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| university:courses:electronics:electronics-lab-4 [23 Mar 2017 15:50] – [Materials:] Doug Mercer | university:courses:electronics:electronics-lab-4 [24 Apr 2017 08:14] – rename Antoniu Miclaus |
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| ===== Background: ===== | ===== Background: ===== |
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| The variable analog outputs supplied by the Discovery hardware are voltages. The BJT collector current is controlled by the base current. The AWG output voltage must be converted into a suitable current to drive the base terminal of the device under investigation. A simple resistor can be used to convert a voltage into a current, as shown in figure 1. However, only if the voltage across the resistor is known or controlled in some way. In this simple circuit, the base current I<sub>B</sub> = (V<sub>AWG1</sub> - V<sub>BE</sub>)/100KΩ. We can set V<sub>AWG1</sub> to known values but we don’t know the exact value of V<sub>BE</sub>. We can of course remove an estimate of the V<sub>BE</sub> mathematically. This is still only an estimate. | The variable analog outputs supplied by the ADALM2000 hardware are voltages. The BJT collector current is controlled by the base current. The AWG output voltage must be converted into a suitable current to drive the base terminal of the device under investigation. A simple resistor can be used to convert a voltage into a current, as shown in figure 1. However, only if the voltage across the resistor is known or controlled in some way. In this simple circuit, the base current I<sub>B</sub> = (V<sub>AWG1</sub> - V<sub>BE</sub>)/100KΩ. We can set V<sub>AWG1</sub> to known values but we don’t know the exact value of V<sub>BE</sub>. We can of course remove an estimate of the V<sub>BE</sub> mathematically. This is still only an estimate. |
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| {{ :university:courses:electronics:a4_f1.png?500 |}} | {{ :university:courses:electronics:a4_f1.png?500 |}} |
| ===== Directions and Setup: ===== | ===== Directions and Setup: ===== |
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| Build the simple curve tracer circuit shown in figure 1. The green boxes indicate where to connect the Discovery Board. Using the custom waveform editor in the Waveforms AWG tool, construct a stair-step waveform with 5 levels. Be sure to reset so that you are starting with flat line at 0%. First set the type to constant. Then with start set to 0%, length set to 20% and offset set to -100% click on generate function. There should now be a line at -100% from 0 to 20%. Next change the start to 20% and the offset to -50% and then click on generate function again. There should now be a line at -50% from 20% to 40%. Next set the offset to +50% and the start to 60% and then click on generate function again. There should now be a line at 0% from 40% to 60% and a line at 50% from 60% to 80%. Finally set the offset to 100% and start to 80% and click on generate function one last time. There should now be a final line at 100% from 80% to 100%. Click on save and your new waveform should be in channel 2. Now at this point set the frequency to 40Hz, the amplitude to 2 V and the offset to 2.6 V. The waveform in the display should start at 0.6V and increase in 1 V increments to 4.6 V (0.6, 1.6, 2.6, 3.6, 4.6) Each step should be 5 mSec long for a total of 25 mSec. In AWG channel 1 configure a triangle wave with an amplitude of 2.5 V and an offset of 2.5V (wave should swing from 0 to 5V). Set the frequency to 200 Hz ( 5 times the 40 Hz of channel 2). Comparing the waveforms in channel 1 and channel 2, the triangle wave in channel 1 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 2. It will probably be necessary to set the phase of channel 1 to 270 degrees to make them line up in this way. | Build the simple curve tracer circuit shown in figure 1. The green boxes indicate where to connect the ADALM2000. Using the custom waveform editor in the Scopy AWG tool, construct a stair-step waveform with 5 levels. Be sure to reset so that you are starting with flat line at 0%. First set the type to constant. Then with start set to 0%, length set to 20% and offset set to -100% click on generate function. There should now be a line at -100% from 0 to 20%. Next change the start to 20% and the offset to -50% and then click on generate function again. There should now be a line