Electronics Demystified, 2nd edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 5

1. In a PIN diode, the intrinsic (I) semiconductor layer reduces the junction capacitance at the P-N junction, compared with an ordinary diode having the same dimensions. The intrinsic layer adds some "extra space" between the P type and N type layers. It's the same effect that we get if we increase the spacing between the plates of a capacitor. The correct choice is D.

2. If we want to intercept modulated-light rays and recover the intelligence from the beam-intensity fluctuations, a silicon photodiode will work well (as long as the modulating frequencies don't get too high). An LED or laser diode won't work; they're intended to emit light, not intercept it. A varactor diode is meant to serve as a variable capacitance in a voltage-controlled oscillator; it has nothing to do with modulated light. The correct choice is C.

3. We can use hot-carrier diodes in a single balanced mixer circuit such as the one shown in Fig. 5-16, but not any of the other three diode types mentioned in this question. Zener diodes work for voltage regulation or limiting. Rectifier diodes convert AC to pulsating DC. Varactor diodes act as variable capacitors in specialized circuits such as voltage-controlled oscillators. The correct choice is B.

4. In the mixer circuit of Fig. 5-16, the local-oscillator (LO) signal mixes with the incoming signal to produce a constant output signal at 9.00 MHz. The LO tunes over the range from 19.00 to 19.50 MHz. If we take the received-signal frequency minus the LO frequency to get 9.00 MHz, we'll "hear" incoming signals over a range 9.00 MHz above the LO range; that's 28.00 to 28.50 MHz. We don't see that range in any of the choices. If we take the LO frequency minus the received-signal frequency to get 9.00 MHz, we'll "hear" incoming signals over a range 9.00 MHz below the LO range; that's 10.00 to 10.50 MHz. Choice A says that, so it's the right answer.

5. A double balanced mixer offers better isolation between the signal ports than a single balanced mixer does, so choice B will work here. Choice A doesn't apply; either a single or double balanced mixer will work over the same range of frequencies given similar diodes and inductor values. Choice C is irrelevant and misleading; both the single and double balanced mixers offer no gain (and in fact introduce some loss) when constructed with semiconductor diodes alone. Choice D is completely wrong. Single and double balanced mixers using diodes alone have no external sources of power other than the applied signals themselves, so they won't oscillate. The best answer is B.

6. In a "crystal-set" radio, the diode extracts the modulating intelligence from the carrier wave for an incoming AM signal by means of envelope detection. The correct choice is C.

7. When an electron "falls" from a particular shell (or energy state) to a lower shell (representing less energy) within an atom, that electron loses energy in the form of a photon. That's the phenomenon that makes photoemission possible. The correct choice is A. The other three choices have nothing to do with the photoemission process.

8. If we apply a small reverse bias to a rectifier diode, the diode won't conduct any current. A narrow depletion region will form at the P-N junction. As we slowly increase the reverse-bias voltage, the depletion region will get wider, causing the junction capacitance to decrease. If we keep on increasing the reverse-bias voltage, this process will continue for awhile; but then, when the reverse-bias voltage reaches the avalanche threshold, the depletion region will vanish and the diode will conduct current in the reverse direction. In effect, the diode will act as a short circuit if we keep the reverse-bias voltage above the avalanche point. As the question is stated, only choice D accurately describes this evolving condition.

9. In a Zener diode, the Zener voltage equals the avalanche voltage. When we look at the graph of Fig. 5-17, we can see that avalanche breakdown occurs with reverse bias of approximately 7.6 V. The correct choice is C.

10. In a Zener diode, the Zener voltage and the avalanche voltage are identical, not only in terms of numerical value but also by definition. The correct choice is B.