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