The Ohmic Region
The slope of the characteristic curve
in the ohmic region is the dc drain-tosource conductance GDS of the JFET.
Thus, the dc drain-to-source resistance is
given by
The JFET as a Variable Resistance
• A JFET can be biased in either the
active region or the ohmic region.
• JFETs are often biased in the ohmic
region for use as a voltage controlled
variable resistor.
• The control voltage is VGS, and it
determines the resistance by varying the
Q-point.
The JFET as a Variable Resistance
• To bias a JFET in the ohmic region,
the dc load line must intersect the
characteristic curve in the ohmic
region.
• Thus, to allows VDS to control RDS,
the dc saturation current ID(sat), is
set much less than IDSS so that the
load line intersects most of the
characteristic curves in the ohmic
region.
The JFET as a Variable Resistance
• Figure shows the operating region
expanded with three Q-points shown
(Q0, Q1, and Q2), depending on VGS.
• As you move along the load line in the
ohmic region, the value of RDS varies as
the Q-point falls successively on curves
with different slopes.
• The Q-point is moved along the load line
by varying
The JFET as a Variable Resistance
• As this happens, the slope of each successive
curve is less than the previous one.
• A decrease in slope corresponds to less ID and
more VDS, which implies an increase in RDS.
• This change in resistance can be exploited in
a number of applications where voltage
control of a resistance is useful.
• ***RDS is the DC drain to source resistance
Q0: ID= 0.270mA, VDS=0.23V
Q1: ID=0.250mA, VDS=0.33V
Q2: ID=0.230mA, VDS=0.44V
Q3: ID=0.210mA, VDS=0.56V
The JFET as a Variable Resistance