Introduction to Electric Circuits

9 Two-port networks

9.1 INTRODUCTION

Figure 9.1

+o I1^1

2-port

network

I2

r

I2

o+

o-

A two-port network has four terminals as shown in Fig. 9.1 and is often called a

four-terminal network. These are made up of:

1 an input pair by which an input current (11 say) enters one terminal and

leaves the other; and

2 an output pair by which an output current (I2 say) enters one terminal and

leaves the other.

The input terminals constitute the input port and the output terminals

constitute the output port. The conventional directions of the currents are as

shown in the diagram.

Some examples of two-port networks are: transistor circuits; amplifier

circuits; filters; power transmission lines.

For linear, passive networks a set of equations can be established which

relate input and output quantities in terms of the network impedances or

admittances. If the input and output currents and voltages (I~, I2, V~ and V2) are

considered in pairs to be the independent variables, the others then being the

dependent variables, there are six possible pairs or sets of equations. These will

be discussed in turn.

9.2 THE IMPEDANCE OR z-PARAMETERS

Set 1" I~ and 12 are the independent variables. The dependent variables are then

given by