Complete Electronics Self-Teaching Guide with Projects

Switching the JFET

37 The use of the junction field effect transistor (JFET) as a switch is discussed in the next few problems. You may want to review problems 28 through 31 in Chapter 3 where this book introduced the JFET.

The JFET is considered a “normally on” device, which means that with 0 volts applied to the input terminal (called the gate), it is ON, and current can flow through the transistor. When you apply a voltage to the gate, the device conducts less current because the resistance of the drain to the source channel increases. At some point, as the voltage increases, the value of the resistance in the channel becomes so high that the device “cuts off” the flow of current.

Questions

A. What are the three terminals for a JFET called, and which one controls the operation of the device? _____

B. What turns the JFET ON and OFF? _____

Answers

A. Drain, source, and gate, with the gate acting as the control.

B. When the gate voltage is zero (at the same potential as the source), the JFET is ON. When the gate to source voltage difference is high, the JFET is OFF.

Project 4.2: The JFET

Objective

The objective of this project is to determine the drain current that flows when a JFET is fully ON, and the gate voltage needed to fully shut the JFET OFF, using the circuit shown in Figure 4.22.

Figure 4.22

General Instructions

After the circuit is set up, change ...