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LM741 Op-Amp

Discussion in 'General Electronics Chat' started by Rush, Dec 28, 2002.

  1. Rush

    Rush New Member

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    How can i in a easy way test a lm741 op-amp if its working,
    with a simple testinstrument?

    and maybe how i can test it in work!


    /Kristian
     
    • Funny Funny x 1
  2. kinjalgp

    kinjalgp Active Member

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    1) Mount 741 on bread board
    2) Connect Vcc and -Vee (+-15V)
    3) Connect both the input terminals to ground. The output should be approximately 0V.
    4) Now connect NI terminal to Vcc. The output should also be Vcc. And if it is connected to -Vee, the output will be -Vee.
    5) You can even test the same thing with NI grounded and I given Vcc or -Vee. The output in this case would be Vcc when I connected to -Vee and -Vee when connected to Vcc.
    Thats it.
     
  3. Rush

    Rush New Member

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    Thanks!

    is NI=noninverted input
    and I=inverted input

    and what is Vee is it pin 4 on a dil package??

    Forgive me for my poor knowledge!!!

    /Kristian
     
  4. dave

    Dave New Member

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  5. kinjalgp

    kinjalgp Active Member

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    Yes NI=Non Inverting
    I=Inverting
    and also -Vee=Pin 4, Vcc=Pin 7
     
  6. pebe

    pebe Member

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    Note that at 3) the amp is running at max gain, and the output may go up to V+ or down to V- depending on the input offsets.
     
  7. kinjalgp

    kinjalgp Active Member

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    If both the inputs are grounded, the output should be ideally zero. I have observed that for 741 in open loop configuration, the output is around 2.5V when both the inputs are at 0V without any offset adjustment.
     
  8. pebe

    pebe Member

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    It depends what voltage 'ground' refers to. You said connect Vcc and Vee to +-15v. I assumed that ground was 0v = half rail. Under those circumstances it is extremely unlikely that the output would also be half rail.

    If ground refers to Vee then there would be no bias current for the input stages, so it would not really be a test.
     
  9. kinjalgp

    kinjalgp Active Member

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    Here ground refers to 0V i.e between +15 and -15. Now if you practically check the output of 741 with both inputs connected to 0V the output is at +2.5V. And even in simulation the output shows 2.439V. This is due to very very very small offset present between two inputs.

    Here if you connect both inputs to -Vee the output swings to +Vcc :) and if both inputs are conneted to Vcc the output swings to -Vee. So this is a test. :) Just check this funny behaviour on breadboard ;)
     
  10. pebe

    pebe Member

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    My original posting was as guidance to Rush that if your item 3) didn't agree with his tests, then his chip was not necessarily faulty. A couple of points of further discourse:

    1. As I said, connecting the inputs to the V- rail will render the input stages inoperative, so that is meaningless as a test.

    2. With both inputs connected to half rail, the inputs are operating in their normal linear mode. The chip has a typical open loop gain of 200,000. There is no DC feedback within the chip, so 10uv differential on the inputs will give 2v swing at the output. The output may go violently high or low depending on the input offsets (1mv typical - 5mv max) but it will never reach full rail because of the bipolar output transistor configurations used.

    In the past I have used this chip operating at almost full gain as a sensitive output detector for a bridge I built. Not only did the offsets need to be nulled to give a valid reading, but it was necessary to constantly adjust the null to compensate for temperature and LF noise drifts. So I find it difficult to understand how you manage to get 2.5v o/p with the inputs shorted together at 0v. Could your test chip have been faulty? You mention results on a simulator. But simulators use idealised conditions, whereas practical circuits live in the real world.

    3. I think the easiest quick check on an op-amp is to wire it as a follower. Connect the NI input to the half rail voltage and connect the o/p to the inverting input. Connect up + and - supplies, and the o/p should read almost the same as NI.
     
  11. kinjalgp

    kinjalgp Active Member

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    Your explaination is correct Pebe. There are chances that my Op-amp was faulty or may be it is due to device to device parameter variations. Op-amps of different make can also cause this problem. Mine is National semiconductors LM741. What about yours?
     
  12. pebe

    pebe Member

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    Thanks. I'm glad you agree with me :) .

    I can't remember whose 741 it was - it's too long ago. It was from either RS Components or Farnell, and could have been from any one of a number of manufacturers. But the results would have been the same for any make.
     

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