Transistor I-V Curve Tracing
This is a workspace set up to perform current-voltage (I-V) curve traces for NPN BJTs. These traces are useful for characterizing these commonly used devices, and for demonstrating that the mathematical models describing them are correct. Other transistors can be used, but may require additional configuration.
The curve tracing makes 15 curves, corresponding to a 16-385 µA base current range. The traces are generated by supplying a 15-step ramping function to the base resistor while supplying the collector with a smooth ramping function running at a frequency 15 times that of the step function's. Thus, for each step of the base voltage the collector resistor voltage is swept from 0 to 5V.
- A Digilent Test and Measurement device that supports WaveForms' Scope and Wavegen instruments.
- An Analog Discovery (Legacy) can also be used.
- WaveForms Installation
- Transistor test circuit: A breadboard, transistor, two resistors, and some jumper wires.
1. Hardware Setup
The workspace included in the ZIP Archive is ready to use, but requires a breadboard, a transistor, two resistors, and some wire leads to perform current-voltage curve tracing. The workspace was configured with the emitter grounded, a 100Ω resistor on both the collector and the base of a 2N3904 BJT. See Figure 1, below, for a schematic of the circuit.
Finish the hardware setup by plugging the Analog Discovery into the computer.
2. Software Setup
Open WaveForms. Upon launch, WaveForms will generate a prompt asking to switch to the device used with the workspace. As this is the device originally used when the workspace was created and archived, select No. Navigate to Settings → Device Manager in the menu bar at the top of the window, to check that the active device is your Analog Discovery.
Open the transistor-i-v-curve-tracer workspace in WaveForms.
Note: Configuration for Different Resistors
If non-100Ω resistors were used in setting up the hardware, the values in the “Collector Current” channel will need to be updated by clicking the equation button at the bottom of each channel menu (see Figure 2, to the right). In the dialog that opens, change the “100” part of the equation to the value in ohms of the collector resistor used.
3. Running the Curve Tracer
After setting up hardware and making any necessary adjustments in the calculations, press Run All at the bottom of the Wavegen 1 window, then Scan at the top of the Scope 1 window. The popped-out window, labeled “XY 1”, is the I-V trace for the transistor.
Note that this workspace was configured using a 2N3904 BJT. The base resistor voltage range (which determines the base current range) was selected from testing various values and comparing the resulting curves. It may be necessary to change the Function Generator Channel 1 offset and amplitude to find a range that generates proper curves. The offset value determines the lowest voltage that will be applied to the base resistor, not the middle of the waveform.
This workspace can be used to perform curve traces on PNP BJTs and n-channel MOSFETs. Regardless of transistor type, be sure to read the data sheet for important characteristics and electrical limitations.
The images below show the results that can be viewed in the Scope instrument.