GROL Amplifier and Oscillator Circuits 2 — Questions and Answers
Question 1: In a Colpitts oscillator, the frequency-determining tank circuit consists of:
- Two inductors and one capacitor
- Two capacitors and one inductor (Correct answer)
- One capacitor and two resistors
- Three resistors in a ladder network
Correct answer: Two capacitors and one inductor
The Colpitts oscillator uses a capacitive voltage divider formed by two capacitors and a single inductor as its tank circuit.
Question 2: What distinguishes a Hartley oscillator from a Colpitts oscillator?
- A Hartley oscillator uses two capacitors in its tank circuit
- A Hartley oscillator uses a tapped inductor in its tank circuit (Correct answer)
- A Hartley oscillator requires a crystal for frequency stability
- A Hartley oscillator uses only resistors and capacitors
Correct answer: A Hartley oscillator uses a tapped inductor in its tank circuit
The Hartley oscillator uses a tapped (center-tapped) coil as its inductive element, while the Colpitts uses two capacitors as its voltage divider.
Question 3: Why are crystal-controlled oscillators preferred in commercial radio transmitters?
- They generate higher output power than LC oscillators
- They provide superior frequency stability compared to LC oscillators (Correct answer)
- They require no external power supply
- They automatically adjust to changing channel conditions
Correct answer: They provide superior frequency stability compared to LC oscillators
Crystal oscillators exploit the piezoelectric properties of quartz to achieve very high frequency stability compared to LC circuits.
Question 4: The Barkhausen criterion states that for sustained oscillation, the total loop gain must be:
- Greater than unity with exactly 90° of phase shift
- Equal to unity with a total phase shift of 0° or 360° (Correct answer)
- Less than unity with any phase shift
- Equal to zero
Correct answer: Equal to unity with a total phase shift of 0° or 360°
For sustained oscillation, the amplifier gain must exactly compensate for feedback network losses, and the total phase shift around the loop must be 0° or a multiple of 360°.
Question 5: Which oscillator circuit is commonly used with a quartz crystal because it requires only one active device and connects the crystal directly in the feedback path?
- Colpitts oscillator
- Hartley oscillator
- Pierce oscillator (Correct answer)
- Clapp oscillator
Correct answer: Pierce oscillator
The Pierce oscillator connects the crystal directly in the feedback path between drain and gate with minimal external components, making it very practical for crystal-controlled designs.
Question 6: In a three-section RC phase-shift oscillator, what is the total phase shift contributed by the RC ladder network?
- 90°
- 180° (Correct answer)
- 270°
- 360°
Correct answer: 180°
Each RC section provides 60° of phase shift; three sections together provide 180°, and the amplifier's inversion provides the remaining 180° for the required 360° total.
Question 7: A Clapp oscillator improves on the Colpitts design primarily by:
- Replacing the transistor with a vacuum tube for higher stability
- Adding a series capacitor with the inductor to reduce the effect of transistor capacitance variations (Correct answer)
- Using a crystal as the primary frequency element
- Eliminating the need for a feedback network
Correct answer: Adding a series capacitor with the inductor to reduce the effect of transistor capacitance variations
The Clapp oscillator adds a small series capacitor to the inductor in the tank circuit, isolating the oscillation frequency from transistor parasitic capacitance changes.
GROL Amplifier and Oscillator Circuits 2
In a Colpitts oscillator, the frequency-determining tank circuit consists of: