# JFET Current Regulator

## Chapter 5 - Discrete Semiconductor Circuits

PARTS AND MATERIALS
• One N-channel junction field-effect transistor, models 2N3819 or J309 recommended (Radio Shack catalog # 276-2035 is the model 2N3819)
• Two 6-volt batteries
• One 10 kΩ potentiometer, single-turn, linear taper (Radio Shack catalog # 271-1715)
• One 1 kΩ resistor
• One 10 kΩ resistor
• Three 1.5 kΩ resistors
For this experiment you will need an N-channel JFET, not a P-channel!experiment you will need an N-channel JFET, not a P-channel! Beware that not all transistors share the same terminal designations, or pinouts, even if they share the same physical appearance. This will dictate how you connect the transistors together and to other components, so be sure to check the manufacturer’s specifications (component datasheet), easily obtained from the manufacturer’s website. Beware that it is possible for the transistor’s package and even the manufacturer’s datasheet to show incorrect terminal identification diagrams! Double-checking pin identities with your multimeter’s “diode check” function is highly recommended. For details on how to identify junction field-effect transistor terminals using a multimeter, consult chapter 5 of the Semiconductor volume (volume III) of this book series. CROSS-REFERENCES Lessons In Electric Circuits, Volume 3, chapter 5: “Junction Field-Effect TransistorsLessons In Electric Circuits, Volume 3, chapter 3: “Diodes and RectifiersLEARNING OBJECTIVES
• How to use a JFET as a current regulator
• How the JFET is relatively immune to changes in temperature
JFET current regulator vsource 1 0 rload 1 2 4.5k j1 2 0 3 mod1 rlimit 3 0 1k .model mod1 njf .dc vsource 6 12 0.1 .plot dc i(vsource) .end
SPICE does not allow for “sweeping” resistance values, so to demonstrate the current regulation of this circuit over a wide range of conditions, I’ve elected to sweep the source voltage from 6 to 12 volts in 0.1-volt steps. If you wish, you can set rload to different resistance values and verify that the circuit current remains constant. With an rlimit value of 1 kΩ, the regulated current will be 291.8 µA. This current figure will most likely not be the same as your actual circuit current, due to differences in JFET parameters. Many manufacturers give SPICE model parameters for their transistors, which may be typed in the .model line of the netlist for a more accurate circuit simulation.
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