virtual ground

Virtual ground is a copy of ground potential created by an amplifier that is not actually connected directly to ground. It is the input node of a current summing negative feedback amplifier. The amplifier produces whatever output voltage is necessary to soak up all the current arriving at the input node from some signal voltage or current source, through some input network, through some other network between the output and the input node. To the input signal, it appears that the input network is connected to ground, but it is just anode forced to stay very close to ground potential by the negative feedback network and the high gain of the amplifier.

One of the uses for it is to sum two signals, with no bleed over of one signal into the other. If each is connected to the virtual ground node through, say, a resistor, each signal voltage is converted to a proportional current, but each input acts as if that resistor were just tied to ground, so there is no cross talk between the two signals, just loading to ground. In the mean time, the amplifier adjusts the output voltage so that the feedback network passes all the current provided by both input signals, in a way that balances the input voltage very close to zero volts. The inputs and outputs just sort of pivot on the input node like a voltage see saw.