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A comparator will output the difference of V+ and V- if it has no supply voltages?

  1. May 5, 2012 #1

    Femme_physics

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    Question at topic title.
     
  2. jcsd
  3. May 5, 2012 #2
    Re: A comparator will output the difference of V+ and V- if it has no supply voltages

    Supply voltage is needed to bias the output transistors. Without it the OPAMP output transistors will be all off and the output node will be have high input impedance. i.e. Whatever voltage the outside circuit tries puts on the output node it will keep that voltage.
     
  4. May 5, 2012 #3

    Femme_physics

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    Re: A comparator will output the difference of V+ and V- if it has no supply voltages

    So no supply voltages = no current at all?

    I'm talking about an amplifier with no feedback btw
     
  5. May 5, 2012 #4

    AlephZero

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    Re: A comparator will output the difference of V+ and V- if it has no supply voltages

    That depends entirely on the chip design, but it's a hypothetical question, because active electronic circuits (e.g. op amps) are only designed to be used when the power supplies ARE connected.
     
  6. May 6, 2012 #5

    jim hardy

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    Re: A comparator will output the difference of V+ and V- if it has no supply voltages

    Just assume all opamps have to be powered. Any that draw their power from input pins instead of V+ and V- pins are special cases.

    When National Semiconductor published their AN31, OP Amp Collection, it didn't show the power pins. They got so many inquiries from newbie experimenters who had neglected to power their circuits they put out a special bulletin to that effect.

    AN31 is really old now but those guys were geniuses and the design of their circuits is quite instructive.
    It's a great introductory learning tool, as is TI's "OpAmps for Everyone".
    One can use modern opamps in these old circuits.

    AN31: http://www.ti.com/general/docs/lit/getliterature.tsp?svadoc=an-31&reg=en

    OPAmps for Everyone: http://www.google.com/url?sa=t&rct=...QxYdkL4XemccjuNYQ&sig2=SsJrqlAmpgf3c7JV7uzt3w

    or search on "opamps for everyone" slod006

    It's a 464 page textbook in pdf that should be printed and kept in one's technical library.
     
  7. May 6, 2012 #6

    NascentOxygen

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    Re: A comparator will output the difference of V+ and V- if it has no supply voltages

    Another puzzle from the questioning mind of Femme_physics! :smile:

    There is no feedforward in an unpowered op-amp (well, none that can be relied upon, anyway!) So the output cannot be powered by the input in the manner you are picturing.

    No supply voltage/s = no usable output
     
  8. May 7, 2012 #7

    Femme_physics

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    Re: A comparator will output the difference of V+ and V- if it has no supply voltages

    :) hehe

    Gotcha! Thanks.

    And also Jimhardy and alephzero--thanks for the "feedbacks":) (my jokes used to be better)..
     
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