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— | university:courses:alm1k:alm-lab-4 [18 Sep 2019 15:31] – [Questions:] Doug Mercer | ||
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+ | ======Activity 4: BJT Characteristic Curves ====== | ||
+ | |||
+ | =====Objective: | ||
+ | |||
+ | The purpose of this activity is to investigate the collector current, I< | ||
+ | |||
+ | =====Notes: | ||
+ | |||
+ | As in all the ALM labs we use the following terminology when referring to the connections to the M1000 connector and configuring the hardware. The green shaded rectangles indicate connections to the M1000 analog I/O connector. The analog I/O channel pins are referred to as CA and CB. When configured to force voltage / measure current -V is added as in CA-V or when configured to force current | ||
+ | |||
+ | Scope traces are similarly referred to by channel and voltage / current. Such as CA-V , CB-V for the voltage waveforms and CA-I , CB-I for the current waveforms. | ||
+ | |||
+ | =====Background: | ||
+ | |||
+ | The variable analog outputs supplied by the ALM1000 hardware are voltages but can also measure current at same time. The Bipolar Junction Transistor can be modeled as a current controlled current source. The BJT collector current, I< | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | |||
+ | If we sweep the voltage on the collector, V< | ||
+ | |||
+ | ====Materials: | ||
+ | |||
+ | ADALM1000 Hardware module\\ | ||
+ | Solder-less Breadboard\\ | ||
+ | 1 - 10KΩ Resistor\\ | ||
+ | 1 - small signal NPN transistor (2N3904, SSM2212) | ||
+ | |||
+ | ====NPN device Directions: | ||
+ | |||
+ | Build the simple characteristic curve measurement circuit shown in figure 1 on your solder-less breadboard. The green boxes indicate where to connect the ALM1000. | ||
+ | |||
+ | ====Hardware Setup:==== | ||
+ | |||
+ | Set the Channel B generator shape to the 10 level stair-step waveform. Set the frequency to 20Hz, the Max to 4.6 V and the Min to 0.6 V. The extra 0.6 volts is an initial estimate of V< | ||
+ | |||
+ | ====Procedure: | ||
+ | |||
+ | The 0.4 V steps in the voltage driving the 10 KΩ base resistor will produce approximately 0.4 V/10 KΩ or 40 uA steps in the base current. Using the scope in XY mode plot channel CA-V on the horizontal axis (V< | ||
+ | |||
+ | ====Questions: | ||
+ | |||
+ | From the measured data calculate the current gain Beta ( ß=I< | ||
+ | Using the curve for the highest base current step, calculate the early voltage (VA) for each device.\\ | ||
+ | Calculate the Beta Early voltage product ( ß*VA) for each device.\\ | ||
+ | Compare your results with manufacturer specifications for each device measured. | ||
+ | |||
+ | ====PNP device Directions: | ||
+ | |||
+ | To measure the PNP device, alter the characteristic curve measurement circuit as shown in figure 2. The connections are not very different but the polarity of the voltage waveforms generated by channel A and channel B must now be negative with respect to the PNP emitter. To accommodate this the emitter is now connected to +5 V. | ||
+ | |||
+ | You can use the same step staircase waveform you used for the NPN device to drive the base current for the PNP device. However, you will need to set the Max to +5 V - V< | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | |||
+ | ====NPN Beta vs Collector Voltage Directions: | ||
+ | |||
+ | Beta (β) is defined as the ratio of the collector current, I< | ||
+ | |||
+ | Change the 10 KΩ resistor in figure 1 to 1 KΩ. Change the shape of Channel B to DC. Change the Channel B Max to 1.5 V, the Min and Freq setting are ignored for the DC shape. Change the Channel A Max to 2 volts. Under the curves drop down select all four traces to be displayed. The current in channel B, the base current, will be small and will be rather noisy so turning on trace averaging is a good idea. You should see something like the time display in figure 3. | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | |||
+ | In the time display of the base current, yellow trace, we see that it increases slightly when the V< | ||
+ | |||
+ | To calculate and plot β vs V< | ||
+ | |||
+ | VBuffA[t] | ||
+ | |||
+ | Set the Y axis math formula to calculate β which is the channel A current divided by the channel B current: | ||
+ | |||
+ | IBuffA[t]/ | ||
+ | |||
+ | Set the Math X axis to V-A. Set the Math Y axis to I-B. Adjust the range and position controls for CA-V to 0.2 V/Div and 1.0 V. Adjust the range and position controls for CB-I to 10.0 mA/Div and 50.0 mA. You should now see something like the X-Y plot shown in figure 4. | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | |||
+ | As we can see the β falls off rapidly as the V< | ||
+ | |||
+ | ====Questions: | ||
+ | |||
+ | Again, from the measured data calculate the current gain Beta ( ß=I< | ||
+ | Using the curve for the highest base current step, calculate the early voltage (VA) for each device.\\ | ||
+ | Calculate the Beta Early voltage product ( ß*VA) for each device.\\ | ||
+ | Compare your results with manufacturer specifications for each device measured. | ||
+ | |||
+ | **Resources: | ||
+ | * LTSpice files: [[ https:// | ||
+ | * Fritzing files: [[ https:// | ||
+ | |||
+ | **For Further Reading:** | ||
+ | |||
+ | [[wp> | ||
+ | http:// | ||
+ | |||
+ | **Return to ALM Lab Activity [[university: | ||