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Questions and Answers
How does neutralization cancel unwanted feedback?
What is a requirement for a 'frequency multiplier' to work?
What is a necessary condition for a sinusoidal oscillation from an amplifier?
What are the conditions for sinusoidal oscillation from an amplifier called?
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What type of component is used in a Hartley oscillator?
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What is a varactor?
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What is the time it takes for a charge carrier to cross from the emitter to the collector called?
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What are the components of a real capacitor?
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What is the purpose of bypass capacitors in RF circuits?
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What is a characteristic of a resonant circuit in RF amplifiers?
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What happens to the Q of a tuned circuit when it is loaded down?
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What is the effect of the Miller effect on an amplifier?
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What frequencies will the output of a mixer contain if the inputs are Va = sin(ωat) and Vb = sin(ωbt)?
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What is the characteristic of a balanced mixer's output?
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What does VFO stand for?
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What is a frequency synthesizer?
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What is a general guideline for conductor lengths in RF circuits?
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How should elements be considered at UHF frequencies and above?
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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.