Unit 2 Oscillator

8/6/2019 Unit 2 Oscillator

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UNIT 2

OSCILLATOR


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BLOCK DIAGRAM OF AN

OSCILLATOR

Oscillator is a device that convert D.C voltage into A.C voltage

without any external source at a particular frequency.

A.C outputD.C inputOSCILLATOR

Figure 2.1: Block diagram of an oscillator


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Oscillator is divided into two classes depending on their waveform :

i. Harmonic oscillator - the sine wave.ii. Non-harmonic oscillator - in the square wave, triangle wave, etc..

Figure 2.2: Harmonic oscillator

Harmonic

oscillator

Figure 2.3 : Non-harmonic oscillator

Non-harmonic oscillator


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REQUIREMENTSOFOSCILLATORCIRCUITS All the basic oscillator circuit consists of three parts:

i. Amplifierii. feedback

iii. Frequency Generation Circuits

keluaran

AMPLIFIER

FEEDBACKFREQUENCYGENERATION

CIRCUITS

Figure 2.4 : Block diagram an Oscillator circuit.


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AMPLIFIER

Generally, an amplifier or simply amp is a device for increasing thepower of a signal.

For electronic amplifier, the input "signal" is usually a voltage or a

current. In this case oscillator requires DC power supply to produce output.

That means this DC power supply is used as an input of the amplifier


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FEEDBACK

Feedback happens when part of the output signal supplied to theinput. Since the oscillator has no input signal, the feedback signalis the input signal for the amplifier in the oscillator circuit.

There are two principles of feedback

Negative feedback

Positive feedback.


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An oscillator using the principle of positive feedback. Figure 2.5 belowshows the basic block diagram of a feedback system used:

Vf

VeVi

Output

Amplifier

AV

Amplifier

Figure 2.5: Block diagram for the basic feedback

F

F = Feedback factorVf=FVo or AFVe

Af= formula for multiple amplifier with positive feedback

FA-1

A

V

V

i

o !

For oscillator it does not require the input voltage, so the input voltage should be Vi = 0.This means that the denominator in the equation | 1 - A F |must be zero or | AF | = 1.


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Let us look at some of the conditions listed below:

i. If | A F | = 1........

The value of Vf= A FVe will be Vf= (1)Ve or Vf= Ve.

That means that the voltage will be equal to the input voltagefeedback to the amplifier.

In this condition the oscillator will operate properly or the inputsignal will always oscillate.(See Figure 2.6 below)

t

Figure 2.6: Condition when| AF | = 1


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ii. If | A F |


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iii. If | A F |> 1..

Example: if | AF | = 2 then Vf= 2Ve. This means that the feedback voltage will be larger than the input

voltage to the amplifier.

In this case the output voltage will be larger until it reaches a stagewhere it will reach the saturation stage and will be cut off. (See

Figure 2.8 below)

t

Figure 2.8: Condition when | AF | > 1


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FREQUENCY GENERATION

CIRCUIT When the power is supply to the electronic circuit, noise signal

with a small value will exist at various frequencies.

These signals are raised and drive to the feedback network, whichconsisting of 'frequency generation circuit' (resonant circuit). But inthis situation, only the signal with the same frequency as theresonance frequency will be fed to the input.


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Generally, the frequency generation circuit of the oscillator in consumerproducts is divided into two:

Oscillator that generating audio frequency: RC oscillator(Resistance-Capacitor) which can produce medium and lowfrequency signals. Examples of types of RC oscillator are the

phase shift oscillator and Wein bridge oscillator.

Oscillator that generates radio frequency: LC oscillator (Inductor-Capacitor). It is also known as tank circuits. It is used to producehigh frequency signals (> 1MHz), and produces a stablefrequency. Examples of the type LC oscillator are Armstrong

Oscillator, Colpitts, Hartley and crystals


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RC oscillator

Phase shift oscillatorWein bridge oscillator

LC oscillator

Armstrong oscillatorColpitts oscillatorHartley oscillatorCrystal oscillator

Types of oscillator


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HARTLEYOSCILLATOR

.

Hartley oscillator is inductively coupled ; variable frequencyoscillators where the oscillator may be series or shunt fed.

Hartley oscillators have the advantage of having one centre tappedinductor and one tuning capacitor.

This arrangement simplifies the construction of a Hartley oscillatorcircuit.

Figure 2.9 : Simple hartley oscillator circuit


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By refering to figure 2.10. Transistorsand resistors R1, R2, R3 and R4 are a

combination of amplifier circuits .

