reverse that.

    Change the resistors to 10K and 22K and use a 1K-ohm pot.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Good catch, however, if the OP measures the voltage at the pot wiper and calls that the input, the voltage at the 741 output pin should still be -2.2 times V(input), regardless of the pot loading. The pot loading would affect the pot linearity, but would not prevent it from producing all voltages from 0V to 9V.

    By my analysis, the pot wiper can not be more than +2.7V without the 741 running into its -Vol limit.
     
    Last edited: Oct 19, 2014
  5. Random3s

    Thread Starter Member

    Jul 30, 2014
    38
    0
    Problem solved, thanks Mike. :)

    input output

    0.4v -0.9v
    1.5v -3.4v
    2.8v -6.2v
    4.2v -6.5v
    5.8v -6.5v

    As you predicted the 741 levelled off at around 6.5v.

    Mr Chips this is a school project, so I am just building the circuits they provide, But point understood thanks.

    Thanks for your help guys.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    So the only problem you have was a gnd? You did not measure all your voltage with respect to gnd?
     
  7. Random3s

    Thread Starter Member

    Jul 30, 2014
    38
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    Exactly Jony, I knew that the output needed to be measured with respect to ground, but I took ground as being 0v on the 18v rail. Silly mistake, but one I am sure not to forget in future.
     
  8. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Sorry, but I do not understand. Did you measure wrt OV or not? o_O
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    He used VEE rail as a GND . And this is why he get 18V at VCC.
    But in this case GND is at the middle.
     
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