HAM Practical Circuits and Components 5 — Questions and Answers
Question 1: In a Colpitts oscillator, what elements form the feedback voltage divider?
- Two inductors in series
- Two capacitors in series across the tank inductor (Correct answer)
- A resistor and capacitor network
- A transformer with tapped secondary
Correct answer: Two capacitors in series across the tank inductor
The Colpitts oscillator uses a capacitive voltage divider (two capacitors in series) tapped across the tank inductor to feed back the correct phase and amplitude.
Question 2: What is the significance of the '1 dB compression point' specification for an RF amplifier?
- The input power level where the noise figure degrades by 1 dB
- The output power level where gain has dropped 1 dB below the linear gain value due to saturation (Correct answer)
- The frequency where the amplifier gain drops 3 dB from maximum
- The power level where the SWR reaches 2:1
Correct answer: The output power level where gain has dropped 1 dB below the linear gain value due to saturation
The 1 dB compression point marks the onset of significant non-linearity — above this level, gain compresses and distortion products increase rapidly.
Question 3: Why is a low-pass filter connected between a transmitter and its antenna?
- To improve the transmitter's frequency stability
- To attenuate harmonic and spurious emissions that could interfere with other services (Correct answer)
- To increase the transmitter output power
- To protect the receiver from strong transmit signals
Correct answer: To attenuate harmonic and spurious emissions that could interfere with other services
Low-pass filters suppress harmonics and other spurious signals above the transmit frequency, keeping the transmitted spectrum within FCC limits.
Question 4: What happens to the resonant frequency of an LC tank circuit if the inductor core is partially removed?
- Resonant frequency decreases because inductance increases
- Resonant frequency increases because inductance decreases (Correct answer)
- Resonant frequency remains unchanged
- Resonant frequency depends only on the capacitor value
Correct answer: Resonant frequency increases because inductance decreases
Removing core material reduces the effective permeability, lowering inductance; since f = 1/(2π√LC), lower L raises the resonant frequency.
Question 5: A cascode amplifier consists of what transistor configuration?
- Two common-emitter stages in series
- A common-emitter stage driving a common-base stage (Correct answer)
- A common-collector stage driving a common-emitter stage
- Two common-base stages in parallel
Correct answer: A common-emitter stage driving a common-base stage
The cascode pairs a common-emitter (or common-source) input with a common-base (common-gate) output, reducing the Miller effect and extending bandwidth while maintaining gain.
Question 6: What is the primary purpose of a step-start or soft-start circuit in an HF linear amplifier power supply?
- To regulate the output voltage under varying load conditions
- To limit inrush current through filter capacitors when power is first applied, protecting rectifiers and fuses (Correct answer)
- To provide automatic shutdown if the SWR is too high
- To reduce audio hum on the output voltage
Correct answer: To limit inrush current through filter capacitors when power is first applied, protecting rectifiers and fuses
Large filter capacitors draw a very high inrush current when uncharged; a step-start circuit inserts a series resistor initially then bypasses it, limiting stress on components.
Question 7: Which semiconductor device exhibits negative resistance and is used in microwave oscillator circuits?
- Zener diode
- Schottky diode
- Gunn diode (Correct answer)
- PIN diode
Correct answer: Gunn diode
The Gunn diode exhibits negative resistance due to the transferred-electron effect in gallium arsenide, sustaining oscillations in microwave resonant cavities.
HAM Practical Circuits and Components 5
In a Colpitts oscillator, what elements form the feedback voltage divider?