“The current and voltage characteristics of the emitter-base junction of the NPN transistor can be measured using the ADALM2000 laboratory hardware and the following connections. Using a breadboard, connect the waveform generator W1 to one end of the resistor R1. Also connect the oscilloscope input 2+ here. Connect the base and collector of Q1 to the other end of R1 as shown. The emitter of Q1 is grounded. Connect oscilloscope input 2 and oscilloscope input 1+ to the base-collector node of Q1. Oscilloscope input 1- can also choose to ground.
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Authors: Analog Devices｜Doug Mercer, Consulting Researcher; Antoniu Miclaus, System Application Engineer
Simple NPN diode connection
Target:
The purpose of this experiment is to study the forward/reverse current and voltage characteristics when a bipolar junction transistor (BJT) is connected as a diode.
Material:
ADALM2000 Active Learning Module
Solderless Breadboard
A 1 kΩ resistor (or other similar value)
A small signal NPN transistor (2N3904)
instruction:
The current and voltage characteristics of the emitter-base junction of the NPN transistor can be measured using the ADALM2000 laboratory hardware and the following connections. Using a breadboard, connect the waveform generator W1 to one end of the resistor R1. Also connect the oscilloscope input 2+ here. Connect the base and collector of Q1 to the other end of R1 as shown. The emitter of Q1 is grounded. Connect oscilloscope input 2 and oscilloscope input 1+ to the base-collector node of Q1. Oscilloscope input 1- can also choose to ground.
Figure 1. NPN diode connection diagram.
Hardware settings:
The waveform generator is configured as a 100 Hz triangle wave with a peak-to-peak amplitude of 6 V and an offset of 0 V. The differential channel 2 (2+, 2-) of the oscilloscope is used to measure the current in the resistance (and transistor). Connect oscilloscope channel 1 (1+) to measure the voltage across the transistor. The current flowing through the transistor is the voltage difference between 1+ and 1- divided by the resistance value (1 kΩ).
Figure 2. NPN diode breadboard circuit.
step:
Load the captured data into a spreadsheet and calculate the current. Plot the current and the voltage across the transistor (VBE) Curve. There is no reverse current flow. In the forward conduction region, the voltage-current relationship is logarithmic. If the current curve is drawn in a logarithmic coordinate system, the result should be a straight line.
Figure 3. XY curve of NPN diode.
Figure 4. NPN diode waveform.
Reverse breakdown characteristics
Target:
The goal of this experiment is to study the reverse breakdown voltage characteristics of the emitter-base junction when the BJT is connected as a diode.
Material:
A 100 Ω resistor
A small signal PNP transistor (2N3906)
instruction:
Using a breadboard, connect the waveform generator output to one end of a 100 Ω series resistor R1 and the base and collector of Q1, as shown in Figure 2. The emitter is connected to a -5 V fixed power supply. Connect oscilloscope channel 1 (1+) to the base-collector node and 1- to the emitter node. Oscilloscope channel 2 is used to measure the voltage across R1, thereby measuring the current through Q1.
The reason why PNP 2N3906 is chosen instead of NPN 2N3904 is that the PNP emitter-base breakdown voltage is less than the maximum +10 V that ADALM2000 can generate, while the breakdown voltage of NPN may be higher than 10V.
Figure 5. PNP emitter-base reverse breakdown configuration.
Hardware settings:
The waveform generator is configured as a 100 Hz triangle wave with a peak-to-peak amplitude of 10 V and an offset of 0 V. Oscilloscope channel 1 (1+) is used to measure the voltage across the resistor. The setting should be configured to connect channel 2 to both ends of resistor R1 (2+, 2-). Both channels should be set to 1 V per division. The current flowing through the transistor is the voltage difference between 2+ and 2- divided by the resistance value (100 Ω).
Figure 6. PNP emitter breadboard circuit.
step:
The laboratory hardware power supply limits the maximum voltage available to less than 10V. The emitter-base reverse breakdown voltage of many transistors is greater than this voltage. In the configuration shown in Figure 6, voltages between 0 V and 10 V (W1 peak-to-peak swing) can be measured.
Figure 7. PNP emitter waveform.
Capture the oscilloscope waveform and export it to a spreadsheet. For the PNP transistor 2N3906 used in this example, the emitter-base junction breakdown voltage is approximately 8.5V.
