Trouble with op amps

Discussion in 'The Projects Forum' started by bluewhistled, Nov 23, 2008.

  1. bluewhistled

    Thread Starter Member

    Nov 23, 2008
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    Yesterday I decided I needed to amplify a ~20mv DC signal to 5v so I looked around the net and found that an operational amplifier is the best route. I found some sample schematics and decided to give it a shot. Since I didn't have any I went to ratshack to discover they only have one type and it's the old 741 (I despise that store). I spent about an hour trying to get the thing to work until I realized that it requires a negative voltage source, I just assumed ground was equivalent. This really hinders my simple circuit. Are there alternatives to this? FYI I am amplifying a binary signal to a 5v square wave so it's really nice how the op amp will trim the edges so to speak when it hits the 5v ceiling due to the 5v V+. I *sort* of have it working with the 741 but it's really not how I want it at all. I can't stand the idea of having to get this thing a negative voltage when it shouldn't ever need it. Also for some reason the output is defaulting to 5v with the way I have it now, and only dropping during input fluctuations that go from high to low. It's really strange. I can ground the input, give +5 to the input, and the output stays at +5. Or I can give -5 to the input and then it finally drops. So needless to say none of this is how I want this to work. Anyone have any suggestions on what I should do? Maybe a more modern op amp to try, or even a completely different approach? I'd really appreciate any help.

    Thanks a lot
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    The output of the 741 will never reach the +5 Volt (it is not rail to rail as they call it).
    You want to amplify the signal as input of a digital circuit.
    Perhaps you can use this circuit and lower R1 to say 2.2K for more gain.
    http://www.elecfree.com/electronic/ic-4049-linear-10x-amplifier/
    I know it is misuse of the 4049 but it works.
    To make a square signal you can place an extra port afther the "amplifier".

    Greetings,
    Bertus
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It sounds to me like you want to produce a square wave output based upon a signal traversing across a 20mV threshold level.

    Opamps are great, but when your design calls for open-loop (maximum) gain, you really should be looking at comparators. Opamps aren't designed to be operated with their outputs in constant saturation; that causes high current consumption and heating, and reduced service life.

    See the attached.
    R1 and D1 establish a "somewhat regulated" low voltage around 400mV, to help stabilize the reference voltage. Pot R2 provides adjustment for the reference voltage. V2 represents a 20mV sinewave signal that varies from 0v to 40mV.

    The LM339 quad comparator has open-collector outputs, therefore a pull-up resistor is required to obtain an output signal. If your output does not approach 0v, increase R3 to 1.6k, which will require the LM339 output to sink only 3mA.

    Not shown is a 0.1uF bypass cap across the Vcc/GND terminals of the LM339.
    A small cap (1nF-100nF) on the inverting input will help a great deal to stabilize the reference voltage.

    All unused inverting inputs should be tied to ground, and all unused noninverting inputs to Vcc to prevent internal oscillation.
     
  4. bluewhistled

    Thread Starter Member

    Nov 23, 2008
    15
    0
    Thanks a lot! That's a much better idea than the op amp. As you can see I'm very new to this. I'm going to buy a 300 piece linear grab bag from jameco:

    https://www.jameco.com/webapp/wcs/s...-1&catalogId=10001&pa=145242&productId=145242

    That should provide me with a little of everything hopefully. I think I'll look through my bin of electronics for a voltage comparator I can remove, I'd like to play with that today. Anyways I'll keep you posted on how it pans out or if I run into some more problems.

    Thanks again.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Radio Shack does carry LM339's.
    If you're looking through salvage items, also look for LM193/293/393/2903 dual comparators, LM111/211/311 single comparators, LM119/219/319 dual comparators.

    There are quite a few comparators out there.

    You might consider picking up Radio Shack's 1/4W resistor assortment. It's a bit pricey and there are plenty of "gaps", but it'll give you a good start.

    RS also carries 1N4148/1N914 diodes in a 10-pack.

    Digikey is a good source for hobbyists; they'll ship SMALL orders via 1st class mail, which will save you a bunch in shipping charges. Shipping charges add up fast nowadays.

    Electronics Goldmine:
    http://www.goldmine-elec.com/
    is a decent source for "grab bags" of goodies. $10 minimum order.

    Beware of going bonkers ordering stuff. You can wind up with lots of stuff you'll never use.

    If you want a great learning kit, pick up an "Electronics Learning Lab" from Radio Shack, about $65. It's worth that for the project board alone. Board was designed by Forrest M. Mims III, who also wrote the two project books that come with it.

    If you want ideas beforehand, you can go to Radio Shack's site and download both manuals in PDF form. The lab comes with everything you need, except six "AA" batteries. It doesn't need a 9v battery as the website claims.
     
  6. bluewhistled

    Thread Starter Member

    Nov 23, 2008
    15
    0
    Yeah I found an LM393 on some old board. It worked nicely. I already own an assortment of diodes and resistors that I purchased a long time ago. Along with assortments of digital logic IC's and caps and trimmers. I hate searching for items and when I want to build something and it's frustrating when you can't find it, have to order the specific part which costs only a couple dollars at most and then 5 dollars shipping. I figured it's smart to have a grab bag of the most necessary components. I own forest m mimms books they are good. I don't need a kit really, I have everything I need, plenty of breadboards etc. And btw my circuit is working perfectly. I didn't use the diode however, I just read the spec sheet for the voltage comparator, divided the 5v down to .01v along with a trimmer so I can tweak it and used it as the reference voltage. Works like a charm. What was the point of the diode in the circuit you provided?
     
  7. bluewhistled

    Thread Starter Member

    Nov 23, 2008
    15
    0
    I don't see any "grab bags" on this site. What's convenient about jameco is they will dump a couple lbs of a type of component into a bag and slap a huge discount on it. I can grab a linear bag that will have 300 random of the most popular circuits.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Since you were needing to trigger based on a very low voltage (20mV) the Vf of a 1N4148 with low current through it would get you most of the way there.

    At 0.2mA, a 1N4148/1N914 has a Vf of about 530mV, which is much closer to 20mV than 5v is. Using a pot across that lower regulated voltage would give you a much finer degree of control than using the same pot across 5v; nearly 10x.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    They call them "GoldPaks"
    http://www.goldmine-elec-products.com/products.asp?dept=1053

    That can be useful if you spend the time to sort them out and catalog them.

    If you don't know what you have on hand, and have it stored in an orderly fashion, it will take you forever to dig through them all when you need something.
     
  10. bluewhistled

    Thread Starter Member

    Nov 23, 2008
    15
    0
    Very true. And thanks for the link.
     
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