TRUE/FALSE
1. In general, components behave the same at 20 MHz as they do at 1 kHz.
ANS: F
2. Stray capacitance is important in high-frequency amplifiers.
ANS: T
3. Microwave-frequency circuits look much different from circuits designed for 1 MHz.
ANS: T
4. All electronic devices have both capacitive and inductive properties.
ANS: T
5. As frequency increases, capacitive effects decrease.
ANS: F
6. At some frequency, a capacitor will self-resonate with the inductance of its leads.
ANS: T
7. An "unstable" amplifier is one that oscillates, or is close to oscillating.
ANS: T
8. Base-to-emitter capacitance can cause a common-emitter amplifier to oscillate.
ANS: F
9. "Distributed Constants" refers to a mathematical operation to calculate gain.
ANS: F
10. "Shielding" prevents RF signals from coupling between components in a system.
ANS: T
11. A "ground-plane" is a type of shielding.
ANS: T
12. A "gimmick" is a type of shielding.
ANS: F
13. Removing any RF signals off the VCC lines is an example of "decoupling".
ANS: T
14. Decoupling usually involves a "bypass" capacitor.
ANS: T
15. Basically, radio-frequency amplifiers can not be distinguished from other amplifiers.
ANS: F
16. Resonant circuits are common in RF amplifiers.
ANS: T
17. Increasing the load on a tuned amplifier increases its Q.
ANS: F
18. In a common-emitter amplifier, the collector-base capacitance "looks" bigger than it is.
ANS: T
19. The Miller Effect only occurs in common-base amplifiers.
ANS: F
20. The Miller Effect can reduce the bandwidth of an amplifier.
ANS: T
21. The Miller Effect can be reduced using neutralization.
ANS: T
22. A piezoelectric crystal behaves like a very low-Q tuned circuit.
ANS: F
23. The frequency of a crystal oscillator is much more stable than an LC oscillator.
ANS: T
24. Mixers must be nonlinear in order to work.
ANS: T
25. A mixer will produce "sum and difference" frequencies.
ANS: T
MULTIPLE CHOICE
1. The time it takes a charge carrier to cross from the emitter to the collector is called:
a. | base time | c. | charge time |
b. | transit time | d. | Miller time |
ANS: B
2. A real capacitor actually contains:
a. | capacitance and resistance only | c. | capacitance, inductance, and resistance |
b. | capacitance and inductance only | d. | reactance only |
ANS: C
3. Bypass capacitors are used to:
a. | remove RF from non-RF circuits | c. | neutralize amplifiers |
b. | couple RF around an amplifier | d. | reduce the Miller effect |
ANS: A
4. A resonant circuit is:
a. | a simple form of bandpass filter | c. | both a and b |
b. | used in narrowband RF amplifiers | d. | none of the above |
ANS: C
5. Loading down a tuned-circuit amplifier will:
a. | raise the Q of the tuned circuit | c. | "multiply" the Q |
b. | lower the Q of the tuned circuit | d. | have no effect on Q |
ANS: B
6. The "Miller Effect" can:
a. | cause an amplifier to oscillate | c. | reduce the bandwidth of an amplifier |
b. | cause an amplifier to lose gain | d. | all of the above |
ANS: D
7. The Miller Effect can be avoided by:
a. | using a common-emitter amplifier | c. | increasing the Q of the tuned circuit |
b. | using a common-base amplifier | d. | it cannot be avoided |
ANS: B
8. In a BJT, the Miller Effect is due to:
a. | inductance of collector lead | c. | base-to-emitter capacitance |
b. | collector-to-emitter capacitance | d. | base-to-collector capacitance |
ANS: D
9. In RF amplifiers, impedance matching is usually done with:
a. | RC coupling | c. | direct coupling |
b. | transformer coupling | d. | lumped reactance |
ANS: B
10. Neutralization cancels unwanted feedback by:
a. | adding feedback out of phase with the unwanted feedback |
b. | bypassing the feedback to the "neutral" or ground plane |
c. | decoupling it |
d. | none of the above |
ANS: A
11. For a "frequency multiplier" to work, it requires:
a. | a nonlinear circuit |
b. | a linear amplifier |
c. | a signal containing harmonics |
d. | an input signal that is an integer multiple of the desired frequency |
ANS: A
12. A sinusoidal oscillation from an amplifier requires:
a. | loop gain equal to unity |
b. | phase shift around loop equal to 0 degrees |
c. | both a and b, but at just one frequency |
d. | none of the above |
ANS: C
13. The conditions for sinusoidal oscillation from an amplifier are called:
a. | the loop-gain criteria | c. | the Bode criteria |
b. | the Hartley criteria | d. | the Barkhausen criteria |
ANS: D
14. The Hartley oscillator uses:
a. | a tapped inductor | c. | an RC time constant |
b. | a two-capacitor divider | d. | a piezoelectric crystal |
ANS: A
15. The Colpitts VFO uses:
a. | a tapped inductor | c. | an RC time constant |
