CH 5 - Oscillator

8/13/2019 CH 5 - Oscillator

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EE T43: Electronic circuits


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Objectives

Describe the basic concept of an oscillator

Discuss the basic principles of operation of anoscillator

Analyze the operation of RC and LC oscillators

Describe the operation of the basic relaxationoscillator & Crystal Oscillator circuits


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Introduction

Oscillator is an electronic circuit that generates a

periodic waveform on its output without an

external signal source. It is used to convert dc to ac.

Oscillators are circuits that produce a continuous

signal of some type without the need of an input.These signals serve a variety of purposes.

Communications systems, digital systems

(including computers), and test equipment make useof oscillators


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Introduction

An oscillator is a circuit that produces a repetitive signal from

a dc voltage.

The feedback oscillator relies on a positive feedback of theoutput to maintain the oscillations.

The relaxation oscillator makes use of an RC timing circuit togenerate a nonsinusoidal signal such as square wave

Sine wave

Square wave

Sawtooth wave


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In this chapter we will explore the working principle of theoscillator. Generally speaking, the oscillator produces sinusoidal andother waveforms.

Beginning with a detailed circuit analysis of the oscillator, we will

proceed to discuss the conditions and frequency of oscillation. Following this, the different types of oscillatorsTuned

oscillator, Hartley oscillator, Colpitts oscillator, Clapposcillator, Phase-shift oscillator, Crystal oscillator and Wien-

bridge oscillator

will be examined with detailed mathematicalanalysis and illustrations.

The chapter ends with an overview of the applications of theoscillator.

Objectives:


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Oscillators are classified based on the type of the outputwaveform.

If the generated waveform is sinusoidal or close to sinusoidal (with a

certain frequency) then the oscillator is said to be a Sinusoidal

Oscillator.If the output waveform is non-sinusoidal, which refers to square/saw-

tooth waveforms, the oscillator is said to be a

Relaxation Oscillator. An oscillator has a positive feedback with the loop gain infinite.

Feedback-type sinusoidal oscillators can be classified as LC(inductor-capacitor) and RC (resistor-capacitor) oscillators.

CLASSIFICATIONS OF OSCILLATORS:


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The classification of various oscillators is shown in Table 12-1.

CLASSIFICATIONS OF OSCILLATORS:


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Types of oscillators

1. RC oscillators

Wien Bridge

Phase-Shift

2. LC oscillators

Hartley Colpitts

Crystal

3. Unijunction / relaxation oscillators

4. Crystal oscillators


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Basic principles for oscillation

An oscillator is an amplifier with positive feedback.

A

Ve

V f

VsVo

+

(1)fse

VVV

(2)of VV

(3)osfseo VVAVVAAVV


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The closed loop gain is:

osfs

eo

VVAVVAAVV

oso VAAVV b

so AVVA b1

AA

V

VA

s

o

f

1


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In generalAandbare functions of frequency andthus may be written as;

is known as loop gain

ssA1

sAs

V

VsA

s

o

f

ssA


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Thus, the condition for sinusoidal oscillation of

frequencyf0is;

This is known as Barkhausen criterion.

The frequency of oscillation is solely determined bythe phase characteristic of the feedback loop the

loop oscillates at the frequency for which the phaseis zero.

100 jjA


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The feedback oscillator is widely used for

generation of sine wave signals.

The positive (in phase) feedback arrangementmaintains the oscillations.

The feedback gain must be kept to unity to keep theoutput from distorting.


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In phase

Noninverting

amplifier

V f V oA v

Feedbackcircuit


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Design Criteria for Oscillators

1. The magnitude of the loop gain must be unity or

slightly larger

Barkhaussen criterion

2. Total phase shift,of the loop gain mus t beNx360 where N=0, 1, 2,

1A


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Oscillators With LC Feedback Circuits

For frequencies above 1 Mhz, LC feedback

oscillators are used. We will discuss theColpitts, Clapp, Hartley, Armstrong, and

crystal-controlledoscillators. Transistors

are used as the active device in these

types.


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Oscillators With LC FeedbackCircuits

The Hartley oscillator is

similar to the Clapp and

Colpitts. The tank circuit

has two inductors and

one capacitor. The

calculation of the

resonant frequency is

the same.


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Colpitts Oscillator:


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RC Oscillators

RC feedback oscillators are generally limited to

frequencies of 1 MHz or less.