Reduce the effective forward voltage of the diode
Target:
The goal of this experiment is to study a circuit configuration with a forward voltage characteristic smaller than that when the BJT is connected as a diode.
Material:
A 1 kΩ resistor
A 150 kΩ resistor (or 100 kΩ and 47 kΩ in series)
A small signal NPN transistor (2N3904)
A small signal PNP transistor (2N3906)
instruction:
Connect the breadboard, connect the waveform generator W1 to one end of the series resistor R1 and the collector of NPN Q1 and the base of PNP Q2, as shown in Figure 8. The emitter of Q1 is grounded. The collector of Q2 is connected to Vn (5 V). One end of the resistor R2 is connected to Vp (5 V). The other end of R2 is connected to the base of Q1 and the emitter of Q2. The single-ended input of oscilloscope channel 2 (2+) is connected to the collector of Q1.
Figure 8. The configuration diagram required to reduce the effective forward voltage drop of the diode.
Hardware settings:
The waveform generator is configured as a 100 Hz triangle wave with a peak-to-peak amplitude of 8 V and an offset of 2 V. Oscilloscope channel 2 (2+) is used to measure the voltage across the resistor. The current flowing through the transistor is the voltage difference between the oscilloscope input 1+ and 1- divided by the resistance value (1kΩ).
step:
Now, the turn-on voltage of the diode is about 100 mV, while the simple diode connection scheme in the first example is 650 mV. Plot the V of Q1 when W1 is sweptBEcurve.
Figure 9. A breadboard circuit that reduces the effective forward voltage drop of the diode.
Figure 10. Waveform to reduce the effective forward voltage drop of the diode.
VBEMultiplier circuit
Target:
We have discussed a method that can effectively reduce VBEMethod, the purpose of this experiment is to increase VBE, And show greater forward voltage characteristics compared with a single BJT connected as a diode solution.
Material:
Two 2.2 kΩ resistors
A 1 kΩ resistor
A 5 kΩ variable resistor, potentiometer
A small signal NPN transistor (2N3904)
instruction:
Connect the breadboard, connect the waveform generator W1 to one end of the resistor R1, as shown in Figure 11. The emitter of Q1 is grounded. Resistors R2, R3, and R4 form a voltage divider, and the wiper of potentiometer R3 is connected to the base of Q1. The collector of Q1 is connected to the other end of R1 and the top of the voltage divider at R2. Oscilloscope channel 2 (2+) is connected to the collector of Q1.
Figure 11.VBEMultiplier configuration.
Hardware settings:
The waveform generator is configured as a 100 Hz triangle wave with a peak-to-peak amplitude of 4 V and an offset of 2 V. The oscilloscope channel single-ended input 2+ is used to measure the voltage across the transistor. The setting should be configured as channel 1+connect generator W1 to Display output, and channel 2+connect Q1 collector. The current flowing through the transistor is the result of dividing the voltage difference across W1 measured by oscilloscope input 1+ and oscilloscope input 2+ by the resistance value (1 kΩ).
step:
At the beginning, set potentiometer R3 to the midpoint of its range, and the voltage at the collector of Q2 should be approximately VBE2 times. When R3 is set to the minimum value, the voltage at the collector should be VBE9/2 (or 4.5) times. When R3 is set to the maximum value, the voltage at the collector should be VBE9/7 times of that.
Figure 12.VBEMultiplier breadboard circuit.
Figure 13.VBEMultiplier breadboard waveform.
problem:
This VBEHow does the voltage and current characteristics of the multiplier compare with a simple diode-connected transistor?
You can find the answers to the questions on the student zone blog.
About the Author
Doug Mercer graduated from Rensselaer Polytechnic Institute (RPI) in 1977 with a bachelor’s degree in electrical engineering. Since joining ADI in 1977, he has directly or indirectly contributed more than 30 data converter products and holds 13 patents. He was appointed as an ADI researcher in 1995. In 2009, he transitioned from a full-time job and continued to serve as an ADI consultant as an honorary researcher, writing articles for the “Active Learning Program”. In 2016, he was appointed as the Resident Engineer of the RPI ECSE Department. Contact information:[email protected]
