The behaviour of a JFET can be described in terms of a set of Characteristic Curves.
In the region shown with a green background the drain-source voltage is small and the channel behaves like a fairly
ordinary conductor. In this region the current varies roughly in proportion to the drain-source
voltage as if the JFET obeys Ohm's law.
However, as we increase the drain-source voltage and move into the region with a light background we increase the
drain-channel voltage so much that we start to ‘squeeze down’ the channel. Hence a large increase in
drain-source:-
- ‘pulls harder’, trying to drag the electrons more quickly from source to drain.
- ‘squeezes down’ the channel making it harder for the electrons to get through.
These effects tend to cancel out, leaving the current the same at all high drain-source voltage.
Provided that it is above a few volts, changing the drain-source voltage does not alter the channel current very much. We can therefore
simplify the above ‘family’ of curves into a single plot showing how the gate-source voltage controls the
drain-source current.
By looking at these curves we can see that the JFET has two areas of operation. At low (a few volts) drain-source voltages it behaves like a variable resistance whose value is controlled by the applied gate-source voltage. At higher drain-source voltages it passes a current whose value depends on the applied gate-source voltage. In most circuits it is used in this ‘high voltage’ region and acts as a voltage controlled current source.
Note that the curves shown here are for a typical small-signal JFET. Other fets will have curves with similar shapes, but the scales on the graphs might be different. e.g. For a power JFET the currents would be perhaps up to tens of amps rather than a few tens of milliamps.
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