741 op amp offset nulls?

Discussion in 'General Electronics Chat' started by Shockmyselfsmart, Jan 1, 2011.

  1. Shockmyselfsmart

    Thread Starter New Member

    Dec 17, 2010
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    Ok Im just playing with a 741 op amp. I dont understand what the offset nulls do? I did some reading do you just add a pot and hook it to both nulls and you can change the output gain?

    Did read the datasheet not seeing any information on it. Just trying to step up the voltage a bit.
     
    Last edited: Jan 1, 2011
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    It's one of the first op amps ever made and quite antiquated, if I recall the offset nulls were there so you could attempt to balance out the + & - sides of the amp such that their currents were equal and the output with no input was 0 volts.

    Even a few of the newer op amps still have this capability but it's rarely needed due to their inherent precision.
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What datasheet did you read?
    In the intersil there is some information on it.

    [​IMG]

    What is happening is that the potmeter is changing the balance of the resistors R1 and R2.

    [​IMG]

    Bertus
     
  4. Shockmyselfsmart

    Thread Starter New Member

    Dec 17, 2010
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    Ok so some times the op amp has a little output even if there is no input and you use the nulls to offset this back down to 0v? If so that makes sense.

    I purchased it from radio shack finding info on it is a little hard. The only data sheet i found just gave you the pin numbers that is about it.

    Thanks all.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    It is too bad that 741's are still easily come by. The design was a major advance over the 709 op amp. but it is such a poor performer by today's standards that it is close to a waste of money buying one.

    The Texas Instrument op amps like the TL071 0r TL081 are probably less expensive and simply work a lot better in every way. You can learn things from a 741, but you're stuck with the requirement for a dual +/-15 volt supply.

    To check that offset null, set the 741 up with the output tied to pin 2 through a 10K resistor and the pin 3 input grounded. That is a voltage follower with no gain. Use a meter to observe the output at pin 6. It will be some millivolts away from 0.

    Put the nulling pot in circuit (with power off) and turn it back on. Watch the meter as you turn the trimming screw to adjust the output to 0 volts.

    A good modern op amp like the OPA1641 does not have a null adjust, as the output has essentially no offset.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Radio Shack does carry the TL082, which is a dual opamp. It's noisier than the TL072, and can't "see" inputs within 1.5v of +V or within 3v of -V, so it's best used with a dual supply also - however, it's a good one to experiment with.

    If you'd like to experiment with more modern opamps, try something like an LM6132 or LMC6482. You'll have to order them from someplace like Mouser, Digikey, Avnet Express or other authorized distributor.
     
  7. Shockmyselfsmart

    Thread Starter New Member

    Dec 17, 2010
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    Thanks. Ya don't really have a purpose or something im building using this op amp just playing around with my bread board trying to learn.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    You should not experiment with an IC without its datasheet in your hand.
    I get datasheets from www.datasheetarchive.com where I can select the datasheet from many manufacturers of ICs. I always select the datasheet that is the most detailed, usually from the inventor of the IC. Fairchild invented the 741 opamp 42 years ago.
     
  9. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    Yaa......!!
     
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  10. Shockmyselfsmart

    Thread Starter New Member

    Dec 17, 2010
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    Ya I had a fairchild data sheet in my had. It didn't explain the nulls. If I blow it I live and I learn:mad:. I couldn't find a datasheet to match the exact numbers on the radio shack ic so it could be different for all I know.
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Mouser doesn't carry National and while I'm sure some will disagree the TL08x series is virtually as noise free as the TL07x series is. There may be some advantages to it or they wouldn't have given it a higher part #, anyone care to go through all the graphs and tell me why?
     
  12. Audioguru

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    If you tell us the numbers then we can easily find the datasheet for it, but the spec's for all 741 opamps are the same.
    It will also have the logo of its manufacturer. The logo for National Semi is a big N. The logo for Texas Instruments is a map of Texas.
     
  13. beenthere

    Retired Moderator

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  14. Audioguru

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    The first page of Texas Instruments TL07x datasheet lists its low noise. The first page of their TL08x datasheet does not.
    But they both have exactly the same spec's (maybe because of a mistake).

    I believe that a TL07x is a TL08x that is selected for low noise. Maybe sometimes depending on the yield all of them are low noise.

    I have seen TL07x opamps in many audio circuits but I have never seen a TL08x opamp.
     
  15. marshallf3

    Well-Known Member

    Jul 26, 2010
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    If you look through the data sheets (both are drawn differently) it's obvious that at the lowest frequencies the TL07x is a tiny bit quieter than the TL08x series. It's now more my guess that they figured out a cheaper way to make the die for the TL08x at a very slight sacrifice in noise levels so they had to give it a new part #. Since most things I make out of op amps isn't mission critical I usually just get the 08x's because they're a little bit cheaper and often more widely available.

    Compare the noise graphs on both and tell me if it's anything that would be audible even if used in an early preamp stage.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    The TL07x and TL08x opamps are fairly old so are not as quiet as newer audio opamps.
    They are fine for line level.
    Their noise might be heard when used in an old phono preamp for a moving magnet cartridge.
    Their noise is not heard for a normal-level mic preamp but is heard when people whisper and the gain is turned up.
     
  17. Shockmyselfsmart

    Thread Starter New Member

    Dec 17, 2010
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    This is what i have http://www.radioshack.com/product/index.jsp?productId=2062547 number LN741CN when I search alldatasheets all i get is http://www.alldatasheet.net/view.jsp?Searchword=LN741CN I chops a lot of the numbers off and searches ln7??? I don't get it.
     
  18. #12

    Expert

    Nov 30, 2010
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    So...what is the problem?
     
  19. Shockmyselfsmart

    Thread Starter New Member

    Dec 17, 2010
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    No problem. Just wondering what the null offset did and what data sheet matches the radio shack 741. I have the general idea of what is going on.
     
  20. beenthere

    Retired Moderator

    Apr 20, 2004
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    All 741's, regardless of origin, have the same spec's. The only difference is normal variation.
     
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