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 V
d) 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.