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OscillatorsOscillators 1119-04-2319-04-23 06:35 AM06:35 AM
Thought of the DayThought of the Day
Expect to be successful,Expect to be successful,
expect to be liked,expect to be liked,
expect to be popular expect to be popular everywhere you go.everywhere you go.
19-04-2319-04-23 06:35 AM06:35 AM OscillatorsOscillators 22
Oscillators Oscillators D.C. KulshreshthaD.C. Kulshreshtha
OscillatorsOscillators 3319-04-2319-04-23 06:35 AM06:35 AM
Need of an OscillatorNeed of an Oscillator An oscillator circuit is capable of producing An oscillator circuit is capable of producing
ac voltage of desired frequency and ac voltage of desired frequency and waveshape.waveshape.
To test performance of electronic circuits, it To test performance of electronic circuits, it is called is called signal generator.signal generator.
It can produce square, pulse, triangular, or It can produce square, pulse, triangular, or sawtooth waveshape.sawtooth waveshape.
High frequency oscillator are used in High frequency oscillator are used in broadcasting.broadcasting.
Microwave ovenMicrowave oven uses an oscillator. uses an oscillator. Used for Used for induction heatinginduction heating and and dielectric dielectric
heating. heating.
OscillatorsOscillators 4419-04-2319-04-23 06:35 AM06:35 AM
Types of Oscillators Types of Oscillators Sinusoidal or non-sinusoidal.Sinusoidal or non-sinusoidal. An oscillator generating square wave or An oscillator generating square wave or
a pulse train is called a pulse train is called multivibratormultivibrator : :1.1. Bistable multivibrator (Flip-Flop Circuit).Bistable multivibrator (Flip-Flop Circuit).
2.2. Monostable multivibrator.Monostable multivibrator.
3.3. Astable multivibrator (Free-running).Astable multivibrator (Free-running). Depending upon type of feedback, we Depending upon type of feedback, we
havehave1.1. Tuned Circuit (Tuned Circuit (LCLC) oscillators. ) oscillators.
2.2. RCRC oscillators, and oscillators, and
3.3. Crystal oscillators. Crystal oscillators.
OscillatorsOscillators 5519-04-2319-04-23 06:35 AM06:35 AM
Using Positive FeedbackUsing Positive Feedback• The gain with positive feedback is given as
• By making 1 – Aβ = 0, or Aβ = 1, we get gain as infinity.
• This condition (Aβ = 1) is known as Barkhausen Criterion of oscillations.
• It means you get output without any input !
OscillatorsOscillators 6619-04-2319-04-23 06:35 AM06:35 AM
How is it Possible ?How is it Possible ?
Connecting point x to y, feedback voltage drives the amplifier.
OscillatorsOscillators 7719-04-2319-04-23 06:35 AM06:35 AM
• What happens to the output ?
• There are three possibilities.
OscillatorsOscillators 8819-04-2319-04-23 06:35 AM06:35 AM
(1) If Aβ < 1, we get decaying of damped oscillations.
OscillatorsOscillators 9919-04-2319-04-23 06:35 AM06:35 AM
(2) If Aβ > 1, we get growing oscillations.
OscillatorsOscillators 101019-04-2319-04-23 06:35 AM06:35 AM
(3) If Aβ = 1, we get sustained oscillations. In this case, the circuit supplies its own input signal.
OscillatorsOscillators 111119-04-2319-04-23 06:35 AM06:35 AM
Wherefrom comes the starting Wherefrom comes the starting voltage ?voltage ?
Each resistor is a noise generator.Each resistor is a noise generator.The feedback network is a resonant The feedback network is a resonant
circuit giving maximum feedback circuit giving maximum feedback voltage at frequency voltage at frequency ff00, providing , providing phase shift of 0° only at this frequency.phase shift of 0° only at this frequency.
The initial loop gain The initial loop gain AAββ > 1. > 1.The oscillations build up only at this The oscillations build up only at this
frequency.frequency.After the desired output is reached, After the desired output is reached, AAββ
reduces to unity. reduces to unity.
OscillatorsOscillators 121219-04-2319-04-23 06:35 AM06:35 AM
Tank CircuitTank Circuit• LC parallel circuit is called tank circuit.
• Once excited, it oscillates at
OscillatorsOscillators 131319-04-2319-04-23 06:35 AM06:35 AM
OscillatorsOscillators 141419-04-2319-04-23 06:35 AM06:35 AM
The energy keeps oscillating between electric potential energy and magnetic filed energy.
OscillatorsOscillators 151519-04-2319-04-23 06:35 AM06:35 AM
Damped oscillations are produced.
