Port (circuit theory)

In multiport network analysis, the circuit is regarded as a "black box" connected to the outside world through its ports.

Equivalently, the algebraic sum of the currents flowing into the two poles from the external circuit must be zero.

[1] It cannot be determined if a pair of nodes meets the port condition by analysing the internal properties of the circuit itself.

Any two-pole circuit is guaranteed to meet the port condition by virtue of Kirchhoff's current law and they are therefore one-ports unconditionally.

Study of one-ports is an important part of the foundation of network synthesis, most especially in filter design.

Two-element one-ports (that is RC, RL and LC circuits) are easier to synthesise than the general case.

[2] Linear two port networks have been widely studied and a large number of ways of representing them have been developed.

One of these representations is the z-parameters which can be described in matrix form by; where Vn and In are the voltages and currents respectively at port n. Most of the other descriptions of two-ports can likewise be described with a similar matrix but with a different arrangement of the voltage and current column vectors.

Common circuit blocks which are two-ports include amplifiers, attenuators and filters.

In these formats, one pole of each port in a circuit is connected to a common node such as a ground plane.

[4] The one-pole representation of a port will start to fail if there are significant ground plane loop currents.

In reality, the ground plane is not perfectly conducting and loop currents in it will cause potential differences.

The energy transfer at that place is thus more complex than a simple flow from one subsystem to another and does not meet the generalised definition of a port.

A transducer may be a one-port as viewed by the electrical domain, but with the more generalised definition of port it is a two-port.