b. | a two-capacitor divider | d. | a piezoelectric crystal |
ANS: B
16. The Clapp oscillator is:
a. | a modified Hartley oscillator | c. | a type of crystal-controlled oscillator |
b. | a modified Colpitts oscillator | d. | only built with FETs |
ANS: B
17. A varactor is:
a. | a voltage-controlled capacitor | c. | used in tuner circuits |
b. | a diode | d. | all of the above |
ANS: D
18. Crystal-Controlled oscillators are:
a. | used for a precise frequency |
b. | used for very low frequency drift (parts per million) |
c. | made by grinding quartz to exact dimensions |
d. | all of the above |
ANS: D
19. If two signals, Va = sin(wat) and Vb = sin(wbt), are fed to a mixer, the output:
a. | will contain w1 = wa + wb and w2 = wa – wb |
b. | will contain w1 = wa / wb and w2 = wb / wa |
c. | will contain w = (wa + wb ) / 2 |
d. | none of the above |
ANS: A
20. In a balanced mixer, the output:
a. | contains equal (balanced) amounts of all input frequencies |
b. | contains the input frequencies |
c. | does not contain the input frequencies |
d. | is a linear mixture of the input signals |
ANS: C
21. "VFO" stands for:
a. | Voltage-Fed Oscillator | c. | Varactor-Frequency Oscillator |
b. | Variable-Frequency Oscillator | d. | Voltage-Feedback Oscillator |
ANS: B
22. A "frequency synthesizer" is:
a. | a VCO phase-locked to a reference frequency |
b. | a VFO with selectable crystals to change frequency |
c. | a fixed-frequency RF generator |
d. | same as a mixer |
ANS: A
COMPLETION
1. Generally, conductor lengths in RF circuits should be ____________________.
ANS: short
2. At UHF frequencies and above, elements must be considered as ____________________ instead of as being "lumped".
ANS: distributed
3. When one side of a double-sided pc board is used for ground, it is called a ____________________.
ANS: ground-plane
4. Interactions between parts of an RF circuit can be reduced by using ____________________ between them.
ANS: shielding
5. In high-frequency RF circuits, the placement of wires and ____________________ can be critical.
ANS: components
6. A ____________________ circuit is used to remove RF from the DC voltage bus.
ANS: decoupling
7. A ____________________ capacitor is used to short unwanted RF to ground.
ANS: bypass
8. The bandwidth of a tuned-circuit amplifier depends on the ____________________ of the tuned circuit.
ANS: Q
9. A value of ____________________ or more for Q is required for the approximate tuned circuit equations to be valid.
ANS: 10
10. In a class C RF amplifier, the ____________________ extracts one frequency from all the harmonics contained in the device current (e.g. collector current).
ANS: tuned circuit
11. Using additional feedback to compensate for "stray" feedback is called ____________________.
ANS: neutralization
12. A Colpitts oscillator uses a ____________________ voltage divider to provide feedback.
ANS: capacitive
13. Electrically, a piezoelectric crystal has both a ____________________ and a ____________________ resonant frequency.
ANS: series, parallel
14. To produce sum and difference frequencies, a mixer must be a non-____________________ circuit.
ANS: linear
15. At some bias point, a diode or a transistor can act as a ____________________-law mixer.
ANS: square
SHORT ANSWER
1. What inductance would you use with a 47-pF capacitor to make a tuned circuit for 10 MHz?
ANS:
5.4 mH
2. What value of Q is required for a 10-MHz tuned circuit to have a bandwidth of 100 kHz?
ANS:
100
3. A tuned-circuit amplifier with a gain of 10 is being used to make an oscillator. What should be the value of the feedback ratio to satisfy the Barkhausen criteria?
ANS:
0.1
4. What is the advantage of a Clapp oscillator compared to a Colpitts oscillator?
ANS:
It is more stable because it "swamps" the device capacitance with large value capacitors in the feedback divider.
5. If a varactor has a capacitance of 90 pF at zero volts, what will be the capacitance at 4 volts?
ANS:
30 pF
6. An oscillator has a frequency of 100 MHz at 20°C, and a tempco of +10 ppm per degree Celsius. What will be the shift in frequency at 70°C? What percentage is that?
ANS:
50 kHz, 0.05%
7. Two sinusoidal signals, V1 and V2, are fed into an ideal balanced mixer. V1 is a 20-MHz signal; V2 is a 5-MHz signal. What frequencies would you expect at the output of the mixer?
ANS:
15 MHz and 25 MHz
8. Suppose the phase-locked-loop frequency synthesizer of Figure 2.39 has a reference frequency of 1 MHz and a fixed-modulus divider of 10. What should be the value of the programmable divider to get an output frequency of 120 MHz?
ANS:
12
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