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| university:courses:alm1k:alm-lab-4 [08 Jan 2018 15:18] – [Questions:] Doug Mercer | university:courses:alm1k:alm-lab-4 [18 Sep 2019 15:32] – [Activity 4: BJT Characteristic Curves] Doug Mercer |
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| ======Activity 4: BJT Characteristic Curves ====== | ======Activity: BJT Characteristic Curves ====== |
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| =====Objective:===== | =====Objective:===== |
| ====Hardware Setup:==== | ====Hardware Setup:==== |
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| Set the Channel B generator output 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<sub>BE</sub>. The waveform in the display should start at 0.6V and increase in 0.4 V increments to 4.6 V. Set the channel A generator to a triangle wave with a Max of 5.0 V and a Min of 0 V (wave should swing from 0 to 5V). Set the frequency to 200 Hz ( 10 times the 20 Hz of channel B). Comparing the waveforms in channel A and channel B, the triangle wave in channel A should go through one cycle from 0 to 5 V and back to zero during the time of one step in the waveform in channel B. Adjust the phases of channel A and/or channel B to make them line up in this way if they do not. | 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<sub>BE</sub>. The waveform in the display should start at 0.6V and increase in 0.4 V increments to 4.6 V. Set the channel A generator shape to a triangle wave with a Max of 5.0 V and a Min of 0 V (wave should swing from 0 to 5V). Set the frequency to 200 Hz ( 10 times the 20 Hz of channel B). Comparing the waveforms in channel A and channel B, the triangle wave in channel A should go through one cycle from 0 to 5 V and back to zero during the time of one step in the waveform in channel B. Adjust the phases of channel A and/or channel B to make them line up in this way if they do not. |
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| ====Procedure:==== | ====Procedure:==== |
| <WRAP centeralign>Figure 2 PNP I<sub>C</sub> vs. V<sub>CE</sub> characteristic curve measurments</WRAP> | <WRAP centeralign>Figure 2 PNP I<sub>C</sub> vs. V<sub>CE</sub> characteristic curve measurments</WRAP> |
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| ====NPN Beta vs V<sub>CE</sub> Directions:==== | ====NPN Beta vs Collector Voltage Directions:==== |
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| Beta (β) is defined as the ratio of the collector current, I<sub>C</sub>, to the base current, I<sub>B</sub>. Beta is not constant and varies as the operating conditions of the transistor changes. In this experiment we will be plotting β vs V<sub>CE</sub>. | Beta (β) is defined as the ratio of the collector current, I<sub>C</sub>, to the base current, I<sub>B</sub>. Beta is not constant and varies as the operating conditions of the transistor changes. In this experiment we will be plotting β vs V<sub>CE</sub>. |
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| Change the 10 KΩ resistor in figure 1 to 1 KΩ. | 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. |
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| | {{ :university:courses:alm1k:alm_lab4_f3.png?650 |}} |
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| | <WRAP centeralign>Figure 3 NPN V<sub>CE</sub>, I<sub>C</sub>, I<sub>B</sub> Time display</WRAP> |
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| | In the time display of the base current, yellow trace, we see that it increases slightly when the V<sub>CE</sub>, green trace, falls below about 0.5 volts. The collector current, light blue trace, also falls rapidly at this same time. |
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| | To calculate and plot β vs V<sub>CE</sub>, open the X-Y Pot window. Select Math for both the X and Y axis. Set the X axis math formula to display the V<sub>CE</sub> which is the channel A voltage: |
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| | VBuffA[t] |
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| | Set the Y axis math formula to calculate β which is the channel A current divided by the channel B current: |
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| | IBuffA[t]/IBuffB[t] |
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| | 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. |
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| | {{ :university:courses:alm1k:alm_lab4_f4.png?400 |}} |
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| | <WRAP centeralign>Figure 4 NPN Beta vs V<sub>CE</sub> X-Y plot</WRAP> |
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| | As we can see the β falls off rapidly as the V<sub>CE</sub> drops below 0.5 volts. At this point the collector base junction is starting to become forward biased (i.e. no longer reversed biased) and the transistor is entering into what is called the saturation region. In the saturation the β or current gain of the transistor is significantly smaller than in the so called linear region (i.e. collector base junction reversed biased). |
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| ====Questions:==== | ====Questions:==== |
| Calculate the Beta Early voltage product ( ß*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. | Compare your results with manufacturer specifications for each device measured. |
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| | **Resources:** |
| | * LTSpice files: [[ https://minhaskamal.github.io/DownGit/#/home?url=https://github.com/analogdevicesinc/education_tools/tree/master/m1k/ltspice/bjt_char_curves_ltspice | bjt_char_curves_ltspice]] |
| | * Fritzing files: [[ https://minhaskamal.github.io/DownGit/#/home?url=https://github.com/analogdevicesinc/education_tools/tree/master/m1k/fritzing/bjt_char_curves_bb | bjt_char_curves_bb]] |
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| **For Further Reading:** | **For Further Reading:** |
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| http://en.wikipedia.org/wiki/Bipolar_junction_transistor\\ | [[wp>Bipolar_junction_transistor|Bipolar junction transistor]]\\ |
| http://www.physics.csbsju.edu/trace/NPN.CC.html | http://www.physics.csbsju.edu/trace/NPN.CC.html |
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| **Return to ALM Lab Activity [[university:courses:alm1k:alm-labs-list|Table of Contents]]** | **Return to ALM Lab Activity [[university:courses:alm1k:alm-labs-list|Table of Contents]]** |
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