HAM Practical Electronics Circuits 3 — Questions and Answers
Question 1: When using a field-effect transistor (FET) as an RF amplifier, what is the advantage of its high input impedance?
- It allows the circuit to operate at lower supply voltages
- It minimizes loading of the signal source or preceding stage (Correct answer)
- It increases the amplifier's output power
- It reduces thermal noise in the drain circuit
Correct answer: It minimizes loading of the signal source or preceding stage
A high input impedance prevents the FET gate from loading down the signal source, preserving signal amplitude and avoiding bandwidth degradation.
Question 2: What is the function of a limiter stage in an FM receiver's IF chain?
- To amplify weak signals above the noise floor
- To remove amplitude variations before the discriminator (Correct answer)
- To shift the IF frequency to baseband
- To filter adjacent channel interference
Correct answer: To remove amplitude variations before the discriminator
The limiter clips amplitude variations from the IF signal so the discriminator only responds to frequency changes, rejecting AM noise and interference.
Question 3: In a crystal-controlled oscillator, what determines the operating frequency?
- The value of the feedback capacitors only
- The mechanical resonance of the quartz crystal (Correct answer)
- The supply voltage applied to the active device
- The Q factor of the tank inductor
Correct answer: The mechanical resonance of the quartz crystal
The piezoelectric quartz crystal's mechanical resonant frequency determines the oscillation frequency with very high stability.
Question 4: What effect does increasing the emitter resistor value have in a BJT common-emitter amplifier without a bypass capacitor?
- Gain increases and input impedance decreases
- Gain decreases and stability increases (Correct answer)
- Gain increases and distortion increases
- No effect on gain since it only affects the DC bias
Correct answer: Gain decreases and stability increases
An unbypassed emitter resistor applies negative feedback, reducing voltage gain but improving linearity and thermal stability.
Question 5: Which type of amplifier class is most efficient for RF power amplifiers transmitting constant-envelope signals such as FM?
- Class A
- Class AB
- Class C (Correct answer)
- Class B
Correct answer: Class C
Class C amplifiers can exceed 80% efficiency by conducting for less than half a cycle, and the tank circuit restores the full waveform for constant-envelope signals.
Question 6: What is the primary cause of intermodulation distortion (IMD) in an amplifier?
- Thermal noise generated in the transistor junction
- Nonlinear transfer characteristics of the active device (Correct answer)
- Impedance mismatch between stages
- Insufficient supply voltage decoupling
Correct answer: Nonlinear transfer characteristics of the active device
Nonlinearities in the amplifier's transfer function cause input signals to mix, producing sum and difference frequency products known as intermodulation products.
Question 7: A toroid core is preferred over an air-core inductor for HF applications primarily because it:
- Has lower DC resistance in the winding
- Confines the magnetic field internally, reducing radiation and coupling (Correct answer)
- Resonates at a higher self-resonant frequency
- Provides higher inductance per turn at VHF
Correct answer: Confines the magnetic field internally, reducing radiation and coupling
The closed magnetic path of a toroid core concentrates flux inside the core, minimizing stray coupling to nearby components and reducing EMI.
HAM Practical Electronics Circuits 3
When using a field-effect transistor (FET) as an RF amplifier, what is the advantage of its high input impedance?