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.

A. What type of JFET is depicted in the circuit? _____
B. What value of VGS would you need to turn the JFET completely ON? _____
C. What drain current flows when the JFET is completely ON? _____
D. What value of VGS 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? _____
A. N-channel JFET.
B. VGS = 0 V to turn the JFET completely ON.
C. Drain saturation current (IDSS).
D. VGS 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 VGS by using the following equation for the transfer curve:


In this equation, ...

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