Teach Yourself Electricity and Electronics, 5th edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 23

1. The current fluctuates in the channel of a JFET when the voltage between the channel and the gate produces an electric field in the channel. This field constricts the channel to a variable extent, affecting the channel's conductivity. The correct answer is (b).

2. If we maintain a constant drain voltage in a P-channel JFET, an increasing positive gate bias voltage (gate more positive than source) decreases the channel conductivity until, at a certain point, the channel stops conducting because pinchoff occurs. The positive gate voltage must attain a significant value if we expect to get pinchoff. The correct choice is (d).

3. The correct answer here is (b). We can get the most amplification when we bias a JFET in the straight-line region of the drain-current versus gate-voltage curve that "ramps upward to the right." In the book, figure 23-5 and the associated text explain the situation.

4. In an enhancement-mode MOSFET, the channel will not conduct unless we apply the proper bias voltage at the gate. A zero-biased enhancement-mode MOSFET will remain pinched off. The correct choice is (a).

5. Figure 23-14 is a simplified cutaway drawing of a P-channel MOSFET. The correct answer is (c).

6. In Fig. 23-14, the object marked X is a thin layer of insulating, or dielectric, material. The correct choice is (b).

7. Field-effect transistors (more than bipolar transistors) are known for their excellent weak-signal performance, so we can rule out choice (a). Transconductance does not apply to bipolar devices, so we can rule out (c). We would rarely, if ever, want an amplifier to draw the maximum possible amount of power from the signal source, so (d) won't work. The best choice is (b). Bipolar transistors can be wired so as to present a low impedance at the input; this is especially true of the common-base configuration, as we learned in Chap. 22.

8. The resistance between the gate and the source of a properly operating MOSFET is so high that we can compare it to the resistance across a capacitor of similar physical size -- that is, "practically infinite." The correct choice is (c).

9. We never want to see current flow between the gate and the channel in a JFET or in a MOSFET. The correct choice is (d). If current flows between the gate and the channel in a JFET, it usually means that forward breakover or avalanche effect is taking place at the junction. Either of these effects reduces the gain and causes the device to operate in a nonlinear fashion. We can usually correct the problem by ensuring that the JFET receives proper bias. If we observe conduction between the gate and the channel of a MOSFET, we know that the dielectric layer has been compromised, probably as a result of an electrostatic discharge. In that case, the device will no longer function properly under any conditions.

10. The majority carriers are electrons in an N type semiconductor medium. In the case of either a JFET or a MOSFET, the charge carriers flow through the channel. Therefore, we must have an N type channel in any FET if we expect electrons to constitute the majority carriers. The correct choice is (a).

11. The common-gate or common-drain circuits produce an output signal wave that's exactly in phase with the input signal wave. Because "source follower" is another name for a common-drain circuit, we can conclude that (a), (b), and (c) will all work here. Therefore, we should answer this question with choice (d), "All of the above."

12. An electrostatic discharge can wreck a MOSFET by destroying the insulating properties of the dielectric. The correct answer is (c).

13. The MOSFET is known for its high input impedance -- usually higher than the input impedance of a JFET. The correct choice is (a).

14. A depletion-mode MOSFETs appears in a schematic diagram with a solid vertical line inside the circle, such as appear in Figs. 23-7B and 23-8B in the book. A broken vertical line indicates an enhancement-mode MOSFET (Figs. 23-10A and B). The correct answer is (d).

15. In a source follower, we take the output signal from between the source and ground. The book illustrates this situation in Fig. 23-13. The correct answer is (d).

16. If we are to believe that the diagram of Fig. 23-15 portrays a source follower, then we must transpose the input and output terminals! The correct choice is (a).

17. Figure 23-15 shows a P-channel JFET as the transistor. That type of device requires a negative DC drain voltage. However, the diagram shows a positive DC drain voltage. We must reverse the polarity if we expect the circuit to work. The correct choice is (d).

18. Figure 23-16 is essentially a "reprint" of Fig. 23-9. When we look back at Fig. 23-9 and the associated text discussion, we see that the curves represent various DC gate voltages. The correct choice is therefore (c).

19. Again referring back to Fig. 23-9 in the book, we can see that the DC gate bias voltages become progressively less positive (or more negative) as we go down. The correct choice is (a).

20. To answer this question, we must do a little graph-reading exercise. The horizontal axis portrays the relative applied DC drain voltage, getting more positive as we move toward the right. The vertical axis portrays the relative drain current. The curves therefore show drain currents as functions of DC drain voltages. We're told that the graph shows characteristic curves, which always assume no-signal conditions. We can now rule out choices (c) and (d)! If we follow any one of the curves from the origin (lower left intersection of the axes) at a constant speed toward the right, we can see that as we steadily increase the DC drain voltage (horizontal displacement), the drain current (vertical displacement) increases rapidly at first, then more and more slowly. The correct choice is therefore (b).