**integrator**is an op amp circuit, whose output is proportional to the integral of input signal.

An **integrator** is basically an inverting amplifier where we replace feedback resistor with a capacitor of suitable value.

The above figure, is an inverting operational amplifier circuit.

This can be modified to an **op amp integrator** circuit, if we replace the feedback resistor R_{f}by a capacitor C as shown below.

In an ideal op amp the voltage at non inverting input terminal is same as that of inverting input.

Here, in the circuit, as the non inverting input is grounded, the electric potential of inverting input will also be zero as non inverting input. In an ideal op amp, no current entires to the op amp through both inverting and non inverting inputs.

Now, if we apply Kirchhoff current law at node 1 of the above circuit, shown in figure 2, we get,

Integrating both side, we get,

This is the integral function of input voltage.