JFET Animation
How A JFET Works
Using the Animation
Start by clicking and dragging up either of the handles on the VGS or VDS voltage sliders. This applies appropriate voltages to the JFET. Watch the "What´s Happening" box for information about the transistor´s operation as you adjust the VGS and VDS sliders.
The VDS control changes the voltage across the drain and source, which sets up a voltage gradient to attract electrons from the source to the drain creating a drain current ID Notice how the initial increase in current stops rising once the pinch off point is exceeded. The increase in current normally caused by an increase in applied voltage is approximately balanced by the increase in resitance of the conducting channel caused by its width being restricted by the growth of depletion layers around the gate regions. This is due to a reverse bias on the gate PN junctions caused by the P type gate being held at 0V whilst the N type channel (especially nearer the positive Drain end) is becoming more positive
With VDS in the saturation area and VGS at zero volts, maximum current is flowing. This current can be reduced to zero just by increasing the negative gate-source voltage VGS to narrow the conducting channel. As the gat/channel junction is reverse biased the JFET controls a large drain current by using a changing voltage on the gate with practically no gate current. The advantage of a JFET is that, although it´s gain (it´s FORWARD or MUTUAL TRANSCONDUCTANCE) is not very high compared with some other devices, because there is no gate current, the input impedance is extremely high, also unlike a bipolar transistor the current does not have to pass through any PN junctions (which in effect are small capacitors) the high frequency performance is very good.
Page Updated 14th July 2010
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