JFET Applications

JEFT as Analog Switch:

JFET can be used as an analog switch as shown in figure 1.It is major application of a JFET. The idea is to use 2 points on the load line: cut off and saturation. When the JFET is cut off, it is just like an open switch. When it is saturated, it is just like a closed switch.

                                  Figure 1                                                      Fig. 2

When V_GS =0, JFET is saturated and operates at upper end of load line. When V_GS is equal to or more negative than the V_GS(off) , it is cut off and operates at lower end of the load line (open and closed switch).This is shown in the figure 2.

Only these 2 points are used for the operation when used as a switch. The JFET is normally saturated below the knee of the drain curve. For this reason the drain current is much smaller than the I_DSS.

FET as a Shunt Switch:

The FET can be used as a shunt switch as shown in the figure 3. When V_cont=0, the JFT is saturated and switch is closed When V_cont is negative FET is just like an open switch. The equivalent circuit is also shown in the figure 3.

Figure 3

FET as a series switch:

JFET can be used as series switch also as shown in the figure 6. When control is zero; the FET is a closed switch. When Vcon= negative, the FET is an open switch. It is better than shunt switch.

Figure 4

Multiplexing:

One of the significant applications of FET is in analog multiplexer. Analog multiplexer is a circuit that selects one of the output lines as shown in figure 5. When control voltages are more negative all the switches are open and output is zero. When any control voltage becomes zero input is transmitted to the output.

Figure 5

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