subcircuit

A subcircuit is a self-contained portion of an electrical circuit whose internal structure is abstracted from the rest of the design. It represents a function or a device model as a single entity with a defined set of external connection points, or ports, through which it interacts with the surrounding circuit. Subcircuits enable modular design, reuse, and easier management of complex circuits.

In schematic capture and circuit simulation, subcircuits are used to encapsulate recurring building blocks such as operational amplifiers, filters, power regulators, or macromodels of complex devices. They allow designers to replace a block by a single symbol while preserving its internal behavior, and to share a common model across multiple projects.

In SPICE and many EDA tools, a subcircuit is defined with a header that lists its port pins and parameters, ends with an end marker, and is called from the top-level netlist by an instance reference. Parameters can be passed to refine behavior, and internal nets remain local to the subcircuit. Subcircuit definitions live in libraries and are included in the main netlist via include statements.

Key advantages include abstraction, modularity, and easier maintenance, as changes to the subcircuit only need to be made in one place. Proper port ordering and consistent pin naming are essential to ensure compatibility with higher-level schematics. Subcircuits also support hierarchical design, enabling large systems to be built from many blocks.

Limitations include potential performance overhead for deep hierarchies and the need for clear documentation of interfaces. While common in analog modeling and SPICE-based flows, the concept also appears in digital design tools under different names, but always as a reusable, self-contained block with a defined interface.