HAM Practical Circuits and Components 2 — Questions and Answers
Question 1: What is the primary advantage of using a toroidal core inductor over an air-core inductor in an RF circuit?
- Higher Q factor at all frequencies
- Magnetic flux is contained within the core, reducing radiation (Correct answer)
- Lower series resistance
- Greater inductance per turn at VHF
Correct answer: Magnetic flux is contained within the core, reducing radiation
Toroidal cores confine the magnetic field within the ferrite or powdered-iron material, minimizing stray coupling and EMI radiation.
Question 2: A MOSFET used as an RF amplifier exhibits a gate-to-drain capacitance (Cgd). What circuit effect does this primarily cause?
- Increased input impedance
- Miller effect multiplying the effective input capacitance (Correct answer)
- Reduced drain current
- Improved linearity
Correct answer: Miller effect multiplying the effective input capacitance
The Miller effect multiplies Cgd by (1 + voltage gain), dramatically increasing the apparent input capacitance and affecting amplifier bandwidth.
Question 3: Which type of diode is specifically designed to operate in the reverse-breakdown region for voltage regulation?
- Schottky diode
- PIN diode
- Zener diode (Correct answer)
- Tunnel diode
Correct answer: Zener diode
Zener diodes are doped to have a precise, stable reverse breakdown voltage used for voltage regulation.
Question 4: In a crystal-controlled oscillator, what determines the fundamental resonant frequency of the quartz crystal?
- The external tank circuit capacitance
- The physical dimensions and cut angle of the crystal (Correct answer)
- The supply voltage
- The feedback resistor value
Correct answer: The physical dimensions and cut angle of the crystal
The resonant frequency of a quartz crystal is determined by its physical thickness and the crystallographic cut angle used during manufacturing.
Question 5: What is the function of a bypass capacitor placed at the power supply pin of an RF amplifier IC?
- To increase the gain of the amplifier
- To filter out RF signals from the audio output
- To provide a low-impedance path for RF currents to prevent feedback through the supply rail (Correct answer)
- To set the DC bias point of the transistor
Correct answer: To provide a low-impedance path for RF currents to prevent feedback through the supply rail
Bypass capacitors shunt RF currents to ground at the supply pin, preventing them from coupling back through the power supply and causing instability.
Question 6: A class-C RF power amplifier draws collector current only during approximately what portion of each cycle?
- More than 180 degrees
- Exactly 180 degrees
- Less than 180 degrees (Correct answer)
- The full 360 degrees
Correct answer: Less than 180 degrees
Class-C amplifiers conduct for less than 180° of the input cycle, yielding high efficiency but requiring a resonant tank circuit to restore the output waveform.
Question 7: What is the primary purpose of a ferrite bead placed on a component lead or wire in an RF circuit?
- To increase the DC current capacity of the wire
- To provide inductance that suppresses high-frequency parasitic oscillations (Correct answer)
- To act as a low-pass filter for audio signals
- To improve mechanical support of the component
Correct answer: To provide inductance that suppresses high-frequency parasitic oscillations
Ferrite beads present high impedance at RF frequencies, damping parasitic oscillations and suppressing unwanted high-frequency currents on leads.
HAM Practical Circuits and Components 2
What is the primary advantage of using a toroidal core inductor over an air-core inductor in an RF circuit?