Virtual ground

The virtual ground (even virtual zero, VNP ) describes a point in the electronics of an electrical circuit, the currents flowing in spite of having a ground potential, but is not connected directly to the ground. The realization is possible only with a reinforcing element (usually an operational amplifier ), which immediately initiates countermeasures even at the lowest voltage appearing at this point and ensures that the correct sign sum of all currents is zero at this point (node ​​rule).

Example

The inverting amplifier amplifies the input voltage Vin by a factor of:

The op-amp tries to keep to zero with the help of its output voltage Vout, the differential voltage at its inputs. When R1 = R2 = 1 k and 10 k are flowing at Vin = 0.5 V, R1 through current the voltage at the inverting input - of the operational amplifier makes a short time positive (). Because of the very high gain of 100,000 for example changes Ua within microseconds to -5 V, because only then:

  • Through both resistors R1 and R2 flow 0.5 mA.
  • The respective voltage drops be U1 = 0.5 V and U2 = 5 V.
  • (-) Therefore, the voltage at the inverting input of the operational amplifier only 5/100.000 V = 50 microvolts.
  • This voltage is so tiny that we think with all instruments available, this point would be connected to ground - even if changes Ue.

The virtual ground point must not connect directly or via a large capacitor to ground you, because you thereby preventing the rapid rules of the OP.

The same potential at the two inputs of an ideal operational amplifier, in the case of the negative feedback, sometimes referred to as a virtual short-circuit, since the two inputs have the same potential and act as seemingly short-circuited. The potential at the two inputs do not in this case be equal to the ground potential.

  • Electrical circuit
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