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Op Amp virtual ground

Discussion in 'Electronic Basics' started by mahak, Nov 4, 2007.

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  1. mahak

    mahak Guest

    Hi,
    This is mahak.
    can anybody tell me what is virtual ground concept.I refer to Micro
    electronics by Sedra & SMith.
    There it says Virtual Groung is because of infinite gain of amplifier.
    But if that is the case then it should be there for both inverting and
    non inverting amplifier. Please clarify.
     
  2. An opamp with negative feedback connected between output and
    inverting input, tries to produce whatever output that will
    cause the inverting input voltage to match what is applied
    to the non inverting input. This is because the opamp has a
    very high differential voltage gain, so the only way the
    output will not be saturated either positive or negative is
    if the two inputs have an almost perfect match (the
    imperfection in that match providing the signal that is
    amplified by the high differential voltage gain to produce
    the finite output).

    So, when you connect the non inverting voltage to ground,
    the feedback forces the inverting input to be an almost
    perfect copy of that ground voltage. From the viewpoint of
    any signal connected to an input network to the inverting
    input, that signal acts as if the input network were
    connected to ground, even though it is just balanced there
    by gain and feedback. Pretending that the inverting node is
    grounded (virtually grounded) makes it a lot simpler than
    doing the math that includes the tiny voltages at that node
    because the amplifier gain is not actually infinite. It is
    a useful approximation.

    Of course, if you connect the non inverting input to some
    other voltage reference than zero, the inverting input is a
    virtual copy of whatever reference voltage you use, instead
    of ground.
     
  3. BobW

    BobW Guest

    Whew. I feel dizzy. I think it's best that the OP stick with Popelish'
    explanation.

    Bob
     
  4. Phil Allison

    Phil Allison Guest

    "mahak"

    ** Virtual ground refers to the situation where the " + " input of an op-amp
    is connected to circuit ground and the circuit is an inverting amplifier
    stage.

    Then the " - " input becomes a "virtual ground" since ( in all normal
    operation ) there is only the *tiniest* of AC or DC voltages on that pin.

    So, it is "virtually grounded" during operation.

    Calculations and analysis can use that simplifying assumption quite safely.



    ....... Phil
     
  5. Eeyore

    Eeyore Guest

    You do talk the most appalling tripe.

    Graham
     
  6. Eeyore

    Eeyore Guest

    Ignore Jamie's 'explanation'. He's an utter fuckwit.

    Phil Allison has explained it in a nutshell.

    Graham
     
  7. mahak

    mahak Guest

    thanks. its convincing.
     
  8. Jamie

    Jamie Guest

    Hmm. I can't find that reference on the NET that you speak of.

    In my eyes. A virtual ground is no more than a voltage
    divider where the divided network is the ground and the outer of
    the network are connected to the + & - of the single source of energy.
    In this system, the common would be derived from the center divide of
    the network.
    I don't see where the gain of an amp comes into play here unless, they
    are referring to the open-loop gain? That would be the (+) input
    connected to the common/virtual ground tap point, and the (-) being the
    input for example.

    Taking measurements on this type of output with no input on (-)
    should yield 0 volts. This is assuming an ideal amp.
    biasing the (-) would result in very high gain but not infinite since
    in theory, only ideal op-amps have that and there is no such thing as an
    ideal op-amp or amp for that matter.

    This seems to be a general theory question at best.
    Maybe you read it incorrectly ?
     
  9. BobW

    BobW Guest

    Assuming Jamie had ever used an opamp then he would know what "virtual
    ground" is and how to (simply) explain it.

    It's amazing how people like Jamie will spew the limited knowledge that they
    have in an attempt to impress people. It always has the opposite effect, in
    the long run.

    Jamie just needs to go to school. If he's been to school then he needs to go
    back.

    Bob
     
  10. Jamie

    Jamie Guest

    Not understanding is the first sign of interest :)
     
  11. me

    me Guest

    He could have said it "nicer", but you're way off.
    Your entire post is:

    The original question is in reference to an actual book often studied by
    EE's, not the NET.
    get your eyes checked.
    No measurements are involved.
    This is a very specific (and basic) question about the operation of
    opamps. His reading skills are fine...
     
  12. Jamie

    Jamie Guest

    Love how you left out the rest of my post as it did go on with
    the op-amp discussion.

    But that's typical of you any ways you lifeless turd.

    Want to smell my shorts? I haven't changed them in a while! it
    should turn you on! I heard that turns you on!
     
  13. BobW

    BobW Guest

    I think it's time to play -- SPOT THE LOONEY!



    Bob
     
  14. Jamie

    Jamie Guest

    Excuse me sir?

    or should it be jerkelator in your case?

    Limited knowledge you say? Where did get yours,
    the five and dime store?

    I think 30+ years in the field is enough.

    That also covers putting 2 son's through
    college for EE both graduating with a 4.5 grade average, both with
    dyslexia and extreme help from their father.

    The title on this group reads "BASIC" which means basic in terms
    of explanation and laymen descriptions.

    It's assholes like your self, parade around
    in public trying to impress some one.

    Facts are, you can impress those that don't know any better, but
    there are many that you will never impress with that kind of approach!

    Should I go on? or does it seem that some one might be talking about you?

    --------------- Bottom line is.. -----------------

    You'd be the wiser to keep the trap shut, you might even learn
    something to help that lacking personally you're exhibiting.

    Let me drop to your level for a moment.

    "Fucking idiot you are! 100 % Grade A snot nose jerk.

    I bet you're real popular around the boys with bull
    shit like that coming out of that urinal.

    If I didn't know any better, I think you're in bed

    with Mr. Graham, or is that Ham.

    You two were meant for each other.. "

    Now you know how you look on the other side.

    Btw..
    My employer just signed a 3 year contract with me, I'm above
    the 135k mark now. I've been with this employer for years, along
    with doing some of my own side adventures in robotics capacitor
    assemblers that you, you're self may have actually had in your hands,
    components made by automative equipment that I designed.

    Scare's the hell out of you doesn't it?

    I think if my skills were suffering, there would be some issues
    with that?

    Have a good fucking day and may your shorts and faults be many!.


    btw, I've turn down guys many times better than you in interviews
    for EE/Electronics E. Don't take it personally.
     
  15. Jamie

    Jamie Guest

    If that video is appealing to you, I can understand your problem.

    I might had enjoyed watching something like that when I was around 10
    or so how ever, I find that video very poorly acted out and boring to
    say the least. A very poor dictation of english humor, but then again,
    it's not much different than a couple of British posters I see here
    dumping their vile.

    Have a good day.
     
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