Short Notes: Oscillators | Analog Circuits - Electronics and Communication Engineering (ECE) PDF Download

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Oscillators
Oscillators are electronic circuits that generate p erio dic signals (e.g., sine, square, or triangular w a v es)
without an external input. They are essen tial in applications lik e signal generation, clo c ks, and comm u-
nication systems.
1. In tro duction to Oscillators
An oscillator con v erts DC p o w er in to an A C signal b y sustaining oscillations through p ositiv e feedbac k.
The k ey requiremen ts for oscillation are:
• Barkhausen Criteria : F or sustained oscillations:
1. The lo op gain m ust b e unit y: |Aß| = 1 , where A is the amplifier gain and ß is the feedbac k
factor.
2. The total phase shift around the lo op m ust b e 0° or 360°.
2. T yp es of Oscillators
Oscillators are classified based on their output w a v eform or frequency-determining comp onen ts:
• Sin usoidal Oscillators : Generate sine w a v es (e.g., LC, crystal oscillators).
• Relaxation Oscillators : Generate non-sin usoidal w a v es (e.g., square, triangular w a v es).
3. LC Oscillators
LC oscillators use an inductor (L ) and capacitor (C ) to set the oscillation frequency . Common t yp es
include:
• Hartley Oscillator : Uses a tapp ed inductor or t w o inductors in the resonan t circuit.
• Colpitts Oscillator : Uses a tapp ed capacitor in the resonan t circuit.
The resonan t frequency is:
f
0
=
1
2p
v
LC
where L is the equiv alen t inductance and C is the equiv alen t capacitance.
Example (Colpitts) : The equiv alen t capacitance for t w o capacitors C
1
and C
2
in series is:
C =
C
1
C
2
C
1
+C
2
The frequency is:
f
0
=
1
2p
v
L·
C1C2
C1+C2
4. R C Oscillators
R C oscillators use resistors and capacitors for lo w- to medium-frequency applications. Common t yp es
include:
• Phase-Shift Oscillator : Uses an R C net w ork to pro vide a 180° phase shift, com bined with a 180°
shift from the amplifier.
• Wien Bridge Oscillator : Uses a balanced R C bridge for lo w distortion.
F or a Wien bridge oscillator, the oscillation frequency is:
f
0
=
1
2pRC
The gain m ust b e sligh tly greater than 3 to sustain oscillations, often stabilized using a dio de or A GC
circuit.
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Oscillators
Oscillators are electronic circuits that generate p erio dic signals (e.g., sine, square, or triangular w a v es)
without an external input. They are essen tial in applications lik e signal generation, clo c ks, and comm u-
nication systems.
1. In tro duction to Oscillators
An oscillator con v erts DC p o w er in to an A C signal b y sustaining oscillations through p ositiv e feedbac k.
The k ey requiremen ts for oscillation are:
• Barkhausen Criteria : F or sustained oscillations:
1. The lo op gain m ust b e unit y: |Aß| = 1 , where A is the amplifier gain and ß is the feedbac k
factor.
2. The total phase shift around the lo op m ust b e 0° or 360°.
2. T yp es of Oscillators
Oscillators are classified based on their output w a v eform or frequency-determining comp onen ts:
• Sin usoidal Oscillators : Generate sine w a v es (e.g., LC, crystal oscillators).
• Relaxation Oscillators : Generate non-sin usoidal w a v es (e.g., square, triangular w a v es).
3. LC Oscillators
LC oscillators use an inductor (L ) and capacitor (C ) to set the oscillation frequency . Common t yp es
include:
• Hartley Oscillator : Uses a tapp ed inductor or t w o inductors in the resonan t circuit.
• Colpitts Oscillator : Uses a tapp ed capacitor in the resonan t circuit.
The resonan t frequency is:
f
0
=
1
2p
v
LC
where L is the equiv alen t inductance and C is the equiv alen t capacitance.
Example (Colpitts) : The equiv alen t capacitance for t w o capacitors C
1
and C
2
in series is:
C =
C
1
C
2
C
1
+C
2
The frequency is:
f
0
=
1
2p
v
L·
C1C2
C1+C2
4. R C Oscillators
R C oscillators use resistors and capacitors for lo w- to medium-frequency applications. Common t yp es
include:
• Phase-Shift Oscillator : Uses an R C net w ork to pro vide a 180° phase shift, com bined with a 180°
shift from the amplifier.
• Wien Bridge Oscillator : Uses a balanced R C bridge for lo w distortion.
F or a Wien bridge oscillator, the oscillation frequency is:
f
0
=
1
2pRC
The gain m ust b e sligh tly greater than 3 to sustain oscillations, often stabilized using a dio de or A GC
circuit.
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5. Crystal Oscillators
Crystal oscillators use a piezo electric crystal (e.g., quartz) for high stabilit y . The resonan t frequency is
determined b y the crystal’s mec hanical prop erties:
f
0
=
1
2p
v
L
s
C
s
where L
s
and C
s
are the equiv alen t inductance and capac itance of the crystal.
6. Oscillator Design Considerations
• Startup : The lo op gain m ust b e sligh tly greater than 1 to initiate oscillations.
• Stabilit y : F requency stabilit y dep ends on comp onen t qualit y (e.g., crystal oscillators are highly
stable).
• Amplitude Con trol : Nonlinear elemen ts (e.g., dio des) or automatic gain con trol prev en t excessiv e
amplitude.
7. F requency Resp onse and Qualit y F actor
The qualit y factor (Q ) of the resonan t circuit affects the purit y of the output signal:
Q=
?
0
L
R
(for LC circuits)
Higher Q results in sharp er resonance and cleaner sin usoidal output.
8. Applications
• Signal generation in RF and audio systems.
• Clo c k signals in digital circuits.
• Lo cal oscillators in comm unication receiv ers.
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