1、5.5 Two-Port Networks 5.5.1 Introduction Many circuit networks have the structure as shown in Fig. 1. N1I1I 2I2I Figure 1: Structure of two-Port network. The network N shown in Fig. 1 includes two ports and the input current of each port is equal to the output current. We call the network a two-port
2、 network. For example, an ideal transformer is a two-port network. 12:NN Figure 2: Ideal transformer. The introduction of the concept of two-port networks has two advantages. One is beneficial to circuit design, which means we can divide a circuit into a number of simple two-port networks, then desi
3、gn each of them independently. The other advantage is that when we analyze a circuit, the two-port networks constituting the circuit can be regarded as black boxes. As long as we know the relation of voltages and circuits of the ports, it is unnecessary to learn the details inside the two-port netwo
4、rks. 5.5.2 Two-Port Parameters For a resistor, it is a one-port network and it is characterized by the parameter of resistance R. Figure 3: Resistor. Similarly, for a two-port network shown in Fig. 4, it is characterized by two-port parameters. N1I 2I1V 2V Figure 4: Two-Port network. Frequently-Used
5、 two-port parameters include impedance parameter, admittance parameter, hybrid parameter and transmission parameter. (1) Impedance Parameter If the relation of voltages and currents of a two-port network shown in Fig. 4 can be expressed as =+1 11 1 12 22 21 1 22 2V z I z IV z I z I (1) then = 11 122
6、1 22zzz zz (2) is called the impedance parameter of the two-port network. Lets take an example. Find the impedance parameter of the two-port network shown in Fig. 5. 1I 2I1V 2V22j Figure 5: Two-Port network for finding the impedance parameter. From Fig.5, ( )( )222jj= + += +1 1 1 22 1 2V I I IV I I
7、(3) Reorganizing the equation (3), we get ( )2 + 2 222jjjj=+1 1 22 1 2V I IV I I (4) From (1), (2) and (4), the impedance parameter is 2 2 222jj+ = z (5) (2) Admittance Parameter If the relation of voltages and currents of a two-port network shown in Fig. 4 can be expressed as =+1 11 1 12 22 21 1 22
8、 2I y V y VI y V y V (6) then = 11 1221 22yyy yy (7) is called the admittance parameter of the two-port network. (3) Hybrid Parameter If the relation of voltages and currents of the two-port network shown in Fig. 4 can be expressed as =+1 11 1 12 22 21 1 22 2U h I h VI h I h V (8) then = 11 1221 22h
9、hh hh (9) is called the hybrid parameter of the two-port network. (4) Transmission Parameter If the relation of voltages and currents of a two-port network shown in Fig. 4 can be expressed as ()()= + = + 1 1 1 2 1 2 21 2 1 2 2 2 2U a U a II a U a I (10) then =11 1221 22aaa aa (11) is called the tran
10、smission parameter of the two-port network. 5.5.3 Interconnection of Two-Port Networks A complex circuit network may be divided into a number of relatively simple two-port networks for convenience of analysis and design. The relatively simple two-port networks can be regarded as the building blocks
11、of the complex circuit network. Through interconnection, the relatively simple two-port networks may constitute a complex circuit network. There are many kinds of interconnection forms of two-port networks, such as series, parallel, series+parallel, parallel+series, and cascade. Here we only introdu
12、ce the commonest cascaded two-port networks, as shown in Fig. 6. N1N2 Figure 6: Cascaded two-port networks. We put a dashed frame on the cascaded two-port networks, as shown in Fig. 7. It is obvious that a larger two-port network forms. N1N2 Figure 7: Cascaded two-port networks form a larger two-port network. If the transmission parameters of N1 and N2 are a1 and a2 respectively, the transmission parameter of the larger two-port network is 1 1=a a a (12)
