In electronics circuit design it is often necessary to calculate the input and output impedance. For example for designing RF circuits and other circuits where amplifiers are involved it is often necessary to calculate the coupling capacitor values. For this we need to calculate the input and output impedances. Also when coupling transfomers has to be designed then we need to the input and output impedances to calculate the turn ratio. Calculating these impedance is tedious and it becomes harder if the circuit is complex and the value of the component parameters such transistor parameters or operational amplifier are unknown. If you have access to electronics design software you can calculate these input and output impedances easily. Here it is shown how to use Proteus professional electronics design software to calculate the input and output impedance.

For the purpose of illustration we will use an am modulator circuit which was used in the previous tutorial AM Transmitter with Crystal Oscillator. The am modulator circuit diagram is shown below.

To measure the input impedance of the modulator we place a sine voltage signal source and current probe at the base input and place a voltage probe at the collector output. In this case we want input signal frequency range upto 100MHz. So in the voltage sine signal source, we set the input voltage as 1V and the frequency of 100MHz. Note that setting the voltage of the source should be 1V so that later we can use the expression of ohm law.

Then a Frequency response graph is used to plot the input impedance. In the frequency response graph properties, we set the reference as the input voltage which is labelled as ROOT_Vin in this case. We set the appropriate frequency range for the graph which is set to 0 10 to 150MHz.

Then we add the probe signal for the graph. To do this we can right click on the graph and select Add Trace. In the add trace window, we give a trace name called Zin which is the input impedance. In the probe P1 field we select the input current probe ROOT_Iin and then in the expression we write 1/P1. This is because we are using the fact that Zin = Vin/Iin but Vin=1V so Zin = 1/Iin = 1/P1 since Iin=P1.

Next we can simulate the graph and obtain the input impedance Zin frequency response as shown below.

This graph shows how the input impedance of the circuit varies with frequency. At 100MHz we can see that the input impedance Zin is 630Ohm. So in this way we can easily calculate the input impedance of a circuit using proteus. Note that we can also get the phase response with the same graph if we put the probe Zin on the right side of the graph.

We can verify this calculation using BJT Amplifier Design Online Calculator. The input impedance from this calculator is 660â„¦.

So the input impedance values from the graph analysis in proteus and the online calculator are approximately equal. With this value for input coupling capacitor can be calculated which is around 24pF at 100MHz input frequency.

Similarly to measure the output impedance of a circuit we place a source at the output directing into the circuit and place a current probe also directing input the circuit as shown below. The voltage source is named Vout and the current probe is named Iout. Notice that a small resistor 0f 0.5Ohm is added in series with inductor this time because the proteus prospice needs a path to calculate at finite value while performing the circuit analysis(real inductors do have small resistance).

Unlike in the input impedance measurement, the sine voltage source frequency is set to 1Hz but the voltage is still 1V.

The setting for the frequency range for the graph is same as for the input impedance. For the trace setting, we add a new trace and name the trace Zout, select ROOT_Iout as probe P1 and in the expression field write 1/P1. This is Zout=1/P1=1/Iout since Vout is 1V.

The measured output impedance frequency response graph is shown below.

From the graph the output impedance is 158Ohm at 100MHz. To know how to calculate the coupling capacitor values see the tutorial How to design BJT amplifier.

So in this way we can determine the input and output impedances using Proteus electronics design software.