Electronics Demystified, 2nd edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 8

1. If we want to make an amplifier oscillate, we should start out with a class-A, grounded-emitter or grounded-source circuit that produces considerable gain. Then we should introduce positive feedback from the output to the input. None of the choices A, B, or C given here follow those guidelines; in fact, all three of those suggestions run contrary to our intent! The correct choice is therefore D, "None of the above."

2. A Pierce oscillator employs a quartz crystal to obtain its signal. We'll always find a crystal in this type of oscillator, regardless of the other details. The correct choice is C.

3. Figure 8-13 shows a Hartley oscillator with a varactor diode to adjust the frequency. The voltage across the varactor should, under ideal circumstances, control the frequency, so the circuit constitutes a voltage-controlled oscillator (VCO). The correct choice is A.

4. As Fig. 8-13 is drawn, the voltage across the varactor can't control the frequency as it should, because the positive voltage pole shorts out to ground through the inductor! To get rid of this short circuit while allowing the coil inductance and the varactor capacitance to control the oscillator frequency, we should place a fixed capacitor between the varactor cathode (represented by the double line at the point of the arrow) and the top of the coil. Once we've done that, the control voltage will influence the capacitance across the coil, thereby allowing us to adjust the oscillator frequency. The correct choice is D.

5. In a phase-locked loop (PLL) system, the output frequency from the programmable divider is constantly compared with the frequency of a reference oscillator. If the two frequencies start to drift apart, their relative phase changes. The phase comparator detects this change, and introduces a DC error voltage to the varactor diode in the VCO. The resulting change in the varactor capacitance returns the VCO frequency its proper value, so that the programmable divider output frequency comes back into agreement with the reference oscillator output frequency. The correct choice is C.

6. The hallmark of a Hartley oscillator is the presence of a tapped inductor with a variable capacitor connected to its end terminals, forming a tunable LC circuit. The resonant frequency of this LC circuit determines the frequency of oscillation. The correct choice is B.

7. The hallmark of a voltage-controlled oscillator is the presence of a varactor diode as part of the capacitance in a tunable LC circuit that determines the frequency of oscillation. The correct choice is C.

8. In a conventional Colpitts oscillator, a pair of series-connected capacitors forms the capacitive portion of the tuned LC circuit. The extent of the feedback depends on the ratio of these two capacitances, while the oscillation frequency depends on their net capacitance. It's difficult to find a dual variable capacitor that maintains the correct ratio of capacitance throughout the range of its settings. For that reason, most conventional Colpitts oscillators employ variable inductors to control the frequency; the capacitive portion of the tuned LC circuit comprises a pair of fixed capacitors in series. The Clapp is a variation of the Colpitts scheme that allows the use a single variable capacitor to adjust the frequency. The correct choice is B.

9. A Pierce oscillator uses a quartz crystal to determine the oscillation frequency. A Hartley oscillator uses a tuned LC circuit for frequency determination and adjustment. Quartz crystals offer better frequency stability than LC circuits do; that's the outstanding advantage of the Pierce circuit over the Hartley circuit (or, for that matter, the Colpitts and Clapp circuits). The correct choice is A.

10. The hallmark of a Colpitts oscillator is the existence of a split capacitance (that is, a series-connected pair of capacitors) in the tuned LC circuit that determines the oscillation frequency. The correct choice is A.