Help with 741 amp or any other

Discussion in 'The Projects Forum' started by ibrahim abukharmeh, Dec 13, 2015.

  1. ibrahim abukharmeh

    Thread Starter New Member

    Nov 10, 2015
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    Hello,
    i am trying to make ua741 op amp works as non inverting amp
    i have connected the very basic circuit consisting of two resistor Rf and Rin
    and the gain now should be 1+(Rf/Rin) but the problem is that gain is always one
    i have connected it in inverting and tried different values for the resistor but it still not working
    Well, honestly i think i burnt the amp by directly connecting +Vcc to 12 v 600mA source and -Vcc to gnd
    and i later calculated that i would need 288 ohm resistor to be connected (not sure from that)
    i calculated by looking at the datasheet the maximum power for that amp were 500mW and my input is 12 v
    so the max current is 41,66mA and the resistor would be 288 ohm.

    now before trying to connect a new amp, i am trying to realize what could went wrong
    from my power supply what circuitry should be connected before the amp
    do i need +12 and - 12 even though i am just trying to amplify a positive signal all the time (out put from Wheatstone bridge)

    what wrong with the following simulation circuit, why its not working

    hala8abibi_650x495.png

    Thanks A lot
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    I doubt you hurt your opamp. When connected to a power supply the opamp will only draw the current it needs, no need to add any resistors.

    I am surprised you get an output from that circuit. The TI LM741 datasheet says that it needs at least a ±10V power supply, and your schematic shows the equivalent of ±6 V. The LM741 was designed in the days of ±15 volt supplies being common and was not intended to operate from a single power supply. Many modern opamps work beautifully with single low voltage power supplies. Even old war horses such as the LM358/LM324 aren't bad at all for many applications.

    The input voltage range (or "common mode input voltage range" in the datasheet) is specified to include 3 volts above the negative power supply and 3 volts below the positive power supply. Your circuit biases the + input at the edge of what would work if you had enough voltage.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Also the feedback is confusing. R1 is not doing anything, so you can replace it with a piece of wire, and when you do that you realize R2 is doing nothing as well, so you can delete it. You end up with a voltage follower.
     
    Last edited: Dec 13, 2015
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  4. DickCappels

    Moderator

    Aug 21, 2008
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    Woops! kubeek just pointed to a large error -why the gain is always one!

    R1 should be between the output and the inverting input, not between the R2 and the inverting input. Rf in the circuit below is your R1.


    [​IMG]
     
  5. ibrahim abukharmeh

    Thread Starter New Member

    Nov 10, 2015
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    Well, thank you for the simulation error it caused me headache this morning and now i am laughing on my self, on the bread board i didn't do that error and i was not getting the needed gain, any recommendation for an amplifier that would work with 12 or even 5 volt (other than lm358) .
     
    Last edited: Dec 13, 2015
  6. DickCappels

    Moderator

    Aug 21, 2008
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    I would think the LM358 is one of the easier opamps to find most places in the world.

    Here are some along with their minimum power supply voltages. You will need to run them at a higher voltage than specified to assure operation. Also check the datasheets to see whether the opamp has sufficient output swing for your experiments. Some of these might be damaged with a power supply voltage of more than six volts, here again the datasheet is your friend.

    ICL7650 10V
    LF412 10V
    TL031/32/34 10V
    TL051/52/53 10V
    LM118/218/318 10V
    TLC2272/2274 9V
    NJM2068 8V
    LM833 8V
    NJM2068 8V
    TL070/71/72/74/75 7.5V
    TL081/82/84 7.5V
    TL01/72/74 7V
    NE5532-3 6V
    OP07 6V
    TLC2201/2/4 5V
    L272M 4V
    TL061/62/64 4V
    TLC2652 3.8V
    LM324 3V
    TLC27L2 3V
    TLC272/277 3V
    LMR321/358/324 2.7V
    LMV321/358/324 2.7V
    TLV2270/2771/2772/2773/2774/2775 2.5V
    NJU7051/7052/7054 1.0V
     
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  7. dl324

    Distinguished Member

    Mar 30, 2015
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    Would you care to elaborate on why you think LM358 are to be avoided?

