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