In some applications, describing a device by embedding the required elements in the main circuit file, as is done for the amplifier in Fig. 26.1, is not desirable. A hierarchical approach may be taken by using subcircuits. An example of a subcircuit statement is shown in the second circuit file:

X1 Amp_In 0 Amp_Out

Amplifier Subcircuits are always identified by a device label beginning with `X'. Just as with other devices, all of the connected nodes are specified. Notice, in this example, that three nodes are used. Finally, the name of the subcircuit is specified. Elsewhere in the circuit file, the simulator looks for a statement of the form:

.subckt <Name> <Node1> <Node2> <Node3> ...

This statement specifies that the lines that follow are part of the Amplifier subcircuit, and that the three nodes listed are to be treated wherever they occur in the subcircuit definition as referring, respectively, to the nodes on the main circuit from which the subcircuit was called. Normal device, model, and comment statements may then follow. The subcircuit definition is concluded with a statement of the form:

.ends <Name>