HAM Practical Electronics Circuits 5 — Questions and Answers
Question 1: What is the primary function of a low-noise amplifier (LNA) placed at the front end of a receiver?
- To filter out-of-band interference before the antenna
- To establish the noise figure of the entire receive chain (Correct answer)
- To convert the RF signal to a lower intermediate frequency
- To provide automatic gain control for strong signals
Correct answer: To establish the noise figure of the entire receive chain
The first amplifier in a cascade dominates overall noise figure per Friis's formula, so an LNA with low noise figure minimizes the receiver's noise floor.
Question 2: In a Colpitts oscillator, the feedback network consists of:
- Two inductors and one capacitor forming an L-network
- A tapped inductor with a single capacitor
- Two capacitors forming a voltage divider with an inductor (Correct answer)
- A crystal in series with a feedback resistor
Correct answer: Two capacitors forming a voltage divider with an inductor
The Colpitts oscillator uses a capacitive voltage divider (two capacitors in series) with an inductor to set frequency and provide feedback.
Question 3: What does a spectrum analyzer display that a conventional oscilloscope cannot directly show?
- The instantaneous peak voltage of a waveform
- Signal amplitude as a function of frequency (Correct answer)
- The time-domain waveshape of a modulated carrier
- The DC operating point of an amplifier stage
Correct answer: Signal amplitude as a function of frequency
A spectrum analyzer plots amplitude versus frequency, revealing harmonics, spurious outputs, and modulation sidebands that are hidden in a time-domain oscilloscope view.
Question 4: Which type of logic family typically has the lowest power consumption at low switching frequencies?
- TTL (74xx series)
- ECL (emitter-coupled logic)
- CMOS (complementary MOS) (Correct answer)
- I²L (integrated injection logic)
Correct answer: CMOS (complementary MOS)
CMOS draws negligible static current because one transistor in each complementary pair is always off, making it far more power-efficient than bipolar logic families at low frequencies.
Question 5: What is the purpose of a tank circuit in an RF power amplifier output stage?
- To match the transistor's output impedance to the antenna
- To reconstruct the full RF waveform and filter harmonics (Correct answer)
- To provide DC blocking between the transistor and antenna
- To measure the output power delivered to the load
Correct answer: To reconstruct the full RF waveform and filter harmonics
The tank circuit stores energy and resonates at the carrier frequency, restoring the sinusoidal waveform from the amplifier's current pulses and suppressing harmonics.
Question 6: In a balanced modulator used for DSB-SC generation, what is cancelled at the output?
- Both sidebands, leaving only the carrier
- The carrier, leaving only the two sidebands (Correct answer)
- The upper sideband only
- The audio modulating signal
Correct answer: The carrier, leaving only the two sidebands
A balanced modulator is designed so the carrier signals from its two halves cancel at the output, producing a double-sideband suppressed-carrier (DSB-SC) signal.
Question 7: What is the role of a varactor diode in a voltage-controlled oscillator (VCO)?
- To provide temperature-compensated bias to the active device
- To vary the tank circuit capacitance with applied voltage, tuning the frequency (Correct answer)
- To stabilize the oscillator amplitude via automatic gain control
- To multiply the oscillator output frequency by an integer
Correct answer: To vary the tank circuit capacitance with applied voltage, tuning the frequency
A varactor's junction capacitance changes with reverse bias voltage, allowing electronic tuning of the LC tank circuit resonant frequency.
HAM Practical Electronics Circuits 5
What is the primary function of a low-noise amplifier (LNA) placed at the front end of a receiver?