After running a simulation with the Run command, we need to find the name assigned by LTspice to the points (nodes) where we are going to detect the amplifier’s output voltage and current. To do this, we must drag the amplifier slightly downwards; the connection wires to the loudspeaker are uncovered and automatically resized. We deactivate the handle by right-clicking and place the cursor (voltmeter probe) on the positive wire. We click and note down the name of the trace that appears, for example V(n001). We then move the cursor to the amplifier’s ‘+’ output: the probe becomes an ammeter and by clicking, another trace appears. We note down the name, for example Ix(x4:+). We can now delete the voltage and current traces we have just generated and type in the correct expression for the impedance display: we right-click on the box and select “Add Traces”. In the line “Expression(s) to add:” we type (in this case): V(n001)/-Ix(x4:+).
The impedance curve graph can now be obtained using the instrument built into the ‘amplifier’ block; to use the instrument, the block must be rotated 180° and connected to the loudspeaker via the ‘impedance TEST’ connection. After running the simulation, the probe placed on the positive wire (not the node) of the loudspeaker returns the impedance curve. Since this is a voltage measurement, the value is returned in volts rather than ohms.
The vertical axis display must be set to Bode and Linear.