A powerful feature of SPICE is the ability to define subcircuits that can be referenced in a data statement. Suppose, for example, that we desire to store the equivalent circuit for the op amp used in Example 4.16 and recall it whenever needed in a solution. This can be done by generating a file such as OPAMP .CKT containing the circuit definition desired for the op amp and then calling it using a data statement within a circuit file employing the X command (subcircuit call). Let us begin by using the subcircuit definition feature for file OPAMP .CKT, which can be written as