Radio-Frequency Circuits Chapter 2
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Questions and Answers

How does neutralization cancel unwanted feedback?

  • by bypassing the feedback to the 'neutral' or ground plane (correct)
  • by adding feedback in phase with the unwanted feedback
  • by amplifying the unwanted feedback
  • by decoupling it

    • What is a requirement for a 'frequency multiplier' to work?

  • a nonlinear circuit (correct)
  • an RC time constant
  • a sinusoidal input signal
  • a linear amplifier

    • What is a necessary condition for a sinusoidal oscillation from an amplifier?

  • loop gain equal to zero
  • phase shift around loop equal to 90 degrees
  • loop gain equal to unity and phase shift around loop equal to 0 degrees (correct)
  • none of the above

    • What are the conditions for sinusoidal oscillation from an amplifier called?

    <p>the Barkhausen criteria</p> Signup and view all the answers

    What type of component is used in a Hartley oscillator?

    <p>a tapped inductor</p> Signup and view all the answers

    What is a varactor?

    <p>a voltage-controlled capacitor</p> Signup and view all the answers

    What is the time it takes for a charge carrier to cross from the emitter to the collector called?

    <p>transit time</p> Signup and view all the answers

    What are the components of a real capacitor?

    <p>capacitance, resistance, and inductance</p> Signup and view all the answers

    What is the purpose of bypass capacitors in RF circuits?

    <p>to remove RF from non-RF circuits</p> Signup and view all the answers

    What is a characteristic of a resonant circuit in RF amplifiers?

    <p>it is used in narrowband RF amplifiers</p> Signup and view all the answers

    What happens to the Q of a tuned circuit when it is loaded down?

    <p>it decreases</p> Signup and view all the answers

    What is the effect of the Miller effect on an amplifier?

    <p>all of the above</p> Signup and view all the answers

    What frequencies will the output of a mixer contain if the inputs are Va = sin(ωat) and Vb = sin(ωbt)?

    <p>ω1 = ωa + ωb and ω2 = ωa – ωb</p> Signup and view all the answers

    What is the characteristic of a balanced mixer's output?

    <p>It does not contain the input frequencies</p> Signup and view all the answers

    What does VFO stand for?

    <p>Variable-Frequency Oscillator</p> Signup and view all the answers

    What is a frequency synthesizer?

    <p>A VCO phase-locked to a reference frequency</p> Signup and view all the answers

    What is a general guideline for conductor lengths in RF circuits?

    <p>They should be short</p> Signup and view all the answers

    How should elements be considered at UHF frequencies and above?

    <p>As distributed elements</p> Signup and view all the answers

    Study Notes

    Neutralization

    • Cancels unwanted feedback by adding feedback out of phase with the unwanted feedback

    Frequency Multiplier

    • Requires a nonlinear circuit to work
    • Nonlinear circuit is necessary to generate harmonics and produce an output frequency that is a multiple of the input frequency

    Sinusoidal Oscillation

    • Requires a loop gain equal to unity and a phase shift around the loop equal to 0 degrees
    • Both conditions must be met at just one frequency

    Conditions for Sinusoidal Oscillation

    • Called the Barkhausen criteria
    • Necessary for an amplifier to produce a sinusoidal oscillation

    Oscillator Types

    • Hartley oscillator: uses a tapped inductor
    • Colpitts VFO: uses a two-capacitor divider
    • Clapp oscillator: a modified Colpitts oscillator

    Varactor

    • A voltage-controlled capacitor
    • Used in tuner circuits and has multiple applications

    Crystal-Controlled Oscillators

    • Used for precise frequency control
    • Used for very low frequency drift (parts per million)
    • Made by grinding quartz to exact dimensions

    Radio-Frequency Circuits

    • Transit time: the time it takes a charge carrier to cross from the emitter to the collector
    • Real capacitors: contain capacitance, inductance, and resistance
    • Bypass capacitors: used to remove RF from non-RF circuits

    Resonant Circuits

    • A simple form of bandpass filter
    • Used in narrowband RF amplifiers

    Loading Down a Tuned-Circuit Amplifier

    • Lowers the Q of the tuned circuit

    Miller Effect

    • Can cause an amplifier to oscillate
    • Can cause an amplifier to lose gain
    • Can reduce the bandwidth of an amplifier
    • Caused by collector-to-emitter capacitance in a BJT
    • Can be avoided by using a common-base amplifier

    Impedance Matching

    • Usually done with transformer coupling in RF amplifiers

    Mixer Output

    • Contains the sum and difference frequencies of the input signals

    Balanced Mixer

    • Output does not contain the input frequencies

    VFO

    • Stands for Variable-Frequency Oscillator

    Frequency Synthesizer

    • A VCO phase-locked to a reference frequency

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    Description

    Test your knowledge of radio-frequency circuits with this multiple choice quiz. Topics include transit time, capacitor properties, and bypass capacitors.

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