OP AMPS 101 and offset balancing

Discussion in 'The Projects Forum' started by ChrisChemist116, Aug 17, 2010.

  1. ChrisChemist116

    Thread Starter Active Member

    Mar 13, 2009
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    Howdy:)

    I am in the process of reviewing basics in OP AMPS, and i figured out that things doesn't seem to work as expected in SPICE simulators such as in Labcenter's PROTEUS.

    But beyond such scope, i would like to know if i am correct in wiring the eponymous UA741 and LM301. Okay i know they're old, but i have to acknowledge they're great for learning. While QUAD op amps are good option as well. As mentioned in the heading, for now my need is just to do a 101 review.

    How to properly balance the offset in 741 and 301?. Is such procedure of
    grounding the terminals (inverting and non inverting) the most accurate or the most preferred?.

    I checked the datasheets of both op amps.

    But still clueless about it. Is that the correct way to do it?. I attached two images, the first one it is an example of the 741 configured as an inverting amplifier (with a gain of 10) using a 10k potentiometer as an offset. It seems to work more or less fine in PROTEUS. The second picture it is the same (741) as inverting amplifier, but configured to set up the offset balancing. THIS is the part i am not sure if i am doing correctly.:confused:

    The same example is given with the 301, as an inverting amplifier and the second image is supposed to set it as offset balancing.

    I would apreciate if anyone could give me a hand on this.

    Cheers

    [​IMG][​IMG][​IMG][​IMG]
     
  2. Wendy

    Moderator

    Mar 24, 2008
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  3. ChrisChemist116

    Thread Starter Active Member

    Mar 13, 2009
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    Its nice you have replied to me quickly. I did read the thread mentioned but i still dont get it.

    To me it seems to mention the speed of the most popular op amps out there, but it really doesnt seem to explain more than that.

    My question is more like to "input offset balance for dummies" -> is there any tutorial or instructions on how to do this?.

    I found this site -> Balancing the input offset but i believe it doesnt seem to cover all. Such as, how to use it or if is it really important.

    Still is some hard to me to know if what i am doing is correct.
    Whatever, any suggestion on this matter is kindly appreciated.:rolleyes:
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Have you a breadboard? Setting up an op amp and simply using the correct pot to trim out the initial output offset is instructive.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    If I understand you correctly, you want to know whether you should adjust offset when you have an input signal present, or when the input is zero volts. The answer is the latter.
    If you tried to do it with signal present (a DC voltage), you would have to set or measure the input and output very accurately. Feedback resistor tolerances will cause the output to be other than what you calculated it should be.
    If you zero the input, then you don't have to measure anything except the output, and the desired result is zero, so you don't have to calculate the desired result. Resistor tolerances will not change those facts.
    Does that make sense?
     
  6. gootee

    Senior Member

    Apr 24, 2007
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  7. ChrisChemist116

    Thread Starter Active Member

    Mar 13, 2009
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    Of course i do have a breadboard. In fact i reproduced the experiment as mentioned in the site given above using a +/- 9 V supply, but i didn't noticed much difference in the output voltage while balancing the offset, i barely seen changes between +/- 0.6 V but not ±3 and ±4 volts as they mention. The changes increased up to ±1 Volts when the supply was incresed to +/- 18 volts on the input.

    I did grounded the inverting and non inverting inputs preceded by 1K resistors, as they do in their schematic.

    I noticed in the article described on this site
    about offset null in op amps. What they do is to directly ground both inputs (non inverting and inverting) and just adjust the offset with the potentiometer tied to the negative supply. But isn't that dangerous?.
    I tried to do the same with my 741 but noticed a strange behavior.

    Yes and No. Okay i will explain myself. But first, thank you for having some patience to read my reply.

    I don't want to adjust the offset when an input signal is present, instead when the input is zero volts.

    About resistor tolerances, okay i get it. I know it doesn't change those facts. But what is it exactly i am supposed to notice in the output?. A voltage different from zero. Is that correct?.

    So i placed a potentiometer between pins 1 and 5, and a resistor from the non inverting input to the output and two resistors for both INV and NON INV tied to GND. (As seen here)
    I followed these instructions, but the things didn't worked out as they mention. Maybe my IC is damaged or maybe the explanation given on that site is wrong.:confused:
     
  8. ChrisChemist116

    Thread Starter Active Member

    Mar 13, 2009
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    Thanks, I was aware of the existence of AN-31 as it is some sort of op amps collection circuits and different applications, but not of AN-20. The latter its more like the explanation version of the AN-31. I had read some pages of the book you mentioned.

    I found it useful, they describe the offset adjustment (internal method), however there is a warning...

    "To make use of this feature, two pins are joined by a potentiometer, and the wiper goes to one of the supplies through a resistor"... "if the wiper is accidentally connected to the wrong supply, the op amp will probably be destroyed"
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The input offset adjustment range on a 741 is typically only ±15 millivolts. At the output, where you measure, the offset will be amplified by the ratio Rf/Rin. You want to make the output zero volts.
    Why do you want to adjust the offset voltage? In many applications, adjustment is not required.
    If you are wanting to insert an intentional offset, using the input offset adjustment pot is not the correct way to do it.
     
  10. gootee

    Senior Member

    Apr 24, 2007
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    OK. But we could put a warning on almost everything. There are plenty of other ways to "let the magic smoke out" of electronic components.

    Many warnings of that type are given only to remind you to understand what you are doing and to be careful, not to prevent you from trying something.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Yeah, I agree with Tom (Gootee).

    Originally Posted by ChrisChemist116:
    What's your point, Chris?
     
  12. ChrisChemist116

    Thread Starter Active Member

    Mar 13, 2009
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    Thank you Ron, that was exactly the answer i was looking for. you hit the nail in the head for my question.
    I just figured out my 741 was defective and i replaced it with a new one and it worked as described. Not as in the page mentioned. But the offset was amplified by the ration of Rf/Rin as you said.
    What do you mean by this?

    Is there any examples on where offset voltage adjustment is mandatory?

    Overall, it became clear to me that offset voltage balancing is optional, or depends upon the application.

    Well..., i must say it was to make clear in the book http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html they mention about being careful with the use of internal method in offset adjustment. Not a big deal.

    By the way. Is it always necessary to fed op amps with dual supplies? +/- Vcc and Vee
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Whether or not you need offset adjustment depends on the application. In a precision DC circuit, you might need it. Cancelling offset voltage in some op amps will also minimize offset voltage drift as a function of temperature. In an AC circuit, you will not need to adjust the offset voltage.
    See this thread on another forum for ideas on how to eliminate the need for dual supplies.
    Also, there are op amps, called rail-to-rail, in which the output voltage swing and/or the allowable input voltage range (common mode range) includes or closely approaches the supply rails. You can use these in some single-supply applications. Of course, you can also use non-rail-to-rail op amps in applications where the input voltage remains within the common mode range, and the output swing is adequate. All this information is in the datasheets.
     
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