1. The Ohmic Region The slope of the characteristic curve in the ohmic region is the dc drain-to- source conductance G DS of the JFET. Thus, the dc drain-to-source resistance is given by

2. 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 V GS, and it determines the resistance by varying the Q-point.

3. To bias a JFET in the ohmic region, the dc load line must intersect the characteristic curve in the ohmic region. Thus, to allows V DS to control R DS, the dc saturation current ID(sat), is set much less than I DSS so that the load line intersects most of the characteristic curves in the ohmic region. The JFET as a Variable Resistance

4. Figure shows the operating region expanded with three Q-points shown (Q 0, Q 1, and Q 2 ), depending on V GS. As you move along the load line in the ohmic region, the value of R DS varies as the Q-point falls successively on curves with different slopes. The Q-point is moved along the load line by varying

5. 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 I D and more V DS, which implies an increase in R DS. This change in resistance can be exploited in a number of applications where voltage control of a resistance is useful. ***R DS 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

6. The JFET as a Variable Resistance