OscillatorsOscillators 161619-04-2319-04-23 06:35 AM06:35 AM
Tuned Collector Oscillator
OscillatorsOscillators 171719-04-2319-04-23 06:35 AM06:35 AM
Same circuit from ac point of view.
OscillatorsOscillators 181819-04-2319-04-23 06:35 AM06:35 AM
Tuned-Drain Oscillator
OscillatorsOscillators 191919-04-2319-04-23 06:35 AM06:35 AM
Building of oscillations using gate-leak biasing
OscillatorsOscillators 202019-04-2319-04-23 06:35 AM06:35 AM
How to take Output ?How to take Output ?
OscillatorsOscillators 212119-04-2319-04-23 06:35 AM06:35 AM
OscillatorsOscillators 222219-04-2319-04-23 06:35 AM06:35 AM
Hartley OscillatorHartley Oscillator
Note that in the collector-tuned Note that in the collector-tuned circuit, two inductor coils are used.circuit, two inductor coils are used.
One end of these coils is grounded.One end of these coils is grounded. If we make the If we make the tickler coiltickler coil an integral an integral
part of the circuit, we get Hartley part of the circuit, we get Hartley Oscillator. Oscillator.
OscillatorsOscillators 232319-04-2319-04-23 06:35 AM06:35 AM
Hartley Oscillator
OscillatorsOscillators 242419-04-2319-04-23 06:35 AM06:35 AM
• When the tank circuit resonates, the circulating current flows through L1 in series with L2. Hence the equivalent inductance is
The feedback factor is
OscillatorsOscillators 252519-04-2319-04-23 06:35 AM06:35 AM
Colpitts OscillatorColpitts Oscillator
An excellent circuit.An excellent circuit.Widely used in commercial signal Widely used in commercial signal
generators.generators.Uses two capacitors instead of the Uses two capacitors instead of the
inductive voltage divider. inductive voltage divider.
OscillatorsOscillators 262619-04-2319-04-23 06:35 AM06:35 AM
Colpitts Oscillator
OscillatorsOscillators 272719-04-2319-04-23 06:35 AM06:35 AM
Its AC Equivalent
OscillatorsOscillators 282819-04-2319-04-23 06:35 AM06:35 AM
OscillatorsOscillators 292919-04-2319-04-23 06:36 AM06:36 AM
Solution :
OscillatorsOscillators 303019-04-2319-04-23 06:36 AM06:36 AM
OscillatorsOscillators 313119-04-2319-04-23 06:36 AM06:36 AM
RCRC Oscillators Oscillators
Two types :Two types :1.1. RCRC Phase shift Oscillator. Phase shift Oscillator.
2.2. Wein Bridge Oscillator. Wein Bridge Oscillator.
OscillatorsOscillators 323219-04-2319-04-23 06:36 AM06:36 AM
RCRC Phase shift Oscillator Phase shift Oscillator(Using phase-lead circuits)(Using phase-lead circuits)
OscillatorsOscillators 333319-04-2319-04-23 06:36 AM06:36 AM
RCRC Phase shift Oscillator Phase shift Oscillator(Using phase-lag circuits)(Using phase-lag circuits)
OscillatorsOscillators 343419-04-2319-04-23 06:36 AM06:36 AM
• A phase-lead or phases-lag circuit A phase-lead or phases-lag circuit can provide phase shift between 0° and can provide phase shift between 0° and 90°.90°.• For total phase shift 180°, we use For total phase shift 180°, we use three identical sections each giving a three identical sections each giving a phase shift of 60°.phase shift of 60°.
&
• It means in the beginning the gain of the FET amplifier must be greater than 29.
• Not very popular, as the frequency cannot be adjusted over large range.
OscillatorsOscillators 353519-04-2319-04-23 06:36 AM06:36 AM
Wien Bridge OscillatorWien Bridge Oscillator
OscillatorsOscillators 363619-04-2319-04-23 06:36 AM06:36 AM
The two arms on the left of the bridge The two arms on the left of the bridge make lead-lag circuit.make lead-lag circuit.
The two arms on the right, are 2The two arms on the right, are 2RRt t and and RRtt, , making a potential divider.making a potential divider.
It has both positive and negative feedback It has both positive and negative feedback paths.paths.
Initially, when switched on, there is more Initially, when switched on, there is more positive feedback than negative feedback.positive feedback than negative feedback.
Oscillations build up.Oscillations build up.Negative feedback increases, making Negative feedback increases, making Aβ
= 1.
