PSpice 计算机仿真 Simulation Program with Integrated Circuit Emphasis

CH4 ADDITIONAL DC ANALYSIS 附加直流分析

4.1 Computing the Thevenin Equivalent 戴维南等效 4.2 Sensitivity Analysis 灵敏度分析 4.3 Simulating Resistor Tolerances 误差电阻的仿真

4.1 Computing the Thevenin Equivalent The open-circuit voltage The short-circuit current

Fig.35

Example 4 Use PSpice to find the Thevenin equivalent with respect to terminals a, b for the circuit shown in Fig.35 (P29).

To compute the short-circuit current We inserted a resistor(R2) between nodes a and b (node 0) whose value is 10e-6. (Fig. 36)

Fig. 36 short-circuit current

Do not forget! F1 的属性需要修改 : 选中 F1 元件, ( 粉色表示选中 !) Edit ->Properties … Gain 项 : 3 (Fig. 36a)

Fig. 36a Edit Properties

在 OrCAD Capture 运行环境下 : PSpice->Run 在 OrCAD PSpice A/D Demo 运行环境下 : View->Output file (Fig. 37)

Fig. 37 output file

The short circuit current, which is the current through the voltage source V3, is 2 A.

To compute the open-circuit voltage We have two options: Connect a resistor between nodes a and b, in his case 10e6 (Fig. 38) Connect a capacitor between nodes a and b The capacitor behaves like an open circuit during dc analysis and, therefore, does not influence the dc Thevenin equivalent.

Fig. 38 open-circuit voltage

OrCAD Capture 环境 : PSpice->Run OrCAD PSpice A/D Demo 环境 : View->Output file (Fig. 39)

Fig. 39 output file

Note that node a in the original circuit (Fig. 35) is assigned as node 54 (It may be different with yours.) by PSpice, so the open-circuit voltage is 12 V.

The Thevenin equivalent Open-circuit voltage 12V Short-circuit current 2A Therefore, the Thevenin resistance is 12/2= 6 Ω and the Thevenin equivalent circuit is as shown in Fig. 40.

Fig. 40 Thevenin equivalent

4.2 Sensitivity Analysis( 灵敏度分析 ) Example 5 illustrates how to perform sensitivity analysis to predict the behavior of an unloaded voltage-divider circuit( 空载分压电路 ).

Fig. 41(P33)

Example 5 Use PSpice to study the sensitivity of the output voltage Vo in the voltage divider circuit shown in Fig. 41.

Fig. 42

Components Analog/R Source/VDC 注意 : Out 为节点的名字, Place/Net Alias …

Simulation PSpice/New Simulation Profile (Fig. 43)

Fig.43_setting

PSpice/Run PSpice/View Output File (Fig. 44)

Fig.44_Output file

From the sensitivity data, we deduce that If R1 increases by 1Ω, Vo will decrease by 0.8V, that is, Vo=99.2V If R1 increases by 1%, Vo will decrease by 0.2V to 99.8V

If R2 increases by 1Ω, Vo will increase by 0.2V, that is, Vo=100.2V If R2 increases by 1%, Vo will increase by 0.2V to 100.2V

If V1 increases by 1V, Vo will increase by 0.8V, that is, Vo=100.8V If V1 increases by 1%, Vo will increase by 1V to 101V

Question? If R1 increases by 1Ω, R2 decreases by 1%, and V1 increases by 0.5V, Vo=?

Answer! We have a linear circuit, so the principle of superposition ( 叠加原理 ) applies, and therefore, Vo= =99.4 V

4.3 Simulating Resistor Tolerances ( 误差电阻的仿真 )

Example6 Replace the 100Ω resistor in the circuit in Fig.41 with a resistor having the same value but with a 10% tolerance. Use PSpice to discover the range of output voltage values that you can expect with this more realistic model of a resistor.

Fig. 45

R2 属性修改： 选中电阻 R2 ， ( 粉色表示被选中 !) 单击鼠标右键，选择 Edit Properties …, Fig.45 在 Tolerance 项，写入 “ 10% ”, Fig. 46 背景； 选中 Tolerance 项, 单击 Display 按钮，出现 Fig. 46 对话框；

Fig. 46

DC sweep analysis Monte Carlo/Worst Case

Fig. 46a DC sweep analysis

Fig. 47 Monte Carlo/Worst Case

Fig. 48 maximum

Fig. 49 output file

You can confirm this result using the voltage division equation for the circuit in Fig. 41 when the R2 resistor has the value 110 Ω.

Fig. 50 minimum

Fig. 50a output file

You can confirm this result using the voltage division equation for the circuit in Fig. 41 when the R2 resistor has the value 90 Ω.

Thus, we see that when the R2 resistor in Fig. 41 has a tolerance of 10%, the actual output( 实际输出 ) voltage may range from V to V.