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Op-Amp question

  1. Jun 7, 2009 #1
    If I run a 741C op amp with Vcc=+9v and Vee=0, will it affect the functioning of the op amp? Can I still use it as I normally would? For a 741C Vcc=|Vee|=13v.
     
  2. jcsd
  3. Jun 7, 2009 #2

    vk6kro

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    You can still use it, but you have to put a series pair of identical resistors in as a voltage divider across the power supply and connect the non inverting input to this junction. It must also be bypassed with a suitable capacitor at this point to ground.

    This becomes a sort of artificial ground for the op amp.

    The output will not swing around zero like it does with a split power supply. It will be at about half the supply voltage, so you would couple out with a blocking capacitor if this was a problem.
     
    Last edited: Jun 7, 2009
  4. Jun 7, 2009 #3
    Here is a data sheet for the national LM741C:
    http://www.national.com/ds/LM/LM741.pdf [Broken]
    You can use it up to +/- 18 volts (36 volts total). For +/- 15 volt operation, the inputs should be between +/- 12 volts. I have used this as a gain of 1 voltage follower.
     
    Last edited by a moderator: May 4, 2017
  5. Jun 7, 2009 #4

    Born2bwire

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    I'm not sure what Vcc and Vee are in terms of the 741, but if it is the supply voltages then I agree with vk6kro. Your supply voltages will always limit the range of the output voltages. So as long as you are interested in output voltages betwen 9 and 0 V (-/+ a little due to the fact that the opamp will saturate before the limits) it should be fine. If you need positive and negative voltages on the output, then you can use a virtual ground comprised of resistors and capacitors. This will give you a range of 4.5 to -4.5 volts to work with. Tangentaudio has some virtual ground circuits, http://tangentsoft.net/elec/vgrounds.html , the resistor divider is the one that has been previously described and it was built for operation with one or two 9V batteries so it would suit your purposes fine.
     
  6. Jun 8, 2009 #5
    I basically want to use it as a non inv unity gain buffer and in a ckt to process the signal from my guitar. I think the voltage swing levels are less than 1 v total, so I guess it should work fine. Is the voltage divider absolutely necessary? If so why?
     
  7. Jun 8, 2009 #6

    vk6kro

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    No it isn't necessary for a unity gain non inverting amplifier.
    You connect the output of the opamp back to the inverting input and drive the non inverting input.
    What you get on the output will be what you put on the input.

    BUT what if your input is swinging about zero volts by half a volt?
    The output can't go negative because there is no negative supply so the output will be distorted.

    We could use a capacitor? Could we?
    Have a look at this:
    http://en.wikipedia.org/wiki/Opamp
    See the circuit of a 741. It has an open base of a transistor as the input marked non inverting.
    If you just have an open base directly driven with a capacitor and no bias resistor, the transistor would never turn on.

    This is why you need a resistive divider. It is two resistors and a capacitor. No big deal.

    .
     
  8. Jun 9, 2009 #7
    Basically we're inserting a d.c. level at the non inverting terminal so that the net voltage swing is > 0 right? And there is a capacitor at the output to remove the dc voltage? Also whats a 3m9 resistor?
     
  9. Jun 9, 2009 #8

    berkeman

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    Where do you see that?
     
  10. Jun 9, 2009 #9

    vk6kro

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    A 3M9 resistor is 3.9 Megohm.

    Because the decimal point can be missed, the multiplier is substituted in circuit diagrams.

    So, you have 1k0 for 1000 ohms. 47R for 47 ohms, 4R7 for 4.7 ohms, 5M6 for 5.6 megohms etc.
     
  11. Jun 14, 2009 #10
  12. Jun 16, 2009 #11
    Two capacitors, one to decouple input, one to decouple output. Still no big deal.
     
  13. Jun 16, 2009 #12
    There are plenty of single supply opamps available as well.
     
  14. Jun 16, 2009 #13

    vk6kro

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    It gets worse, though. Op amp circuits seem to end up as a rats nest of extra components.

    Active filters, especially, use lots of extra bits and when you start using chips that have 4 op amps in them (like the LM324), it becomes a nighmare trying to fit the components anywhere near the chip.

    These chips can share a divider network between opamps but there are sure to be lots of other components. I sometimes just use a couple of the op amps and leave the others unused or use chips with only one or two op amps in them, like the TL071 and TL072.

    Anyway, this query was about 741s.
     
  15. Jun 19, 2009 #14
    Well, of course vk6kro is talking practice, when we (at least I) was talking theory.

    "In theory, theory and practice are the same. In practice, theory and practice are practically unrelated". :smile:
     
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