Finding output impedance for a circuit
Assume with the circuit below that the output impedance is wanted from the marked voltage
node.
R1
V4
V3
DC = 10V
C1
DC = 10V
1
R2
1n
C1
0
V6
X2
BS170
1n
1Vac
0Vdc
R6
Rload
2
R3
R4
V5
DC = 10V
Step 1: Remove the input V6 and set it to ground or the input values to zero.
Step 2: Replace the load resistor, R6 with a test source (Current or Voltage for out
example) and place V marker and I marker at the node you want to find the resistance.
R31
V11
v9
DC = 10V
DC = 10V
1
I
R22
C4
0
V
x3
BS170
1n
V8
2
R32
AC = 1
R41
V10
DC = 10V
Step 3: Simulate using an AC sweep and plot the current coming out of the source V8.
Step 4. Determine the output impedance vs frequency value =>
a. The following plot comes up of I and V on it.
1.0
0.8
0.6
0.4
0.2
1.0Hz
I(V8)
10Hz
V(x3:d)
100Hz
1.0KHz
10KHz
Frequency
100KHz
1.0MHz
10MHz
Note that the current is I(V8) and the voltage is V(x3:d) =>
Get to the Modify Trace window (either by double clicking on the variable I(V8) or a
different variable or through the menus. Replace the Trace Expression with V/I to give
you the graph of resistance vs frequency. Trace Expression: V(x3:d)/I(V8) {your node
voltages will be different than mine}. The resultant plot should look something like the
form below:
2.62
2.61
2.60
2.59
2.58
1.0Hz
10Hz
V(x3:d)/I(V8)
100Hz
1.0KHz
10KHz
100KHz
1.0MHz
10MHz
Frequency
where this shows Rout = 2.62 when it is under 200 Hz.
Plotting to create a VGS x-axis:
Step 1. Create graph of Vout vs VG. Double click on the x-axis to get the following:
Under the x-axis tab => click on axis variable.
Change the Trace Expression to be the VG node – VS node (depends on your circuit).
Click OK and this changes the axis to be the above expression instead of just VG.