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158 2. Tutorial
Stimulus Sources for Two-Port Calculations
A particular two-port matrix can be computed by connecting appropriate stimulus sources to the
ports and solving for the opposite port quantities. For instance, if we want to compute a Z-matrix we
must connect current sources to the ports, set up circuit equations and solve for the corresponding
port voltages. Likewise, a Y-matrix is obtained by connecting voltage sources V1 and V2 to the ports
and determining the port currents I$V1 and I$V2 in terms of these voltages.
I$V1
I$V2
V1
Y
V2
Figure 9.9: Voltage source excitation for Y-matrix calculation
Note that a straightforward combination of the two-port circuit and the excitation sources as shown
in Figure 9.9 will not yield an entirely correct result. The resulting Y-parameters would have a wrong
sign because the positive reference directions for the branch currents of the voltage sources are not
oriented according to the reference directions for two-port terminal currents.
Remember that the positive reference direction for the current and voltage in a circuit branch is always
determined by the order of the nodes in the corresponding netlist entry (see Chapter 2.2).
The reference direction for the port currents can be aligned with the two-port terminal current
directions by reversing the order of the nodes in the netlist entries belonging to the stimulus sources.
However, the voltage reference directions, whose initial settings were correct, are changed by this
operation as well. Hence, to compensate this unwanted effect, the voltages must be given negative
values in the netlist (see Figure 9.10).
I$V1
I$V2
-V1 Y -V2
Figure 9.10: Correct voltage source excitation for computing Y-parameters
Computing Y-Parameters
Let’s demonstrate the procedure for two-port analysis by calculating the Y-matrix of the circuit shown
in Figure 9.11.