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.