UNIVERSITY of PENNSYLVANIA

DEPARTMENT OF ELECTRICAL ENGINEERING

SPICE Example: DC Sweep, and Thevenin Equivalent Circuit

This is an example of a SPICE input file that will calculate the DC Node voltages, the Thevenin Equivalent Circuit and also sweep the input voltage source.

Input file: (the lines starting with a "*" are comment lines and are inserted for clarity only).

Spice Example: DC Sweep, .TF, .OT analysis *

* We are interested in finding the following characteristics:
* 1. Node voltages v12, v2 and current i4 when vin=10V
* 2. Thevenin equivalent voltage and resistance, seen
* at the output terminals v(3,0)
* 3. Sweep the input from 0 to 20V in steps of 2V and plot
* the voltages v12, v2, vo and the current i4.
* ------------------------------------------------------
VIN 1 0 DC 10
F1 0 3 VMEAS 0.5
VMEAS 4 0 DC 0
*VMEAS is a 0V source to measure i4
R1 1 2 1K
R2 2 3 10K
R3 1 3 15K
R4 2 4 40K
R5 3 0 50K
.OP
* --------------------------------------------------
* The operating point .OP command will tell Spice
* print all DC node voltages, currents through voltage
* sources and voltages over current sources.
* --------------------------------------------------
.TF V(3,0) VIN
* --------------------------------------------------
* The .TF statement tells Spice to calcuate the ratio of
* Vout/Vin and the resistance seen at the terminals of
* Vout=V(0,3) and Vin (corresponding to the output and
* input resistances). This statement is used to calculate
* the Thevenin equivalent circuit.
* --------------------------------------------------
.DC VIN 0 20 2
* --------------------------------------------------
* The .DC statement instructs Spice to sweep the voltage source
* Vin from 0 to 20 V in steps of 2V.
* --------------------------------------------------
.PLOT DC V(1,2) V(2,4)
.PLOT DC I(VMEAS)
* --------------------------------------------------
* The above Plot statement instruct Spice to plot the
* node voltages and current in the Vmeas voltage source,
* obtained as a results of the DC sweep.
* --------------------------------------------------
.END

The result of the SPICE simulation is given below.

DC Sweep:

a: v(1,2)
b: v(2,4)

```
volt       v(1,2)
(ab      )       0.            5.0000       10.0000       15.0000      20.0000
+             +             +             +             +
0.         0.    -2------+------+------+------+------+------+------+------+-
2.0000    54.507m a    b +      +      +      +      +      +      +      +
4.0000   109.015m a      +   b  +      +      +      +      +      +      +
6.0000   163.522m a      +      + b    +      +      +      +      +      +
8.0000   218.029m +a     +      +      +b     +      +      +      +      +
10.0000   272.537m +a     +      +      +     b+      +      +      +      +
12.0000   327.044m +a     +      +      +      +    b +      +      +      +
14.0000   381.551m +a     +      +      +      +      +  b   +      +      +
16.0000   436.059m +a     +      +      +      +      +      + b    +      +
18.0000   490.566m +a     +      +      +      +      +      +      b      +
20.0000   545.073m-+-a----+------+------+------+------+------+------+----b-+-
+             +             +             +             +

volt       i(vmeas)
(a       )       0.          200.0000u     400.0000u     600.0000u    800.0000u
+             +             +             +             +
0.         0.    -a------+------+------+------+------+------+------+------+-
2.0000    48.637u +  a   +      +      +      +      +      +      +      +
4.0000    97.275u +      a      +      +      +      +      +      +      +
6.0000   145.912u +      +  a   +      +      +      +      +      +      +
8.0000   194.549u +      +      a      +      +      +      +      +      +
10.0000   243.187u +      +      +  a   +      +      +      +      +      +
12.0000   291.824u +      +      +     a+      +      +      +      +      +
14.0000   340.461u +      +      +      +  a   +      +      +      +      +
16.0000   389.099u +      +      +      +     a+      +      +      +      +
18.0000   437.736u +      +      +      +      +  a   +      +      +      +
20.0000   486.373u-+------+------+------+------+-----a+------+------+------+-
+             +             +             +             +

```
***** operating point status is all simulation time is 0.

node =voltage node =voltage node =voltage
+0:1 = 10.0000 0:2 = 9.7275 0:3 = 9.4340 +0:4 = 0.

**** voltage sources

Currents in Voltage sources:

element 0:vin 0:vmeas
volts 10.0000 0.
current -310.2725u 243.1866u
power 3.1027m 0.

total voltage source power dissipation= 3.1027m watts

NOTICE: The power dissipation givin in the output is the one of the independent voltage sources only, and does not include the power generated in the dependent sources or the independent current sources.

**** current-controlled current sources

element 0:f1
current 121.5933u

**** small-signal transfer characteristics

v(3)/vin = 943.3962m
input resistance at vin = 32.2297k
output resistance at v(3) = 5.6604k

The THEVENIN Equivalent resistance is given by the above output resistance RT=5.66 KOhm. The DC Thevenin voltage is given by the node voltage at the open circuit terminal: VT=9.434V (see operating point solution, above).

Jan Van der Spiegel;
jan@ee.upenn.edu
Created Oct. 29, 1995; Updated Oct. 29, 1995