Opamp gain

Discussion in 'General Electronics Chat' started by vladtess, Jun 9, 2011.

  1. vladtess

    Thread Starter Member

    Jan 5, 2011
    43
    1
    Hello there! I've attached a circuit that I've built. For some reason the output pin has more than just gain of two. Can someone explain why this may be so?

    I expect output of somewhere 2.5, but get ~3.8..

    Thanks much!!
     
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  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The problem has to do with the fact that you are trying to use an opamp with a single supply and referencing the 10K resistor to ground. Reference the resistor to a reference voltage equal to the positive supply divided by 2 and you should get a lot closer to a gain of 2.

    hgmjr
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The LM741 really needs 10v or more for its' supply voltage. Even with a very light load (>10K Ohms), the output swing is from Vee+4v to Vcc-4v; with a single 10v supply that's only a swing of 2v.

    The LM741 design is over 40 years old, and has many shortcomings. They are still used in educational institutions, as they perform so poorly that no one would steal them.

    You would do better to use an LM324 quad opamp. They are also very old and slow, but are rated for single supply operation, and inputs and outputs can go nearly to the ground rail. Wire the unused opamps as voltage followers; each channels' output to its' respective inverting (-) input, and the noninverting (+) inputs to ground.
     
  4. vladtess

    Thread Starter Member

    Jan 5, 2011
    43
    1
    Well this is very informative!! I am waiting for LM339N to come (I ordered it a couple of days ago). Would this one work well with the common rail and operate @ 5VDC? Thanks much!!
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    The LM339 is a quad comparator, not an operational amplifier aka opamp.

    Although the schematic symbols for comparators and opamps are virtually identical, they serve different purposes. Opamps are compensated, so that the gain of the opamp can be adjusted via feedback paths, and the output will follow the input multiplied by the gain. Comparators are typically not compensated, are typically designed with open-collector outputs (not all are), and are used where rapid response and very high open-loop gain is desired. If you attempt to use a comparator as an opamp, you will find that the output will be rather non-linear. Comparators are basically designed to transition the output state as rapidly as possible. Your LM339 comparator will need pull-up resistors on the outputs in order for you to see a signal out. A "rule of thumb" for calculating the LM339 pull-up resistors is Rpullup >= Vcc/5mA, as 6mA current is the maximum the comparator can sink; so if your Vcc is 5v, then Rpullup >= 1k Ohms.

    Opamps don't perform as well in open-loop mode as comparators do; their output voltage swing is typically far slower than that of comparators. Opamps can be used as very slow comparators, and comparators can be used as very poor opamps - but...

    You can install screws using a hammer, but the results are so much nicer if you use a drill and a screwdriver. Use the proper tools for the job at hand.

    Also, please see the "decoupling or bypass capacitor" thread, here:
    http://forum.allaboutcircuits.com/showthread.php?t=45583
    You will need at least one 0.1uF cap per power rail.
     
  6. vladtess

    Thread Starter Member

    Jan 5, 2011
    43
    1
    ok I understand, thanks so much!! I found a 741 in radio shack, will get i there!!
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    A 741 won't work unless you have a "dual rail" supply of at least +/-5v, preferably +/-15v.

    The only opamp that Radio Shack carries that will work with a single 5v supply is the LM324 quad opamp, as I recommended in my 1st reply.
    Link: http://www.radioshack.com/product/index.jsp?productId=2062592&filterName=Type&filterValue=IC-Analog

    As mentioned in my 1st reply, you will need to connect the unused channels as voltage followers.

    If you try to use any other analog IC from Radio Shack, your circuit will not work properly.
     
  8. nigelwright7557

    Senior Member

    May 10, 2008
    487
    71
    Look for a "rail to rail" input and output op-amp.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    One of these would certainly work (provided the supply range was sufficient), however they are not stocked at any local Radio Shack stores.

    One would really have to order such an opamp, and it would take at least a few days to be delivered. v A few examples would be the LM6132/LM6134, LM6588, LM8272, LMC6482/LMC6484, LMV321/2/4 - there are lots more out there.
     
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