Q. Show Operation on JFET?

The junction in the JFETis reverse-biased for normal operation.No gate current flows because of the reverse bias and all carriers flow from source to drain. The corresponding drain current is dependent on the resistance of the channel and the drain-to-source voltage v_DS.As v_DS is increased for a given value of v_GS, the junction is more heavily reverse-biased, when the depletion region extends further into the conducting channel. Increasing v_DS will ultimately block or pinch off the conducting channel. After the pinch-off, the drain current i_D will be constant, independent of v_DS.

It is the active region beyond pinch-off that is useful for the controlled-source operation, since only changes in v_GS will produce changes in i_D. Figure illustrates the JFET characteristics. Part (a) shows the idealized static characteristics with two regions separated by the dashed line, indicating the ohmic (controlled-resistance or triode) region and the active (controlled-source) region beyond pinch-off. Note that i_D is initially proportional to v_DS in the ohmic region where the JFET behaves much like a voltage-variable resistance; i_D depends on v_GS for a given value of v_DS in the active region. In a practical JFET, however, the curves of i_D versus v_DS are not entirely flat in the active region but tend to increase slightly with v_DS, as shown in Figure(b);

when extended, these curves tend to intersect at a point of -VA on the v_DS axis. Another useful characteristic indicating the strength of the controlled source is the transfer characteristic, relating the drain current i_D to the degree of the negative bias v_GS applied between gate and source; a cutoff region exists, indicated by the pinch-off voltage -VP , for which no drain current flows, because both vGS and vDS act to eliminate the conducting channel completely.

Mathematically, the drain current in the active controlled-source region is approximately given by:

where I_DSS, known as the drain-source saturation current, represents the value of i_D when v_GS = 0.