D.C Analysis

 DC analysis is one of the standard analyses that we can perform using

PSpice. Other standard analyses include transient, AC, and Fourier.

Under DC analysis, there are two kinds of simulation that PSpice can

execute: DC nodal analysis and DC sweep.

PSpice allows dc nodal analysis to be performed on sources with an

attribute of the form DC value and provides the dc voltage at each

node of the circuit and dc branch currents if required. To view dc node

voltages and branch currents requires adding two kinds of additional

parts, shown in Fig. D.11. The symbol VIEWPOINT is connected to

each node at which the voltage is to be viewed, while the symbol

IMPROVE is connected in the branch where the current is to be dis-

played. This necessitates modifying the schematic. For example, let us

consider placing voltage VIEWPOINTS and current IPROBES to the

schematic in Fig. D.8(c). To add VIEWPOINTS, we take the follow-

ing steps:

1. Click Draw/Get New Part (or type <Ctrl- G>).

2. Type VIEWPOINT in the Part Browser Basic box.

3. Click OK (or type <Enter>).

4. DRAG to locate VIEWPOINT above V1 and CLICK.

5. DRAG to locate VIEWPOINT above R2 and CLICk.

6. CLICKR to end placement mode.

Figure D.12 shows the two voltage VIEWPOINTS. Since the IPROBE

symbol must be connected in series with a branch element, we need to

move R2 down by clicking and dragging R2 and the wires. Once this

is done, we add IPROBE as follows:

1. Click Draw/Get New Part (or type <Ctrl- G>).

2. Type IPROBE in the Part Browser Basic box.

3. Click OK (or type <Enter>).

4. DRAG to locate IPROBE above R2 and CLICK.

5. CLICKR to end placement mode.

6. Use wiring to join all gaps.

The schematic becomes that shown in Fig. D.12. We are ready to sim-

ulate the circuit. At this point, we must save the schematic—PSpice

will not run without first saving the schematic to be simulated. Before

learning how to run PSpice, note the following points:

1. There must be a reference node or ground connection (part

AGNES) in the schematic. Any node can be used as ground, and

the voltages at other nodes will be measured with respect to the

selected ground.

2. Dependent sources are found in the Parts library. Obtain them by

selecting Draw/Get New Part and typing the part name. Figure D.13

shows the part name for each type, with the gain. E is a voltage-

controlled voltage source with gain e; F is a current-controlled

current source with gain f; G is a voltage-controlled current source

with a transconductance gain g; and H is a current-controlled volt-

age source with transresistance gain h.

3. By convention, we assume in dc analysis that all capacitors are

open circuits and all inductors are short circuits.

We run PSpice by clicking Analysis/Simulate. This invokes the

electric rule check (ERC), which generates the netlist. The ERC per-

forms a connectivity check on the schematic before creating the netlist.

The netlist is a list describing the operational behavior of each com-

potential in the circuit and its connections. Each line in the netlist rep-

resents a single component of the circuit. The netlist can be examined

by clicking Analysis/Examine Netlist from the Schematics window.

If there are errors in the schematic, an error window will appear. Click

OK (or type <Enter>) to display the error list. After noting the errors,

exit from the error list and go back to Schematics to correct the errors.

If no errors are found, the system automatically enters PSpice and per-

forms the simulation (nodal analysis). When the analysis is complete,

the program displays Bias point calculated, and creates the result/

output file with extension .out.

To examine the output file, click Analysis/Examine Output from

the Schematics window (or click File/Examine Output from the

PSpice window). To print the output file, click File/Print, and to exit

the output file, click File/Exit.

We can also examine the results of the simulation by looking at

the values displayed on the VIEWPOINTS and IPROBES parts of the

schematics after the simulation is complete. The values displayed with

VIEWPOINTS and IPROBES should be the same as those in the out put file.

DC nodal analysis allows simulation for DC sources with fixed volt-

ages or currents. DC sweep provides more flexibility in that it allows

the calculation of node voltages and branch currents of a circuit when

a source is swept over a range of values. As in nodal analysis, we

assume capacitors to be open circuits and inductors to be short circuits.

Suppose we desire to perform a DC sweep of voltage source V1

in Fig. D.12 from 0 to 20 volts in 1-volt increments. We proceed as

follows:

1. Click Analysis/Setup.

2. CLICK DC Sweep button.

3. Click Name box and type V1.

4. Click Start Value box and type 

5. Click End Value box and type 

6. Click Increment box and type 

7. Click OK to end the DC Sweep dialog box and save parameters.

8. Click Close to end the Analysis Setup menu.

Figure D.14 shows the DC Sweep dialog box. Notice that the default

setting is Voltage Source for the Swept Var. Type, while it is Linear for

Sweep Type. If needed, other options can be selected by clicking the

appropriate buttons.

To run DC sweep analysis, click Analysis/Simulate. Schematics

will create a netlist and then run PSpice if no errors are found. If errors

are found in the schematic, check for them in the Error List and cor-

rect them as usual. If no errors are found, the data generated by PSpice

is passed to Orcad PSpice. The Orcad PSpice window will appear, dis-

playing a graph in which the X axis is by default set to the DC sweep

variable and range, and the Y axis is blank for now. To display some

specific plots, click Trace/Add in the Orcad PSpice menu to open the

Add Traces dialog box. The box contains traces, which are the output

variables (node voltages and branch currents) in the data file available

for display. Select the traces to be displayed by clicking or typing them,

and click OK. The selected traces will be plotted and displayed on the screen. As many traces as you want may be added to the same plot or

on different windows. Select a new window by clicking Window/New.

To delete a trace, click the trace name in the legend of the plot to high-

light it and click Edit/Delete (or press <Delete>).

It is important to understand how to interpret the traces. We must

interpret the voltage and current variables according to the passive sign

convention. As parts are initially placed horizontally in a schematic as

shown typically in Fig. D.3, the left-hand terminal is named pin 1 while

the right-hand terminal is pin 2. When a component (say R1) is rotated

counterclockwise once, pin 2 would be on the top, since rotation is about

pin 1. Therefore, if current enters through pin 2, the current I(R1)

through R1 would be negative. In other words, positive current implies

that the current enters through pin 1, and negative current means that the

current enters through pin 2. As for voltage variables, they are always

with respect to the ground. For example, V(R1:2) is the voltage (with

respect to the ground) at pin 2 of resistor R1; V(V1:) is the voltage

(with respect to the ground) at the positive terminal of voltage source

V1; and V(E 2:1) is the voltage at pin 1 of component E 2 with respect

to ground, regardless of the polarity.