Selecting an Op amp

Discussion in 'General Electronics Chat' started by easilyconfused, Mar 26, 2015.

  1. easilyconfused

    Thread Starter New Member

    Mar 25, 2015
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    I'm an electronics dummy and I find it all a bit daunting, but I'm hoping to pick up a bit of knowledge along the way.

    I've been looking at the Non inverting operational amplifier, and how I can use it to double a DC input voltage.

    I have come up with the following circuit diagram
    [​IMG]
    If V1 is between 0 & 2.5v and both resistors are equal, I think Vout should be between 0 & 5.0v, correct ?

    Will it operate on varying voltage, or does it have to be fixed ?

    How do I determine which Op amp I should use ?

    Also, I have some 1 meg ohm resistors..... are they too large to use to provide my 'gain' ?

    Sorry for the basic questions, and thanks for any input
     
  2. MCU88

    Member

    Mar 12, 2015
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    It is textbook and will work.

    Correct

    Can be varied, but ideally you want something stable and a few volts above the maximum output swing.

    The TL072 dual FET operational amplifier IC is generally all round good for the hobbyist / experimenter. It is certainly better than an LM358.

    Personally I would use 100K for good measure. The 100K is my favorite value of resistor for some reason.
     
  3. #12

    Expert

    Nov 30, 2010
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    I think he meant the input voltage. Yes, it can vary all over between 0 and 2.5 volts. The power supply should be a higher voltage than the output. How much higher depends on which chip you are using. The TL072 needs about 2 extra volts on each end of the power supply. 0v to 5v out would need -2v and +7v for a power supply. It will behave well with 1 meg resistors.
     
  4. easilyconfused

    Thread Starter New Member

    Mar 25, 2015
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    Sorry, I should have been more clear. I don't really know the right terminologies.
    When I asked if it will operate with varying voltage, I meant V1. Will the op amp function correctly if V1 varies ?
    The supply voltage for the op amp will be 12V
    Does the TL072 have 2 amplifiers ?
     
  5. MCU88

    Member

    Mar 12, 2015
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    Are we going to warn him about the fake ICs on eBay? ... The TL072CP from China has notable side effects when you start and try and do real things with it. They can latch up if you draw close to the maximum 20mA from the output. They behave strange when configured as differential amps... I had a nightmare with them with an power supply that I made based around 4 of them. They seem ok with some of the more basic OPAMP configurations / building blocks though...
     
  6. MCU88

    Member

    Mar 12, 2015
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    Can vary no problems.

    Yes!
     
  7. easilyconfused

    Thread Starter New Member

    Mar 25, 2015
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    Sorry, but why would I need a package with 2 amps in it ? (am I missing something ?)

    I looked at the TL071 spec sheet that #12 put up (thanks) ....... whoa, that'll do your head in ! Am I right in assuming a TL071(single amp) with no letter affixes would suit my purposes ? It appeared the affixes related mainly to ambient air operational capabilities !
     
  8. MCU88

    Member

    Mar 12, 2015
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    Hello...

    Most of them come in dual just to use up the pins. Tie unused pin inputs to GND.
     
  9. #12

    Expert

    Nov 30, 2010
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    The TL0 series comes with 1, 2, and 4 amplifiers in a package.
    Do not ground the extra output pins. This can cause excess current. Better to configure unused amplifiers as voltage followers and have them follow some unimportant DC voltage. It keeps them from getting bored. :rolleyes:
     
  10. #12

    Expert

    Nov 30, 2010
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    I said that because some amplifier chips get irritated when you just ground the inputs and let the DC offset voltage lock their outputs to one or the other voltage supply. This may be a throwback to the 1970's, but it was a safe bet then, and it seems like a safe bet now.
     
  11. MCU88

    Member

    Mar 12, 2015
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    Yeah I would make the unused amps voltage followers / buffers too they're all going to be locked up pin 3 to 5 you'll be out in 4.
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If you are using a TL07x and 12V for Vdd and 0V for Vss, then the input cannot go all the way to 0V, because the TL07x's output pin can only pull to about 2V (not 0v). If you need to go all the way to 0V in/out, you will have to use a negative supply for Vss.

    Or you could use the LM358, which allows inputs to go to -0.5V and pulls very close to 0V even when its Vss is tied to 0V (gnd).
     
  13. easilyconfused

    Thread Starter New Member

    Mar 25, 2015
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    Okay, thanks. Input will never be zero, but it could go lower than 0.5V....... so that would result in a Vout of less than 1.0V (which would rule out TL07x as a viable option)

    MCU88 doesn't appear to have a very high opinion of the LM358 but your post has made it look more suitable for my needs.

    I will always have + input voltage and a + Vout, if that makes any difference to the op amp I can use.
     
  14. MCU88

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    Mar 12, 2015
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    It is alright. I have stock inventory of 100+ pcs if that says anything. The TL072 definitely for audio work though.
     
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If you are using a single power supply, you must have a opamp with an input common mode range that includes the negative power potential. For a single-supply circuit, this is called having a common mode range that includes ground. The TL071 data sheet posted does not state explicitly that it can do this.

    Fortunately for the universe, the LM358 does....!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    ak
     
    Roderick Young likes this.
  16. easilyconfused

    Thread Starter New Member

    Mar 25, 2015
    16
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    If I use the LM358 can I also run V1 to the 2nd amp (with a lower gain) and have a 2nd Vout just using a single power supply ?

    eg if Vout1 & Vout2 both have maximum voltages of less than 5.0Vea, then I should be okay with a single 12VDC supply, correct ?
     
  17. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    An LM358 operated on 12V and 0V, its output will pull down within mV of 0V. It will pull up to ~10V. The common-mode range of its inputs is about -0.5V (below the Vss pin) to ~10.5V.

    358.gif

    X-axis is V(in)
    Y-axis is V(out1) and V(out2)
     
    Last edited: Mar 28, 2015
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