Contact us

Откроется новое окно Откроется новое окно

What is the virtual short of an op-amp?

Virtual short is a circuit node with a steady voltage that is not directly connected to a voltage reference such as ground or a DC voltage source.

The concept of virtual short is commonly used for op-amps. Figure 1 shows a negative feedback amplifier using an op-amp.

Suppose that it is the ideal op-amp, which has:

1.infinite open-loop gain (AV)

1.infinite input impedance

1.zero output impedance

Because the input impedance is infinite, all of the current (i1)

flowing through R1 flows through R2.

i1 = (Vi – V) / R1 = (V - Vo) / R2           (1)

The output voltage of the op-amp is given by the equation:

Vo = AV(V+ - V)    (2)

From Equation 1 and Equation 2, V+ is calculated as follows:

V+ = {1 + (R1 + R2) / (AV * R1)} * V - R2 * Vi / (AV * R1)

Because AV is infinite, we obtain V = V.

Hence, the voltage at the negative inverting input (V) is equal to

the voltage at the grounded noninverting input (V+).

 

Roughly speaking, in the event of there being a large difference in voltage between the two inputs, the output voltage becomes abnormally large because an op-amp has a very large open-loop gain (AV).

Even real op-amps have an open-loop gain (AV) around 100 dB (105). This means that, in order to obtain a typical output voltage of several volts, the two inputs must have almost the same voltage (with a difference less than several 10–5 volts).

What is the virtual short of an op-amp?

To Top
·Before creating and producing designs and using, customers must also refer to and comply with the latest versions of all relevant TOSHIBA information and the instructions for the application that Product will be used with or for.