at -50% from 20% to 40%. Next set the offset to +50% and the start to 60% and then click on generate function again. There should now be a line at 0% from 40% to 60% and a line at 50% from 60% to 80%. Finally set the offset to 100% and start to 80% and click on generate function one last time. There should now be a final line at 100% from 80% to 100%. Click on save and your new waveform should be in channel 2. Now at this point set the frequency to 40Hz, the amplitude to 2 V and the offset to 2.6 V. The waveform in the display should start at 0.6V and increase in 1 V increments to 4.6 V (0.6, 1.6, 2.6, 3.6, 4.6) Each step should be 5 mSec long for a total of 25 mSec. In AWG channel 1 configure a triangle wave with an amplitude of 2.5 V and an offset of 2.5V (wave should swing from 0 to 5V). Set the frequency to 200 Hz ( 5 times the 40 Hz of channel 2). Comparing the waveforms in channel 1 and channel 2, the triangle wave in channel 1 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 2. It will probably be necessary to set the phase of channel 1 to 270 degrees to make them line up in this way. |
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| You should export your newly created stair-step waveform to a .csv file for future use. | You should export your newly created stair-step waveform to a .csv file for future use. |
| ===== Materials: ===== | ===== Materials: ===== |
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| Analog Discovery Lab Instrument Hardware\\ | ADALM2000 Active Learning Module\\ |
| Solder-less Breadboard\\ | Solder-less Breadboard\\ |
| 1 - Dual Op AMP (such as ADTL082)\\ | 1 - Dual Op AMP (such as ADTL082)\\ |
| ===== Directions and Setup: ===== | ===== Directions and Setup: ===== |
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| Below are schematics for both a common emitter and a common base BJT curve tracer test circuit for use with the Discovery Lab hardware. It uses one dual opamp (ADTL082) powered from the +/- 5 Volt board supplies. In the common emitter configuration, one opamp serves as a virtual ground at the base terminal to convert the voltage steps from waveform generator W2 into base current steps through a 10KΩ resistor. The collector voltage is swept using a ramp from generator W1. V<sub>CE</sub> is measured differentially by scope inputs 1+, 1-. The collector current, I<sub>C</sub> is measured by scope inputs 2+, 2- differentially across a 100Ω resistor. A ratio of 100 for the base and collector resistors is used because beta, the collector current to base current gain, is often approximately 100. The voltage on the base terminal can be offset to either +2.5V or -2.5V (or 0V) to increase the possible V<sub>CE</sub> swing (by -2.5 for NPN or +2.5 for PNP). The +2.5V is generated by a voltage divider from the +5V supply and the -2.5V is generated by inverting the +2.5V with the second opamp in the dual op-amp (ADTL082). The base and emitter connection can be interchanged to configure the device under test (DUT) in the common base mode. The resistor values are changed to 1KΩ for both in this configuration. This ratio of one is appropriate given that alpha, the ratio of emitter current to collector current, is very close to one. The voltage from W2 now sets the emitter current and the ramp on W1 sweeps the V<sub>CB</sub> and is measured differentially with 1+, 1-. The collector current I<sub>C</sub> is measured differentially across the 1KΩ resistor with 2+, 2-. | Below are schematics for both a common emitter and a common base BJT curve tracer test circuit for use with the ADALM2000. It uses one dual opamp (ADTL082) powered from the +/- 5 Volt board supplies. In the common emitter configuration, one opamp serves as a virtual ground at the base terminal to convert the voltage steps from waveform generator W2 into base current steps through a 10KΩ resistor. The collector voltage is swept using a ramp from generator W1. V<sub>CE</sub> is measured differentially by scope inputs 1+, 1-. The collector current, I<sub>C</sub> is measured by scope inputs 2+, 2- differentially across a 100Ω resistor. A ratio of 100 for the base and collector resistors is used because beta, the collector current to base current gain, is often approximately 100. The voltage on the base terminal can be offset to either +2.5V or -2.5V (or 0V) to increase the possible V<sub>CE</sub> swing (by -2.5 for NPN or +2.5 for PNP). The +2.5V is generated by a voltage divider from the +5V supply and the -2.5V is generated by inverting the +2.5V with the second opamp in the dual op-amp (ADTL082). The base and emitter connection can be interchanged to configure the device under test (DUT) in the common base mode. The resistor values are changed to 1KΩ for both in this configuration. This ratio of one is appropriate given that alpha, the ratio of emitter current to collector current, is very close to one. The voltage from W2 now sets the emitter current and the ramp on W1 sweeps the V<sub>CB</sub> and is measured differentially with 1+, 1-. The collector current I<sub>C</sub> is measured differentially across the 1KΩ resistor with 2+, 2-. |
