Using the Probe Tab
This tutorial features a more complex circuit in order to illustrate several features regarding probing voltages and currents.
If your circuit is too large for the screen, you can use the “+” and “-” magnifier buttons to zoom in and out. Alternately, you can use the sliders along the bottom and side to reposition your view.
Initially, this circuit has no probes. Running a Transient Analysis will display all of the nodes in the Waveform Viewer.
Displaying all of the waveforms may be quite busy and contain too much information. In that case, go to the use the Probes tab to enable this mode.
In Probes Mode, moving the cursor over any wire makes the Voltage Probe icon appear. Right Click to select that net for the Waveform Viewer. If you probe nets Vinp, Vinn, and Vout, you’ll now have three nets in your probe list.
Running the simulator will display the three nets you previously selected.
Now, suppose you’d like to see the difference the Vinp and Vinn voltages. You can remove the probes for these nets and drag a Voltmeter into the schematic. Wire it between the two nets and rerun the simulation.
Now, you should see the difference between the inputs centered around 0V and the output driving up to about 1V. This amplifier is used to convert a differential input signal to a 3V output. So we need to increase the output.
The current bias of the bipolar differential amplifier is set to 1 mA. Double click on the current source and boost its level to 3 mA, or so. Now Vout swings much higher, closer to the desired level of 3V. You can continue to boost the bias current and see its effect on Vout.
You can probe the current through any two-terminal device. As you hover over the device, the Current Probe icon appears next to the “hand” cursor. Click to add this current to the Waveform Viewer.
You can also probe the current into a specific pin. As you hover over the pin, the Current Probe icon appears and the “hand” cursor changes to a “cross hairs” cursor. Click to add to the Waveform Viewer.
If you add the current in R1 to the probe list and rerun simulation, you will see the currents added to the Waveform Viewer. Because the current is mimicking the output voltage, the plots overlap due to scaling on both plots has been matched.
On the current output, you can see 2.9 milliamps. If you hide the current output, then you’ll see 2.93V because it’s driving into a 1 kohm resistor.
Probing Three-Terminal Devices for Current
Probing a three-terminal device for current is similar to the two-terminal device.
Choose the Probes tab. You won’t be able to hover over the three-terminal device directly, but you can hover over its individual pins as in the two-terminal case. Similarly, the Current Probe icon appears and the “hand” cursor changes to a “cross hairs” cursor
For instance, you can measure the current into the collectors of Q1 and Q4.
Removing some of the other probes, you can see clearly view the collector waveforms.
Another way to add probes is via the drop-down list of names on the Probe tab. For example, select the emitter voltage “Vem” and click the “Add Probe” button. You can always remove probes from this list by clicking the “x” button.