Junction Field Effect Transistor – JFET

JFET transistors come as either N channel or P channel. N channel conducts +drain to -Source while P channel conducts -drain to +source. A low enough voltage (even lower than the source terminal) to the gate of a N channel JFET lowers the current flow until it is off, while a positive enough signal to the P channel JFET lowers the current until it turns off.

JFET Properties

  • Conduction is controlled by voltage instead of current. (BJTs are controlled by current and are usually learned about first.)
  • No gate current when wired properly. Avoid gate forward bias (observe the “diodes”) – High input impedance.
  • Depletion component – Normally on, until turned off.
  • Easy to make a constant current source/sink.
  • Unfortunately, FET (MOSFET/JFET) schematic symbols are not uniform.

J310 JFET Maximum values

  • VDS … 25V (Drain more positive than source max value)
  • VGS … -25V (Gate more negative than Source max value)
  • IGF … 10mA (Avoid passing current through the gate from +drain and/or +source

JFET Switch

J310 N channel JFET pin layout and demo switch circuit schematic diagram by electronzap electronzapdotcom
J310 N channel JFET pin layout and demo switch circuit schematic diagram by electronzap electronzapdotcom

 

J310 N channel JFET transistor switch demo circuit using TLE2426 virtual ground rail splitter

 

JFET current source/sink

N channel JFET Idss tested for component based current sink source by electronzap

Light Dependent Resistor LDR controlled J310 N channel JFET switch circuit by electronzap
Light Dependent Resistor LDR controlled J310 N channel JFET switch circuit by electronzap

Light dependent resistor LDR J310 JFET n channel switch circuit by electronzap

Quick video series: Brief N channel JFET current sink circuit – J310 constant current source

J310 N Channel Depletion JFET Current Source Circuit Diagram by Electronzap Electronzapdotcom
J310 N Channel Depletion JFET Current Source Circuit Diagram by Electronzap Electronzapdotcom

Transistor circuit 16 J310 n channel depletion JFET current source demo and multimeter measurements

A resistor from source to ground, while the gate is connected directly to ground, will provide a certain amount of constant current below the particular JFET’s Idss as long as enough voltage is provided.

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A lot of the components that I use are in this diode and transistor semiconductor kit.


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