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OscillatorsOscillators 1120-04-2320-04-23 05:23 AM05:23 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.

20-04-2320-04-23 05:23 AM05:23 AM OscillatorsOscillators 22

Oscillators Oscillators D.C. KulshreshthaD.C. Kulshreshtha


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.


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.


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 !


How is it Possible ?How is it Possible ?

Connecting point x to y, feedback voltage drives the amplifier.


• What happens to the output ?

• There are three possibilities.


(1) If Aβ < 1, we get decaying of damped oscillations.


(2) If Aβ > 1, we get growing oscillations.


(3) If Aβ = 1, we get sustained oscillations. In this case, the circuit supplies its own input signal.


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.


Tank CircuitTank Circuit• LC parallel circuit is called tank circuit.

• Once excited, it oscillates at


The energy keeps oscillating between electric potential energy and magnetic filed energy.


Damped oscillations are produced.


Tuned Collector Oscillator


Same circuit from ac point of view.


Tuned-Drain Oscillator


Building of oscillations using gate-leak biasing


How to take Output ?How to take Output ?


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.


Hartley Oscillator


• When the tank circuit resonates, the circulating current flows through L1 in series with L2. Hence the equivalent inductance is

The feedback factor is


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.


Colpitts Oscillator


Its AC Equivalent


Solution :


RCRC Oscillators Oscillators

Two types :Two types :1.1. RCRC Phase shift Oscillator. Phase shift Oscillator.

2.2. Wein Bridge Oscillator. Wein Bridge Oscillator.


RCRC Phase shift Oscillator Phase shift Oscillator(Using phase-lead circuits)(Using phase-lead circuits)


RCRC Phase shift Oscillator Phase shift Oscillator(Using phase-lag circuits)(Using phase-lag circuits)


• 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.


Wien Bridge OscillatorWien Bridge Oscillator


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.


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 :


• 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,


Solution :


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


Cm (mounting capacitance) = 3.5 pF;

Cs = 0.0235 pF; L = 137 H; R = 15 kΩ


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.


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..


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 484820-04-2320-04-23 05:23 AM05:23 AMThe fo is between 411 kHz and 412

kHz.


Crystal Oscillator Circuit.Crystal Oscillator Circuit.


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

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