    A true craftsman will have a lot of tools in his tool box and be proficient in the use of all of them. I've found that LM358 can be used for most of my opamp circuits. You need to understand the capabilities and deficiencies of any components at your disposal, know how to utilize them effectively, and make appropriate choices.

    Using a high bandwidth, high slew rate, high output current, low bias current, low offset voltage, and/or rail-to-rail input/output opamp when it isn't necessary is one option; but it doesn't show any finesse.
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    There is some other things going on that you need to be cognizant of...

    Look at this simple LTSpice sim of two different versions of your circuit using a LM358 and a single 5V supply. The plot is V(out) vs V(in), i.e. a plot of the transfer function.

    1591.gif

    Here are some things about the sim you should be able to answer:

    1. Why can't V(out1) or V(out2) go below zeroV?
    2. Why can't V(out1) or V(out2) go above ~4V?
    3. Why is the gain (the slope) of V(out1) = V(out2)?
    4. Why is Vout(1) shifted upward by 2.5V as compared to V(out2)?
     
  9. ibrahim abukharmeh

    Thread Starter New Member

    Nov 10, 2015
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    Well, as it turned out that my amp were labeled as ua741, but i was not expecting that since i asked for lm741
    anyway, yesterday i connected two 9 volt batteries, to overcome the issue with vcc and vee. but the problem still persist,
    maximum gain i was able to get, was about 80% of the calculated gain, will try to buy some other amps today and see if the problem
    is from it. otherwise i am doing something wrong on the breadboard and in that case i will try to attach some photo
    so it can be clearer situation, thank all for help, will keep updated.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What are you connecting to the output of the opamp as a load (nothing to do with the gain setting resistor)?
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    There is no difference. One was manufactured by Fairchild and the other was manufactured by National Semiconductor. They're both long gone so there is nobody to complain to.
     
  12. ibrahim abukharmeh

    Thread Starter New Member

    Nov 10, 2015
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    I am not connecting any load resistor to the amp, i am just trying the basic functionality right now, and then i will try to connect that to the analog input of micro controller.
     
  13. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    non inverting mean if input is +ve it will output -ve, vice versa
     
  14. ibrahim abukharmeh

    Thread Starter New Member

    Nov 10, 2015
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    Sorry i dont get what you mean, as far as i am concerned an non inverting means that that output signal of my amp would have the same polarity of my input in the direction, and the magnitude would be the gain multiplied by the input signal which is
    k = (1 +(Rf/rin))
     
  15. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    I have use 741 in 5 year past.
     
  16. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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  17. ibrahim abukharmeh

    Thread Starter New Member

    Nov 10, 2015
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    Could you please do me a favour and prove that mathematically or from simulation
    basic control system analogy the output is the transfer function multiplied by the input, here our transfer function is the gain
    and since that the gain equation is proven to be 1+(Rf/Rin) then we would never get flipped sign from the input
    (last time i checked there was no negative resistors on EBAY):)

    the site you copied what you said from, if you scroll down a bit, you would see in the mathematical example that the input was 0.5 V, the gain was 11 and the output is 5.5 volt,

    could you please tell me where the switch in sign come from.
     
  18. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    sorry, i was telling of INVERTING AMPLIFIER. This means that if the voltage going into the741 chip is positive, it is negative when it comes out of the 741.
     
  19. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Since you are actually breadboarding the circuit, measure the resistors with an Ohmmeter. Perhaps one of them is not what you think it is...
     
  20. ibrahim abukharmeh

    Thread Starter New Member

    Nov 10, 2015
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    i manged to make my circuit work by replacing the ua741 with lm324 same setup same everything, but with correct gain, thanks all for help :)
     
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