At high frequencies (e.g., microwave frequencies), the use of power and energy variables is more appropriate, and the two-port current–voltage approach is replaced by an approach based upon scattering parameters.Ī 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. These current and voltage variables are most useful at low-to-moderate frequencies. The difference between the various models lies in which of these variables are regarded as the independent variables. , current into port 2which are shown in figure 1. They are usually expressed in matrix notation, and they establish relations between the variables These are all limited to linear networks since an underlying assumption of their derivation is that any given circuit condition is a linear superposition of various short-circuit and open circuit conditions. The common models that are used are referred to as z-parameters, y-parameters, h-parameters, g-parameters, and ABCD-parameters, each described individually below. The analysis of passive two-port networks is an outgrowth of reciprocity theorems first derived by Lorentz.In two-port mathematical models, the network is described by a 2 by 2 square matrix of complex numbers. Any linear circuit with four terminals can be regarded as a two-port network provided that it does not contain an independent source and satisfies the port conditions.Įxamples of circuits analyzed as two-ports are filters, matching networks, transmission lines, transformers, and small-signal models for transistors (such as the hybrid-pi model). For example, transistors are often regarded as two-ports, characterized by their h-parameters (see below) which are listed by the manufacturer. It also allows similar circuits or devices to be compared easily. This allows the response of the network to signals applied to the ports to be calculated easily, without solving for all the internal voltages and currents in the network. A two-port network is regarded as a "black box" with its properties specified by a matrix of numbers. The two-port network model is used in mathematical circuit analysis techniques to isolate portions of larger circuits. In a two-port network, often port 1 is considered the input port and port 2 is considered the output port. The ports constitute interfaces where the network connects to other networks, the points where signals are applied or outputs are taken. 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. 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.