Why this topic is important for ECET?
Oscillators are the backbone of electronic circuits used in communication, computers, signal generation, and embedded systems.
In ECET, you’ll face conceptual + formula-based problems from oscillators, especially on Barkhausen criterion, frequency of oscillation, and circuit types (RC, LC, Crystal).
Mastering this topic ensures fast scoring in both objective and numerical questions.
📘 Concept Notes
1. What is an Oscillator?
An oscillator is an electronic circuit that generates continuous AC signals (sinusoidal or non-sinusoidal) without any external input.
It uses positive feedback + an amplifier.
Key role: Converts DC power → AC signal.
2. Classification of Oscillators
- Sinusoidal Oscillators
- Produce sine waves.
- Examples: RC Phase Shift, Wien Bridge, Hartley, Colpitts, Crystal Oscillator.
- Non-Sinusoidal Oscillators
- Produce square, triangular, or sawtooth waveforms.
- Examples: Multivibrators, Relaxation Oscillators.
3. Barkhausen Criterion
For sustained oscillations:
- Loop Gain Condition:
Phase Condition:
Where:
- A = Amplifier gain
- β = Feedback factor
4. Types of Oscillators
(a) RC Phase Shift Oscillator
- Uses resistor-capacitor networks for feedback.
- Provides 180° phase shift in amplifier + 180° in RC network = 360°.
- Frequency of oscillation:
(b) Wien Bridge Oscillator
- Uses Wien bridge network.
- Produces low distortion sine wave.
- Frequency of oscillation:
(c) Hartley Oscillator
- Uses two inductors + one capacitor.
- Frequency:
(d) Colpitts Oscillator
- Uses two capacitors + one inductor.
- Frequency:
where
(e) Crystal Oscillator
- Uses quartz crystal for stable frequency.
- Frequency:
⚙️ Formulas
- Barkhausen Criterion: ,
- RC Phase Shift Oscillator:
Wien Bridge Oscillator:
Hartley Oscillator:
Colpitts Oscillator:
Crystal Oscillator:
🔟 10 MCQs
Q1. The condition for sustained oscillations in an oscillator is:
a) |Aβ| < 1
b) |Aβ| = 1 and phase = 0°
c) |Aβ| > 1
d) Phase shift = 90°
Q2. Frequency of RC Phase Shift Oscillator is given by:
a) 
b) 
c) 
d) 
Q3. A Wien bridge oscillator uses:
a) 2 capacitors + 1 inductor
b) RC network
c) Quartz crystal
d) Transformer
Q4. In a Hartley oscillator, effective inductance is:
a) L1 × L2
b) L1 + L2
c) L1 / L2
d) L1 – L2
Q5. The Colpitts oscillator equivalent capacitance is:
a) 
b) 
c) 
d) 
Q6. An RC phase shift oscillator requires minimum amplifier gain of:
a) 1
b) 8
c) 29
d) 100
Q7. The frequency of a Wien bridge oscillator with R = 10 kΩ and C = 0.01 μF is:
a) 159 Hz
b) 1.59 kHz
c) 15.9 kHz
d) 159 kHz
Q8. Which oscillator provides the most frequency stability?
a) RC oscillator
b) LC oscillator
c) Crystal oscillator
d) Wien bridge oscillator
Q9. An oscillator is essentially:
a) Rectifier
b) Amplifier with feedback
c) Multiplier
d) Attenuator
Q10. For Colpitts oscillator with L = 10 mH, C1 = 100 pF, C2 = 100 pF, find f.
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | b |
| Q2 | b |
| Q3 | b |
| Q4 | b |
| Q5 | b |
| Q6 | c |
| Q7 | b |
| Q8 | c |
| Q9 | b |
| Q10 | 356 kHz |
🧠 Explanations
- Q1: Barkhausen criterion: |Aβ| = 1, phase = 0°.
- Q2: RC Phase Shift →
. - Q3: Wien bridge uses RC network.
- Q4: Hartley oscillator → effective L = L1 + L2.
- Q5: Colpitts equivalent capacitance = (C1C2)/(C1+C2).
- Q6: For RC phase shift oscillator → min gain = 29.
- Q7:
. - Q8: Crystal oscillator → most stable frequency.
- Q9: Oscillator = amplifier with positive feedback.
- Q10:
.
.
🎯 Motivation / Why Practice Matters
Oscillators form the heart of communication systems and ECET 2026 often asks tricky numericals.
Practicing oscillator problems gives you:
- Speed in solving frequency questions.
- Accuracy in applying Barkhausen criterion.
- Competitive edge since oscillators are GATE-level scoring topics included in ECET.
📲 CTA
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