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single supply opamp

Discussion in 'Electronic Basics' started by [email protected], Mar 28, 2005.

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  1. Guest

    I am simulating a single supply opamp in an inverting opamp
    configuration with gain set to 1. I/P is an ac sine 200mVpp 0Voffset. I
    am using the TLC081 TI single supply opamp spice model.
    Qn) When input sine wave becomes +ve wrt gnd , then o/p should drive
    negative. Of course that can't happen because we have a single supply
    system. In the simulation i see an o/p DC offset(300mV) wrt which the
    o/p sine wave swings up and down. Does this mean that in a single
    supply opamp the o/p always gets a DC offset even if input has no DC in
    it? I don't qualatatively understand what happens when V- input of a
    single supply opamp crosses the V+ input(0V always in this case).
    Thanks
     
  2. Bob

    Bob Guest

    If the + input is more negative than the - input then the output will head
    toward the most negative supply that the opamp's power pins are connected
    to. If the most negative supply is what you're calling "gnd" then that's the
    most negative voltage that the output can get to.

    Typically, in a single-supply opamp that's hooked up in an inverting
    configuration, the + input is biased at the midpoint of the supply. For
    example, if the supply is 9V, then the + input will be set to 4.5V. If you
    then capacitively couple the - input (through the feedforward resistor) then
    the output will move up and down from its idle 4.5V point (as the input is
    driven with a changing signal).

    An interesting thing happens if you capacitively couple the output. Then,
    you can get an output signal that goes more negative than your most-negative
    supply ("gnd", in your case). This is how car audio amplifiers can get
    speakers to move both outward and inward, on those type of amplifiers that
    require one side of the speaker to be connected to the minus side of the
    car's 12V battery (i.e., the car's chassis).

    Bob
     
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