EMT 1255 - Experiment #11 JEFT Biasing PDF

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Summary

Experiment #11:JFET BiasingDue Date: 05/18/ObjectiveThe objective of this experiment is to be able to determine V GS (off) indirectly from aself-bias circuit, specify the source resistor for a self-biased JFET that will provide a specified I D and draw the loads line for the resistor, and also to be...

Description

Experiment #11: JFET Biasing

Due Date: 05/18/2018 Objective The objective of this experiment is to be able to determine V GS (off) indirectly from a

self-bias circuit, specify the source resistor for a self-biased JFET that will provide a specified I D and draw the loads line for the resistor, and also to be able specify the source resistor for a JFET using voltage-divider bias and test the circuit.

Theory Equipment ● ● ● ● ● ●

One 2.2 K ohms resistor One 330 K ohms resistor One 1.0 M ohms resistor One 1.0 K ohms potentiometer One 2N5458 JFET One milliammeter 0- 10 range

Procedure 1. We set up the circuit shown in figure 12-2. 2. Set the potentiometer for zero ohms and record the current reading on the milliammeter

to get I DSS. 3. We determined V GS (off) by increasing the resistance until the milliammeter read 50% of I DSS, at that point we measure the voltage across the gate and source, and ,ultiplied the reading by 3.4 which will give us V GS (off). 4. We recorded the values of V GS (off) and I DSS into table 12-1. 5. We were given a normalized graph of I D verses V GS. Meaning that the values on the curves were set to dimensionless numbers the can be converted to actual measured quantities by a scaling operation. 6. We were instructed by the manual to convert the scale to a practical curve for our JFET by multiplying the values on the x-axis by V GS (off) and the values shown in the y-axis by I DSS ( Refer to plot 12-1.) 7. We determined the value of the nearest standard source resistance by a given formula. Which in our case was 100 ohms 8. With the source resistor we determined in step 7, we drew a load line in plot 12-1 and indicated the Q point on the plot, from this we predicted I D for the circuit. 9. We replaced the potentiometer with the source resistor we found in step 7, we measured the drain current once again and the result was not far from the predicted value that we got in step 8. 10. Assuming we wanted to keep the same Q point as we had before but now in a Voltage divider circuit, we needed to calculate a source resistor that would keep the same gatesource voltage. The divider was shown in figure 12-3 11. We computed the voltage on the gate from the voltage-divider rule and entered the value on table 12-2 12. We added that value to the V GS absolute value obtained in step 3, and this will gives us the desired source voltage, we entered this information on table 12-2 as computed value.

13. The drain current was the same as before, knowing this information we applied ohms law

and calculated the desired source resistance.

Data

Conclusion In conclusion throughout this experiment we were able to comprehend and understand how to determine V GS (off) indirectly from a self-bias circuit, specify the source resistor for a self-biased JFET that will provide a specified I D and draw the loads line for the resistor, and also to specify the source resistor for a JFET using voltage-divider bias and test the circuit. This experiment was quite simple and it ran smoothly, however we had lessons learn at first when we were testing current with our milliammeter we thought there was something wrong with our circuit, and we were stuck there for a couple of minutes without getting any current reading, but shortly on we realized that we forgot to switch the leads on the milliammeter and that was the reason why we could not have any current value....