A
two-port network (a kind of
four-terminal network or
quadripole) is an
electrical network (
circuit) or device with two
pairs of terminals to connect to external circuits. Two terminals constitute a
port if the currents applied to them satisfy the essential requirement known as the port condition: the
electric current entering one terminal must equal the current emerging from the other terminal on the same port. The ports constitute interfaces where the network connects to other networks, the points where signals are applied or outputs are taken. In a two-port network, often port 1 is considered the input port and port 2 is considered the output port.