Virtual mass (electronics)

In electronics, virtual ground (also virtual zero point , VNP ) describes a point in an electrical circuit that, despite flowing currents, has a ground potential , but is not directly connected to ground. The realization succeeds with an amplifying element (usually an operational amplifier ), which immediately initiates countermeasures at the lowest voltage occurring at this point and ensures that the voltage at this point remains almost zero. The voltage at the virtual ground point is regulated to ground potential in this way.

example

Wiring of an inverting amplifier

The inverting amplifier amplifies the input voltage U _e with the factor:

${\displaystyle V=-{\frac {R_{2}}{R_{1}}}\Rightarrow U_{a}=-U_{e}\cdot {\frac {R_{2}}{R_{1}}}}$

With the aid of its output voltage U _a, the operational amplifier tries to keep the differential voltage at its inputs at zero. If R ₁  = 1 kΩ and R ₂  = 10 kΩ, current flows  through R ₁ at U _e = 0.5 V , which briefly makes the voltage at the inverting input (-) of the operational amplifier positive. Because of the very high no-load gain of 100,000, for example, U _a changes to −5 V within microseconds, because only then does the following apply:

• 0.5 mA flow through both resistors R ₁ and R ₂ .
• The respective voltage drops are U ₁  = 0.5 V or U ₂  = 5 V.
• Therefore the voltage at the inverting input (-) of the operational amplifier is only 5 V: 100,000 = 50 µV.
• This voltage is so low that with simple measuring instruments one thinks that this point is connected to ground - even if U _e changes.
• The input resistance of this circuit results (neglecting the finite no-load gain) through the virtual ground as simply R ₁ .

It should be noted that the open loop gain in operational amplifiers is usually not a very precisely specified quantity, since it is only important that it is very large; how big exactly is of minor importance.

The virtual ground point must not be connected to ground directly or via a large capacitor, because this prevents the OP from being regulated quickly.

The equipotentiality between the two inputs of an ideal operational amplifier, in the case of negative feedback , is sometimes also referred to as a virtual short circuit , since the two inputs have the same potential and seem to behave as if they were short-circuited. In this case, the potential at the two inputs does not have to be the same as the ground potential.