A virtual short is a concept that applies to the inputs of an operational amplifier, or op amp, a common electronic device in analog circuits. Like a real short circuit, a virtual short causes the voltage differential (called Vd) between the two inputs to the op amp to drop to 0. Unlike a real short, however, no current is allowed to flow between the inputs--indeed, an ideal op amp has infinite input impedance.

The reason for the occurance of a virtual short is straight forward and easy to understand. An ideal op amp has the following characteristics:

The voltage gain is infinite:
Av = ∞

The voltage transfer characteristic equation is:
Vo = Av·Vd
Where Vo is the output voltage, and Vd is the input voltage differential, as shown in the figure:

 - ---|-\
Vd    |  \___ Vo
      |  /
 + ---|+/

Then, if we express Vd in terms of Vo, we get:
Vd = Vo / Av

And, substituting in Av:
Vd = Vo / ∞ = 0

Of course, in the real world, the voltage gain is not infinite, but only a rather large number. Nevertheless, Vd will only ever be a small fraction of the output voltage.

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