A port is a pair of terminals in an electrical network. It is convenient to think of the input terminals of an electronic circuit as one port and the output terminals as another. so, for analysis purposes, the circuit is regarded as a two-port network. Figure 9-1 shows the input and output ports of a general network: one that might be composed of a single semiconductor device or of any number of interconnected components. such as an amplifier stage. Note the polarities assigned to the input and output voltages in Figure 9-1 Also note that current flowing into each terminal is assumed to be positive. These conventions an! standard in two-port network theory. but they do not mean that actual voltages and currents in a network being analyzed must have those polarities. As usual, a negative value simply that a polarity or direction is opposite to that assumed. Do not be confused by the fact that currents seem to be flowing from open circuits into the ports of the network; as we shall soon see, the terminals at a port arc often shorted for analysis purposes, thus providing a path for current flow. or else they are left open as shown (in which case the currents are zero). When the currents and voltages are ac quantities, the polarities are used as instantaneous references. In most electronic circuits analyzed as two-port networks. one terminal of each port is common to both input null output, Figure illustrates this fact in the circuit of il Conrail-Source!J FE”. using a circuit diagram we have already studied in detail, Note that the source terminal is common to both the input port and the output port. The common terminal of each port is the circuit ground.
Two-Port Network Parameters
In an curlier chapter, we distinguished between inherent physical parameters of a device and derived parameters that arc based on algebraic ratios, such as a and (3 , in a bipolar transistor. Two-port parameters are derived parameters defined by certain ratios of input and output voltages and currents. The specific ratios define each parameter depend on the parameter set from which they are derived. We shall study two important sets of parameters: hybrid, or h, para me CI~ and admittance, or y, parameters, Both of these sets arc widely used to describe character stylistics of electronic devices.
The important feature of two-port parameters is that they completely character inc a network for which their values arc given. In other words, if all the two port parameter values associated with a network are known, then every important characteristic of that artwork can be determined, including its voltage and current gains and its input and output resistances. Furthermore, any circuit having equal parameter values is equivalent to and can be substituted for the actual network. The most important application of this fact is that it is possible to construct an accurate circuit model of an electronic device based on its parameter values. The performance of the device in a larger system can then be evaluated using the equivalent circuit model to represent the device.