Analog Electronics – Problems Page 22
Oscillator Conceptual & GATE Level Problems
This section includes conceptual questions and solved problems on oscillator circuits useful for GATE and PSU examinations.
Problem 1
State the Barkhausen criterion for sustained oscillations.
Solution
The Barkhausen criterion states that for sustained oscillations:
|Aβ| = 1
and the total phase shift around the loop must be:
360° (or 0°)
This ensures that the feedback signal reinforces the input signal.
Problem 2
Why are LC oscillators used for high frequency applications?
Solution
- LC circuits have very low losses
- They provide high Q factor
- They can operate efficiently at radio frequencies
Hence LC oscillators are used in RF transmitters and communication systems.
Problem 3
Calculate the oscillation frequency of an LC oscillator if L = 2 mH and C = 50 pF.
Solution
Formula:
f = 1 / (2π√LC)
Substitute values:
f = 1 / (2π √(2×10⁻³ × 50×10⁻¹²))
Result:
f ≈ 503 kHz
Problem 4
What is the advantage of crystal oscillators compared to RC oscillators?
Solution
Crystal oscillators provide:
- Very high frequency stability
- Very high Q factor
- Low frequency drift
Hence they are used in digital clocks, microprocessors and communication systems.
Problem 5
Which oscillator is commonly used in audio frequency applications?
Solution
RC Phase Shift Oscillator
Because RC networks are suitable for low frequency ranges.
Problem 6
What happens if the loop gain Aβ is greater than 1?
Solution
- Oscillations grow in amplitude
- Signal distortion occurs
- The amplifier eventually saturates
Therefore practical circuits stabilize the gain.
Problem 7
What is the main function of the feedback network in an oscillator?
Solution
The feedback network:
- Provides the required phase shift
- Controls the oscillation frequency
- Maintains the feedback ratio β
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