Virtual ground
From Free net encyclopedia
Virtual ground (sometimes called virtual earth) is a very important concept found in electronic circuit designs. It identifies a point in a circuit as being held close to the circuit's ground or reference level electric potential. Creation of a virtual ground is due to the actions or effects of the parts in the circuit. It is called virtual since this point does not have any real electrical connection to ground. The reference may or may not be the same as the local utility ground or earth.
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Overview
The virtual ground concept aids circuit analysis in operational amplifier and other circuits and provides useful practical circuit effects that would be difficult to achieve in other ways. An ideal virtual ground would be able to sink an infinite amount of current. In practice, the sinking capability is determined by the other circuit impedances and the amplifier used.
In operational amplifier circuits
A virtual ground presents a very low impedance to any signal connected to it and it therefore provides the perfect type of input for current type signal sources. It can therefore be used to sum input signals using an op-amp as shown below. It also provides isolation of the input signals. A typical example of a virtual ground point is found in circuits using operational amplifiers with negative feedback. In an inverting amplifier configuration with the + input grounded, the inverting (-)input is said to be a virtual ground.
How to create a virtual ground point
Since an operational amplifier has very high open loop gain, in negative feedback circuits, the circuit acts automatically to make the potential difference between its inputs tend to zero. If the + input is grounded, then the non inverting input, although not connected to ground, will assume a similar potential.
This circuit is somtimes called an audio mixing circuit (not to be confused with frequency mixer)
With the input resistors, this circuit is suitable for voltage signals. If no input resistor is used, the op amp realises an ideal current to voltage converter.
In electrical distribution networks
Three-phase electrical "Y" (or star)circuits are said to have a virtual ground node when their sources and loads are balanced respectively. The virtual ground in this case exists at the star pointTemplate:Electro-stub