Complete Electronics Self-Teaching Guide with Projects

The JFET as an Amplifier

31 Chapter 3 discussed the JFET in problems 28231, and Chapter 4 discussed the JFET in problems 37241. You may want to review these problems before answering the questions in this problem. Figure 8.30 shows a typical biasing circuit for a JFET.

Figure 8.30

Questions

A. What type of JFET is depicted in the circuit? _____

B. What value of V_GS would you need to turn the JFET completely ON? _____

C. What drain current flows when the JFET is completely ON? _____

D. What value of V_GS would you need to turn the JFET completely OFF? _____

E. When a JFET is alternately turned completely ON and OFF in a circuit, what type of component are you using the JFET as? _____

Answers

A. N-channel JFET.

B. V_GS = 0 V to turn the JFET completely ON.

C. Drain saturation current (I_DSS).

D. V_GS should be a negative voltage for the N-channel JFET to turn it completely OFF. The voltage must be larger than or equal to the cutoff voltage.

E. The JFET is being used as a switch.

32 You can use a JFET to amplify AC signals by biasing the JFET with a gate to source voltage about halfway between the ON and OFF states. You can find the drain current that flows in a JFET biased to a particular V_GS by using the following equation for the transfer curve:

In this equation, ...