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virtual ground

Discussion in 'Electronic Design' started by Danny, Feb 9, 2004.

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  1. Danny

    Danny Guest

    Hi

    In an opamp (inverting amplifier) how does the virtual ground come about?

    Dan
     
  2. Jim Thompson

    Jim Thompson Guest

    Virtually ?:)

    ...Jim Thompson
     
  3. I read in sci.electronics.design that Danny <[email protected]
    The current flowing from the input to the circuit flows through the
    feedback resistor to the output. No appreciable current flows into the -
    input of the op-amp. The very high open-loop gain of the op-amp means
    that almost no voltage difference between the + and - inputs is
    necessary to produce the required output signal. The + input is at
    ground potential, so the - input must be at the same potential.
     
  4. mikem

    mikem Guest

    The "vitrual ground" when referring to an opamp is somewhat of a
    misnomer. What really happens when you configure an opamp to have
    negative feedback, is that it tries to minimize the voltage difference
    between the + and - inputs.

    If you "ground" the + input in an inverting amplifier, and assuming that
    the bias current is small compared to the currents flowing in the gain
    setting resistors, then the voltage on the - input will be "very close"
    to zero volts...

    MikeM
     
  5. Drive a virtual ground rod?
     
  6. N. Thornton

    N. Thornton Guest

    Hi

    In inverting configuration, with the + input grounded, the opamp will
    do its best to keep its - input at ground potential. Hence it is
    referred to as a virtual ground.

    Consider the 2 key resistors in the above setup:
    ______ _____
    -----|______|------O------|_____|-----
    R_in R_feedback

    If 1v is fed in thru R_in, and R_feedback is 10x R_in, the output must
    swing to -10v in order to keep the -input at 0v - this is what happens
    in this config.


    Regards, NT
     
  7. gwhite

    gwhite Guest


    As a negative feedback amplifier, the inverting input is driven by the output to
    be _equal_ to the non-inverting input (it "seeks" to drive the _difference to
    zero_).

    Since the non-inverting input is at ground, and the inverting input is driven to
    the same zero potential, a "virtual" (not real) ground exists. That is, if you
    put a voltmeter probe on that inverting node, you will see the same number of
    volts as if you put the probe on ground. That "same number of volts" is
    specifically zero volts, since the non-inverting input is grounded.
     
  8. Fred Bloggs

    Fred Bloggs Guest

    In modern engineering parlance the word "virtual" is used in a multitude
    of fields to generally mean "acts like for practical purposes but is not
    actually" and can apply to other things besides GND. So a virtual GND
    refers to a circuit node that acts like it's grounded without actually
    being grounded. This implies that the node potential remains at constant
    0V for all circuit conditions to which it is subjected. In the case of
    an operational amplifier, this effect is achieved by negative feedback
    being applied in such a way to keep the virtual GND node at 0V, it does
    this by maintaining a zero sum current at the inverting node. The term
    usually has a context, for example whether the virtual GND is a virtual
    AC-GND or a virtual DC-GND or both.
     
  9. And don't ground the virtual ground ;)
     
  10. Fred Bloggs

    Fred Bloggs Guest

    Well- that would make it an actual GND.
     
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