Virtual ground

Discussion in 'General Electronics Chat' started by pete.york, Mar 15, 2011.

  1. pete.york

    Thread Starter New Member

    Mar 15, 2011
    Can anyone explain, the reason for virtual ground?
    Everyone always tell : " if you have negative feedback, then V+=V- in opamp" , but why? What is the reason?
    How people get to this thing at the beginning?

    Thank you.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
  3. Papabravo


    Feb 24, 2006
    Answer: An operational amplifier amplifies the difference between the two inputs. If they are exactly equal down to a large number of decimal places then the output will be close to zero. Small deviations will produce an output that is close to one of the supply rails.

    So the beginning is to look at the terminal without the feedback and ask "What is happening at this terminal?". The answer is very likely that it is connected via a resistor to ground. Question: What is the voltage drop across a resistor in which there is only a tiny current flowing, say a couple of picoamperes?

    Answer that question and you can get some feel for the virtual ground. It is just a point in a circuit that is at or near ground potential without being connected to ground. In the case of the inverting opamp it is the node where the feedback and the input resistor are connected.