Teach Yourself Electricity and Electronics, 5th edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 20

1. Amateur radio operators and other communications experts use devices called Gunnplexers to carry out low-power communications at microwave frequencies. A Gunnplexer comprises a horn-shaped antenna and a Gunn-diode oscillator circuit. The correct answer is (b).

2. A laser diode produces low-intensity coherent visible-light or IR radiation, usually in a cone-shaped beam. The correct choice is (c). The beam is nowhere near strong enough to burn a hole in steel, nor is it narrow (unless focused by an external lens), so choices (a) and (b) won't work.

3. The circuit diagram of Fig. 20-11 shows two diodes connected across a source of AC in order to limit, or clip, the positive and negative voltage peaks. The correct choice is (d). The reverse-parallel diode combination can't amplify, generate, or rectify AC, so choices (a), (b), and (c) are all wrong.

4. In a crystal-set radio receiver, the diode detects (or demodulates) an incoming amplitude-modulated (AM) signal. The correct answer is therefore (a). The diode can't limit, generate, or amplify signals in this application.

5. The nonlinear characteristics of a diode cause harmonics to appear when we apply an AC signal across its P-N junction (provided that the peak amplitude of the signal is sufficient to overcome the forward breakover threshold of the junction). Therefore, a diode makes an excellent harmonic generator. The correct answer is (d).

6. The expression "negative resistance" refers to a situation in which an increase in the instantaneous applied voltage causes a decrease (rather than the expected increase, as Ohm's Law would imply) in the current through a device. The correct choice is (c). The other choices in this question constitute figments of my imagination. They are irrelevant, even though they might seem sophisticated!

7. When we apply two AC signals having different frequencies to the input of a diode-based mixer circuit, we get the original two signals at the output, along with new signals at the sum and difference frequencies. In this case, all three choices (a), (b), and (c) "qualify," so the correct answer is (d), "All of the above."

8. We would use a rectifier diode to convert AC to pulsating DC for use in powering a small appliance from the 60-Hz utility mains. The correct choice is (a). While any diode will theoretically convert AC to pulsating DC, only a rectifier diode is specifically designed to get DC from 60-Hz AC utility electricity.

9. We could use a Zener diode to regulate the voltage in a power supply designed to obtain DC from utility AC. The correct choice is (d). None of the diodes mentioned in the other three choices can effectively serve this purpose.

10. This question requires careful reading and interpretation. When we apply a negative voltage to a diode's anode and a positive voltage to its cathode, we have the condition of reverse bias. The correct choice is (b). We don't necessarily observe avalanche breakdown with reverse bias, and we don't necessarily observe junction depletion with reverse bias (although we should observe either one or the other of these two states in a reverse-biased diode that hasn't burned out), so neither (a) nor (c) always applies. Choice (d) is completely wrong; forward breakover only takes place in forward-biased diodes!

11. Figure 20-12 shows a frequency-multiplier circuit that uses a diode to generate harmonic energy. (In this case, the circuit multiplies the 7.50-MHz input frequency by a factor of 3.) The diode's nonlinearity causes it to generate energy at harmonics. The correct answer is therefore (c).

12. A crystal set doesn't need a battery or any other external power source (besides the radio signal from the antenna) in order to operate, so the correct choice is (a). You might think that (d) would work until you look back at the text and see what the term "cat's whisker" (in the electronics context) actually means! Choices (b) and (c) definitely apply to the diode in a crystal set, so neither of them answers this question correctly.

13. An IMPATT diode works in much the same way as a Gunn diode does. Both of these devices can generate microwave radio signals. The correct answer is (c). Choices (a) and (b) are completely wrong; application (a) uses a Zener diode while (b) uses a rectifier diode.

14. In a nonlinear circuit, device, or system, the output waveform differs in shape from the input waveform, regardless of the relative amplitudes of the signals. The correct choice is (b). You might suspect that choice (c) could also work, until you realize that the peak-to-peak pulsating-DC output voltage in this case would equal only half of the peak-to-peak AC input voltage.

15. In direct sunlight, the electrical power that a solar panel can provide depends on the power output from each individual cell, the number of cells in the panel, and the angle at which the sunlight strikes the panel's surface (among other things). The correct choice is (d), "All of the above."

16. When electrons "fall" from higher to lower energy states within the atoms of a light-emitting diode (LED) or infrared-emitting diode (IRED), the P-N junction emits visible or IR rays, both of which are forms of radiant energy. The correct answer is (a).

17. Figure 20-13 shows a tuned circuit for use in a voltage-controlled oscillator (VCO). The reverse-biased semiconductor device is a varactor diode. When we vary the reverse-bias voltage across the varactor, assuming that it never experiences avalanche breakdown, its capacitance varies, causing the resonant frequency of the entire circuit to fluctuate. The correct choice is (a).

18. In the VCO circuit of Fig. 20-13, the fixed capacitor prevents the coil from shorting out the reverse-bias voltage that we apply to the varactor. The correct choice is (d).

19. In order to minimize the junction capacitance in a diode to make it function as an RF switch, manufacturers can add a layer of intrinsic (I type) semiconductor material between the P type and N type layers to form a so-called PIN diode. The correct answer is (c).

20. We might expect to find an IRED as an integral part of an optoisolator, which takes advantage of IR or visible light to couple electronic circuits with a minimum of interaction. The correct choice is therefore (b).