Velleman DCA55 - Manual (Page 20)

Atlas DCA User Guide October 2007 – Rev 7
Page 20
Junction FETs (JFETs)
Junction FETs are conventional Field Effect Transistors.
The voltage applied across the gate-source terminals controls
current between the drain and source terminals. N-Channel
JFETs require a negative voltage on their gate with respect to their source, the
more negative the voltage, the less current can flow between the drain and
source.
Unlike Depletion Mode MOSFETs,
JFETs have no insulation layer on the
gate. This means that although the input
resistance between the gate and source is
normally very high (greater than 100MΩ), the gate current can rise if the
semiconductor junction between the gate and source or between the gate and
drain become forward biased. This can happen if the gate voltage becomes
about 0.6V higher than either the drain or source terminals for N-Channel
devices or 0.6V lower than the drain or source for P-Channel devices.
The internal structure of JFETs is
essentially symmetrical about the gate
terminal, this means that the drain and
source terminals are indistinguishable by
the Atlas DCA. The JFET type and the
gate terminal are identified however.
P-Channel
Junction FET
Drain and Source
not identified
RED GREEN BLUE
Gate