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Opamp Virtual Ground Concept

Hari T O0
Genius 3820 points

Hi Team,

May I know 

Why is the concept of virtual ground not applicable for OpAmp in positive feedback configuration or in open loop configuration?

Regards

Hari

  • 0
    TI__Guru**** 164546 points
    Hari,

    You are right, the virtual ground is a concept, mental construct. The virtual ground only exists when the output is in the normal range, not at VOL and VOH level. Once the op amp output gets to maximum low or high level, the gain of the op amp drops to zero. The virtual ground only works if op amp gain is much greater than circuit programmed gain.
  • 0
    TI__Guru 67601 points
    Adding to Ron's comments - a virtual ground (short) concept applies only when the negative feedback drives the inverting input terminal within the input voltage offset (Vos) of the non-inverting input terminal. Since using a positive feedback, or an open-loop configuration, does NOT result in both inputs being at the same voltage potential, the concept of virtual short between the input terminals falls apart.
  • 0 in reply to Marek Lis
    Genius 3820 points
    Hi Marek,
    Thank you very much.

    Why virtual ground concept is not applicable to positive feedback system(My understanding is positive feedback is a non linear operation that is why virtual Ground concept is not applicable to it-Please correct me if I am wrong)

    May I know you have any mathematical proof which shows virtual gnd is applicable to only negative feedback system not for positive feedback.

    Regards
    Hari
  • 0 in reply to Hari T O0
    TI__Guru**** 164546 points

    Hari,

    The answer to this and much more are available in a video series.

    https://training.ti.com/ti-precision-labs-op-amps