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| {{ :university:courses:electronics:a4_f4.png?500 |}} | {{ :university:courses:electronics:a4_f4.png?500 |}} |
| ===== Directions: ===== | ===== Directions: ===== |
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| The circuit below, figure 8, can be used in conjunction with the Analog Discovery Lab hardware board to accurately measure the V<sub>BE</sub> vs. emitter current of an NPN transistor. Emitter current is used in this measurement rather than collector current but I<sub>E</sub> and I<sub>C</sub> are essentially the same given a high beta transistor. The op-amp supplies the base current and any bias current that might flow in the 2+ scope input while forcing the emitter of the transistor to the (virtual) ground potential. Negative voltages applied by waveform generator W1 set the emitter current through the 1KΩ resistor. The same circuit can be used to measure PNP transistors by connecting the collector to Vn rather than Vp. Positive voltages applied by generator W1 set the emitter current through the 1KΩresistor. | The circuit below, figure 8, can be used in conjunction with the ADALM2000 to accurately measure the V<sub>BE</sub> vs. emitter current of an NPN transistor. Emitter current is used in this measurement rather than collector current but I<sub>E</sub> and I<sub>C</sub> are essentially the same given a high beta transistor. The op-amp supplies the base current and any bias current that might flow in the 2+ scope input while forcing the emitter of the transistor to the (virtual) ground potential. Negative voltages applied by waveform generator W1 set the emitter current through the 1KΩ resistor. The same circuit can be used to measure PNP transistors by connecting the collector to Vn rather than Vp. Positive voltages applied by generator W1 set the emitter current through the 1KΩresistor. |
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| {{ :university:courses:electronics:a4_f10.png?500 |}} | {{ :university:courses:electronics:a4_f10.png?500 |}} |
| ====== Alternative Method ====== | ====== Alternative Method ====== |
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| The circuits used earlier to make these measurements use the differential nature of the scope input channels of the Analog Discovery hardware. This was done, in part, to facilitate the conversion of the AWG’s voltage output into a current suitable to drive the transistor’s base. You may not always have access to instruments with differential input capability, such as standard bench top Oscilloscopes or the Analog Explorer student lab system. The following configuration, shown in figure 10, allows both the collector emitter voltage V<sub>CE</sub> and the collector current I<sub>C</sub> to be measured with ground referenced, singled ended, scope inputs while still converting the AWG voltage output into a suitable base current. | The circuits used earlier to make these measurements use the differential nature of the scope input channels of the ADALM2000 hardware. This was done, in part, to facilitate the conversion of the AWG’s voltage output into a current suitable to drive the transistor’s base. You may not always have access to instruments with differential input capability, such as standard bench top Oscilloscopes or the Analog Explorer student lab system. The following configuration, shown in figure 10, allows both the collector emitter voltage V<sub>CE</sub> and the collector current I<sub>C</sub> to be measured with ground referenced, singled ended, scope inputs while still converting the AWG voltage output into a suitable base current. |
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| {{ :university:courses:electronics:a4_f14.png?500 |}} | {{ :university:courses:electronics:a4_f14.png?500 |}} |
