The resistor network circuit in Fig. 3.12 is a potential divider where the ratio of the output voltage to the input voltage is given by

$\frac{{\mathrm{V}}_{\text{out}}}{{\mathrm{V}}_{\text{in}}}=\frac{\mathrm{R}1}{\mathrm{R}1+\mathrm{R}2}$

which can be written as:

${\mathrm{V}}_{\text{out}}={\mathrm{V}}_{\text{in}}\frac{\mathrm{R}1}{\mathrm{R}1+\mathrm{R}2}$

Therefore the output voltage is determined by the ratio of the fixed values of the resistors R1 and R2. If R1 = R2, the output voltage is equal to half of the input voltage.

1. Draw the resistor circuit in Fig. 3.12 and name the nodes as shown by selecting Place > Net Alias (Fig. 3.13).

2. Create a PSpice simulation profile, PSpice > New Simulation Profile and for the Analysis type, select DC Sweep. Select the Sweep variable as the voltage source V1 and set up a linear sweep from 0 to 10 V in steps of 1 V, see Fig. 3.14. Click on OK.

3. Place voltage markers on nodes, in and out (Fig. 3.15).

4. Run the simulation .

5. PSpice will launch and Probe will display the two traces for V(in) and V(out). Fig. 3.16 shows the trace colors are the same as the marker colors in the circuit diagram in Fig. 3.15. Note that the circuit is acting as a potential divider in that the output voltage is half of the input voltage.

6. Delete the ground symbol and re-simulate. You should see the warning message dialog box (Fig. 3.17) and a message will be displayed asking you to check the Session Log.

7. The Session Log is normally open at the bottom of the screen, if not, the session log can be found from the top tool bar, Window > Session Log. The warning message will read:
WARNING [NET0129] Your design does not contain a Ground (0) net.

8. Click on OK in the Warning message (Fig. 3.15) and PSpice will launch. If the Output file is not displayed, then select, View > Output File. The output file is shown in Fig. 3.18. Your circuit will have different node numbers. The removal of a 0 V symbol was covered in the exercises at the end of Chapter 2 and is mentioned here again as a reminder that in order for the analog voltages to be calculated, a 0 V node must exist in the circuit otherwise nodes will be reported as floating.

9. Re-connect a 0 V symbol. Place > Ground or press G on the keyboard and select a 0 V symbol.

10. Remove resistor R2 from the circuit and simulate.

11. PSpice will start and display the output file with an error message:

12. The Error message relates to no DC path to ground at node out, i.e., the end of the resistor R2 is floating. One other requirement in PSpice is that every node in a circuit must have a DC path to ground. If you need to simulate a node as open circuit, then you can always connect a large value resistor, for example, 100 G ohm or 1T ohm from a node to ground which will provide a DC path to ground without adversely affecting the bias conditions of the circuit.
Similarly, a short circuit can be implemented by a very small resistance of say 1μ ohm or less.

Exercise 2

A nested sweep will be performed to display the transistor characteristic family of curves. You will set up the VCE for the transistor as the primary sweep and the base current as the secondary sweep.

In 17.2 you can search for the Q2N3904 transistor using, Place > PSpice Component > Search and entering Q2N3904 in the search field. See Chapter 1, Section 1.3.3, Fig. 1.13. Alternatively follow the following steps.

1. Draw the circuit in Fig. 3.19. The transistor can be found in the bipolar library. In the Place Part menu, select the Add Library icon as shown in Fig. 3.20 or in previous versions, click on Add Library.
This will open up the Browse File window shown in Fig. 3.21. Scroll along or alternatively, type in bipolar.olb in the File name field. Select bipolar.olb and click on Open.

2. The bipolar library will now be added to the list of libraries in the Place Part menu. Select the bipolar library and type in Q2N3904 (not case sensitive) in the Part box, see Fig. 3.22. Double click on the q2n3904 transistor and place in the schematic page.

3. Place the rest of the components as shown in Fig. 3.19. The dc current source, idc is from the source library.

4. You will set up a nested DC Sweep where the VCE will be the primary sweep and the base current the secondary sweep.

5. Create a simulation profile, PSpice > New Simulation Profile and select the Analysis type to DC Sweep. For the Primary Sweep, which is shown by default, select the Sweep variable to be voltage the source VCE. The Sweep type is Linear with a Start value of 0 V, an End value of 12 V and an Increment of 0.1 V, see Fig. 3.23. Click on Apply but do not exit the Simulation Profile.

6. In the Options box, select the Secondary Sweep. The Sweep variable is the current source I1. The Sweep type is Linear with a Start value of 40 μA and an End value of 200 μA, the Increment is 40 μA. Make sure the Secondary Sweep box is checked and click on OK, see Fig. 3.24.

7. Place a current marker on the collector pin of the transistor and simulate.

8. You should see the characteristics curves shown in Fig. 3.25.

9. Select Plot > Axis Settings > YAxis and change the Data Range to User Defined and enter a range from 0 mA to 40 mA. Click on OK and see the change.

10. Select Plot > Axis Settings > YGrid and uncheck Automatic and set the Major Spacing to 10 m. Click on OK and see the change.

11. Select Plot > Axis Settings > XGrid and uncheck both Major and Minor Grids to None. Click on OK and see the change.

12. Select Plot > Axis Settings > YGrid and uncheck both Major and Minor Grids to None. Click on OK and see the change.

13. Select Plot > Label > Text, change the font color to silver and add the base currents as shown in Fig. 3.26.