GROL - General Radiotelephone Operator License Amplifier and Oscillator Circuits Questions and Answers 1 — Questions and Answers
Question 1: For an LC oscillator to begin and sustain oscillation, which condition must be met according to the Barkhausen criterion?
- The total phase shift around the feedback loop must be 0° or 360°, and the loop gain must be slightly greater than 1. (Correct answer)
- The total phase shift around the feedback loop must be 180°, and the loop gain must be exactly 1.
- The feedback signal must be negative, and the loop gain must be less than 1.
- The amplifier must be biased for Class A operation, and the feedback network must have a 90° phase shift.
Correct answer: The total phase shift around the feedback loop must be 0° or 360°, and the loop gain must be slightly greater than 1.
The Barkhausen criterion states two conditions for sustained oscillation: 1) The total phase shift around the amplifier and feedback loop must be 0° or an integer multiple of 360°. 2) The magnitude of the loop gain (|Aβ|) must be equal to or greater than 1. In practice, the gain must be slightly greater than 1 to initiate the oscillation, which then stabilizes at 1 as the amplitude builds.
Question 2: A technician is designing an RF power amplifier for a continuous wave (CW) transmitter. To achieve the highest possible efficiency, which class of amplifier should be selected?
- Class A
- Class B
- Class C (Correct answer)
- Class AB
Correct answer: Class C
Class C amplifiers offer the highest theoretical efficiency (approaching 90%) because the active device (transistor) conducts for less than 180° of the input signal cycle. While this creates high distortion, the use of a tuned LC tank circuit at the output filters out harmonics, making it ideal for high-efficiency RF applications like CW transmitters where linearity is not the primary concern.
Question 3: What is the primary distinguishing feature between a Hartley oscillator and a Colpitts oscillator?
- A Hartley oscillator uses a tapped capacitor in its tank circuit, while a Colpitts uses a tapped inductor.
- A Hartley oscillator is used for audio frequencies, while a Colpitts is exclusively for RF.
- A Colpitts oscillator uses a resistive feedback network, while a Hartley uses a transformer.
- A Hartley oscillator uses a tapped inductor for feedback, while a Colpitts oscillator uses a tapped capacitive divider. (Correct answer)
Correct answer: A Hartley oscillator uses a tapped inductor for feedback, while a Colpitts oscillator uses a tapped capacitive divider.
The fundamental difference lies in how the feedback signal is developed in the LC tank circuit. A Hartley oscillator uses an inductive voltage divider (a single tapped inductor or two separate inductors) to provide feedback. A Colpitts oscillator, conversely, uses a capacitive voltage divider (two capacitors in series) to provide the necessary feedback.
Question 4: In an Armstrong oscillator circuit, how is the essential positive feedback provided to sustain oscillations?
- Through a capacitive voltage divider connected between the collector and base.
- Through an RC phase-shift network that provides a 180° phase shift.
- Through magnetic coupling between a "tickler" coil in the output circuit and the tank coil in the input circuit. (Correct answer)
- Through a direct connection from the emitter output back to the base input.
Correct answer: Through magnetic coupling between a "tickler" coil in the output circuit and the tank coil in the input circuit.
The Armstrong oscillator, also known as a tickler oscillator, achieves feedback through mutual inductance. A transformer magnetically couples energy from the output (collector or drain) circuit back to the input (base or gate) LC tank circuit. The phasing of the transformer windings is critical to ensure the feedback is positive (regenerative).
Question 5: A technician observes that the output of a newly constructed oscillator has significant harmonic distortion and is not a pure sine wave. Which of the following is the most likely cause?
- The resistance in the biasing network is too low.
- The loop gain of the oscillator is significantly greater than 1. (Correct answer)
- The power supply voltage is too low, causing clipping.
- The feedback network is providing negative feedback instead of positive.
Correct answer: The loop gain of the oscillator is significantly greater than 1.
For a pure sine wave, the Barkhausen criterion requires the loop gain to be exactly unity. In practice, the gain is made slightly greater than 1 to start the oscillation. If the loop gain is excessively high, the amplitude of the oscillations will build until the amplifier begins to clip or operate in a non-linear region, which introduces harmonics and distorts the sinusoidal output.
Question 6: Which of the following describes the effect of negative feedback in an amplifier circuit, as opposed to the positive feedback used in an oscillator?
- It increases the overall gain and decreases stability.
- It reduces gain but improves stability and linearity. (Correct answer)
- It causes the amplifier to oscillate at a specific frequency.
- It narrows the amplifier's bandwidth and increases distortion.
Correct answer: It reduces gain but improves stability and linearity.
Negative feedback involves feeding a portion of the output signal back to the input out-of-phase. This process reduces the overall amplifier gain but provides significant benefits, including improved stability, reduced distortion, lower noise, and increased bandwidth. This is in direct contrast to positive feedback, which increases gain and can lead to instability and oscillation.
GROL - General Radiotelephone Operator License Amplifier and Oscillator Circuits Questions and Answers 1
For an LC oscillator to begin and sustain oscillation, which condition must be met according to the Barkhausen criterion?