Capacitors C3 and C4 are used tointercept signals shuttle to earth.

Amplifier will provide different phase ofthe output signal 180 degree

LC circuit in the feedback loop willproduce a phase shift of 180 degree.

So the feedback voltage will be in phasewith the input voltage on the transistor.

Figure 2.10 : Hartley oscillator circuit


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oscillation frequency

Formulas for the resonance frequency of the circuit are:

Since the capacitor in the tank circuit is connected in series, then:

Oscillation occurs when the feedback factor (F ) is given;

And conditions for the oscillation occurs is | AF | = 1

Therefore, to ensure oscillation start his own:

TCLfr

T2

1!

21 LLLT !

2

1

L

L

V

V

out

f!!F

1

2

L

LA

V!

2

1

C

CA V "


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COLPITSOSCILLATORCircuit connection

Figure 2.11 : Colpits oscillator circuit


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Operation of the circuit

Just look at the figure 2.11. Transistors and resistors R1, R2, R3and R4 are a combination of amplifier circuits.

Capacitors C3 and C4 are used to intercept signals shuttle toearth.

Amplifier will provide different phase of the output signal 180degree.

LC circuit in the feedback loop will produce a phase shift of 180

degree. So the feedback voltage will be in phase with the input voltage

on the transistor.


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Oscillation Frequency :

Oscillation frequency of the tank circuit L1, C1 and C2 are as follows:

Since the connections C1 and C2 in the tank circuit are in series, then:

Oscillation will occur when the feedback factor (F ) is given

TLC

fT2

1!

21

21.

CC

CCCT

!

1

2

2

1

C

C

X

X

V

V

C

C

out

f!!!F


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Continue Oscillation Frequency :

To turn off the oscillator that happens, |AF| should be equal to 1. Sothe voltage gain of the amplifier must be:

To ensure oscillation start his own:

2

1

CCAV !

2

1

C

CAV "


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CRYSTALOSCILLATOR

The most stable and precisely oscillator is the oscillator that using apiezoelectric crystal in a feedback circuit.

When a.c voltage is applied to the crystal shuttle, the mechanicalvibrations will occur and these vibrations have a natural resonantfrequencies that depend on the thickness of the crystal.

To obtain a high frequency crystal to be thinner.

Figure 2.12 : Symbol

Figure 2.13: Equivalent Electrical Circuit


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The electrical characteristic of crystals can be expressed with resonant circuit

in Figure 2.12 & Figure 2.13: Inductance , LH represents the mass of the crystal.

Capacitance, Cb represents the elasticity of the crystals.

Resistance, Rh represents the friction in the crystal structure.

Capacitor, Cm represents the capacitance of containers loaded crystal.

Figure 2.14: Crystal Oscillator Circuit


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Oscillation Frequency :

For crystal oscillator, It has two resonant frequencies.

The first frequency generated by the series circuit Rh-LH-Ch.

The second occurs when the frequency of series components same withCm

z

f2f1

f

Series resonance

Parallel resonance

hhCL

fT2

1

1!

CL

f

mT2

1

2!

hm

hm

CC

CCC

!

.

Figure 2.15: Resonant Frequencies Crystal Oscillator


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Operation of the circuit

Resistors R1, R2 and R3 are intended to provide bias voltage to thetransistor.

Capacitors C1 and C2 are used to forestall shuttle signals. Do youremember the connection configuration in the Common Emitteramplifier.

This configuration will provide different phase of the output signal180o.

Therefore the transformer Tr is selected so as to produce a phase shiftof 180o to get a feedback voltage in phase with the input transistors.

Usually the ratio between the coil windings L1 to L2 coils arranged sothat the product of the gain, A, the feedback factor,F , is a (|AF|= 1).


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Oscillation frequency:

Oscillation frequency is determined by the circuit L2.C2

given by:

22L2

1

Cf

T!


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PHASE SHIFTOSCILLATOR (RC)

Phase shift oscillator consists of amplifier and feedback network withthree RC circuits

Circuit Connection


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Circuit Operation:

The output signal from the amplifier have a different phase withthe input signal about 180 degree.

To generate a positive feedback signal, the output signal shouldbe phase shifted by 180 degree to make it in phase with theinput.

RC network produces a phase shift of 180 degree which each RCnetwork will shift phase by 90 degree.

Frequency of Oscillation:

62

1

RCf

T!

Unit 2 Oscillator

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