Measuring Input Impedance
To measure the input impedance of an unknown circuit first set the signal genaerator to a current source of magnitude 1 amp. You also need a shunt resistor of 100 Meg. This setup is useful for measuring amplifiers, filter and oscillator circuits. Use the signal generator as below:
Now perform an AC frequency analysis. Set the start limit to 1 Hz, set the end limit to your choice. Depending on your circuit,1MHz may be OK for audio work, though you can set any start and end limits for filter circuits. As the input is now 1 amp, the y-axis now directly reads the input impedance of your circuit. Cursor B can be scrolled across the spectrum, see example below:
The cursors allow easier reading intermediate points. The input impedance increases at low frequencies due to the 1u input capacitor. The impedance would be infinite at DC or 0Hz. The 100M shunt resistor across the generator is necessary as the output impedance of a current source in circuit maker is infinite, omitting this resistor will result in a simulation error.
Measuring Output Impedance
A similar method is used for measuring output impedance. The signal generator is again set to a current source of
magnitude 1 amp. The generator will be applied to the output circuits terminals, as shown below. In this circuit,
the output has a 10 k load resistor so a high value parallel shunt resistance is superfluous.
When an AC frequency analysis is performed, the graph reads the output voltage across the frequency range. From ohms law V = I x R. As I is set at 1 amp, then the output waveform is now a direct measurement in ohms of the circuits output impedance. Results are shown below:
This is a quick method and an alternative to measuring output voltage divided by circuit output current. Both input and output impedance measurements should be compatible with all versions of circuit maker.