OscillatorsOscillators 373719-04-2319-04-23 06:36 AM06:36 AM
The reason why the loop gain reduces The reason why the loop gain reduces to unity :to unity :
1.1. Initially tungsten lamp has low resistance; Initially tungsten lamp has low resistance; giving low negative feedback.giving low negative feedback.
2.2. Thus, loop gain Thus, loop gain Aβ is greater than unity.3. As oscillations are built up, the tungsten
lamp heats up increasing its resistance.4. Negative feedback increase to make Aβ =
1. With sustained oscillations, the
resistance of the lamp increases to exactly RRt t , so that the gain becomes :, so that the gain becomes :
OscillatorsOscillators 383819-04-2319-04-23 06:36 AM06:36 AM
• At resonance, the voltage ratio or feedback factor of the lead-lag circuit is 1/3.
• Therefore, loop gain becomes unity.
• The oscillation frequency is the same as that of the lead-lag circuit,
OscillatorsOscillators 393919-04-2319-04-23 06:36 AM06:36 AM
OscillatorsOscillators 404019-04-2319-04-23 06:36 AM06:36 AM
Solution :
OscillatorsOscillators 414119-04-2319-04-23 06:36 AM06:36 AM
Crystal OscillatorCrystal OscillatorUsed when accuracy and stability of Used when accuracy and stability of ffoo is is
utmost important.utmost important.Where do you need such high stability Where do you need such high stability
of frequency of oscillations ?of frequency of oscillations ? Instead of an inductor, it uses a crystal Instead of an inductor, it uses a crystal
of of quartz, tourmalinequartz, tourmaline, or , or Rochelle saltRochelle salt. . Piezoelectric effect.Piezoelectric effect.The crystal is suitably cut and then The crystal is suitably cut and then
mounted between two metallic plates.mounted between two metallic plates.The fundamental frequency is given as The fundamental frequency is given as
OscillatorsOscillators 424219-04-2319-04-23 06:36 AM06:36 AM
Cm (mounting capacitance) = 3.5 pF;
Cs = 0.0235 pF; L = 137 H; R = 15 kΩ
OscillatorsOscillators 434319-04-2319-04-23 06:36 AM06:36 AM
Crystals have incredibly high Crystals have incredibly high Q.Q.For the given values, For the given values, QQ = 5500. = 5500.QQ as high as 100 000 can be as high as 100 000 can be
possible.possible.An An LC LC circuit has circuit has Q Q not greater than not greater than
100.100.The extremely high value of The extremely high value of Q Q
makes makes ffoo highly stable. highly stable.
OscillatorsOscillators 444419-04-2319-04-23 06:36 AM06:36 AM
OscillatorsOscillators 454519-04-2319-04-23 06:36 AM06:36 AM
Series and Parallel ResonanceSeries and Parallel ResonanceFirst, resonance occurs at First, resonance occurs at ffss for the for the
series combination of series combination of LL and and CCss..
Above Above ffss the series branch the series branch LCLCssR R has has inductive reactance.inductive reactance.
It then resonates at It then resonates at ffp p , with , with CCmm..For this parallel resonance, For this parallel resonance,
equivalent series capacitance is equivalent series capacitance is CCpp..
OscillatorsOscillators 464619-04-2319-04-23 06:36 AM06:36 AM
OscillatorsOscillators 474719-04-2319-04-23 06:36 AM06:36 AM
Normally, Cs is much smaller than Cm.
Therefore, Cp is slightly less than Cs.
Hence, the frequency fp is slightly greater than fs.
The crystal is inductive only between the frequencies fs and fp.
The frequency of oscillation must lie between these frequencies.
Hence the stability.
OscillatorsOscillators 484819-04-2319-04-23 06:36 AM06:36 AMThe fo is between 411 kHz and 412
kHz.
OscillatorsOscillators 494919-04-2319-04-23 06:36 AM06:36 AM
Crystal Oscillator Circuit.Crystal Oscillator Circuit.
OscillatorsOscillators 505019-04-2319-04-23 06:36 AM06:36 AM
ReviewReview Need of an Oscillator.Need of an Oscillator. Types of Oscillators .Types of Oscillators . Using Positive Using Positive
Feedback.Feedback. Barkhausen Criterion of
Oscillations. Starting Voltage .Starting Voltage . Tank Circuit.Tank Circuit.Tuned Collector
Oscillator. Tuned-Drain Oscillator.
Hartley Oscillator.Hartley Oscillator.
Colpitts Oscillator.
RCRC Phase Shift Phase Shift Oscillator.Oscillator.
Wien Bridge Wien Bridge Oscillator.Oscillator.
Crystal Oscillator.Crystal Oscillator.
Series and Parallel Series and Parallel ResonanceResonance