The types of RC oscillators that we will discuss are

the Wien-bridgeand the phase-shift


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Phase-Shift Oscillator:


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Phase-Shift Oscillator

The phase shift oscillator utilizes three RC circuits

to provide 180 phase shift that when coupled withthe 180 of the op-amp itself provides the necessaryfeedback to sustain oscillations.

The gain must be at least 29to maintain theoscillations.

The frequency of resonance for the this type is

similar to any RC circuit oscillator:

RCfr

62

1


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Wien-bridge Oscillator

It is a low frequency oscillator which ranges from a

few kHz to 1 MHz.


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Circuit Diagram of Wien-Bridge Oscillator:


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Advantages of Wien-Bridge Oscillator: 1. The frequency of oscillation can be easily varied just by changing RC

network

2. High gain due to two-stage amplifier

3. Stability is high

Disadvantages of Wien-Bridge Oscillator The main disadvantage of the Wien-bridge oscillator is that a high frequency

of oscillation cannot be generated.

Wien-Bridge Oscillator:


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LC Oscillators

Use transistors and LC tuned circuits or crystals in

their feedback network.

For hundreds of kHz to hundreds of MHz frequencyrange.

Examine Tuned collector ,Colpitts, Hartley andcrystal oscillator.


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CIRCUIT DIAGRAM OF TUNED OSCILLATOR:


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Crystal Oscillator

Most communications and digital applications require the

use of oscillators with extremely stable output.Crystaloscillators are invented to overcome the output fluctuationexperienced by conventional oscillators.

Crystals used in electronic applications consist of a quartz

wafer held between two metal plates and housed in a apackage as shown in Fig. 9 (a) and (b).


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Crystal Oscillator

Piezoelectric Effect

The quartz crystal is made of silicon oxide (SiO2) andexhibits a property called thepiezoelectric

When a changing an alternating voltage is applied acrossthe crystal, it vibrates at the frequency of the appliedvoltage. In the other word, the frequency of the applied acvoltage is equal to the natural resonant frequency of thecrystal.

The thinner the crystal, higher its frequency of vibration.This phenomenon is called piezoelectric effect.


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Crystal Oscillator

Characteristic of Quartz

Crystal

The crystal can have two resonantfrequencies;

One is the series resonance frequency f1

which occurs when XL= XC. At thisfrequency, crystal offers a very lowimpedance to the external circuit whereZ = R.

The other is the parallel resonance (orantiresonance) frequency f2whichoccurs when reactance of the series legequals the reactance of CM. At thisfrequency, crystal offers a very highimpedance to the external circuit

R

L

C

CM


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Circuit Diagram of CRYSTAL OSCILLATOR:

l ill


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Crystal Oscillator

The crystal is connected as a series element in the

feedback path from collector to the base so that it isexcited in the series-resonance mode

BJT

FET

l ill


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Crystal Oscillator

Since, in series resonance, crystal impedance is the smallest that

causes the crystal provides the largest positive feedback.

Resistors R1, R2, and RE provide a voltage-divider stabilized dc bias

circuit. Capacitor CEprovides ac bypass of the emitter resistor, RE

to avoid degeneration.

The RFC coil provides dc collector load and also prevents any acsignal from entering the dc supply.

The coupling capacitor CChas negligible reactance at circuit

operating frequency but blocks any dc flow between collector and

base. The oscillation frequency equals the series-resonance frequency of

the crystal and is given by:

C

oLC

f2

1

ij i ill


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Unijunction Oscillator

The unijunction transistor

can be used in what iscalled a relaxation oscillatoras shown by basic circuit asfollow.

The unijunction oscillatorprovides a pulse signalsuitable for digital-circuitapplications.

Resistor RTand capacitor CTare the timing componentsthat set the circuitoscillating rate

UJT

U ij i O ill


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Sawtooth wave

appears at the emitterof the transistor.

This wave shows the

gradual increase ofcapacitor voltage

U ij i O ill


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The oscillating frequency is calculated as follows:

where, = the unijunction transistor intrinsic stand-

off ratio

Typically, a unijunction transistor has a stand-off

ratio from 0.4 to 0.6

1/1ln

1

TT

oCR

f


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Oscillators are a common element of almost all electronic circuits. They are usedin various applications, and their use makes it possible for circuits and subsystems to

perform numerous useful functions.

Oscillators are commonly used in communication circuits. All thecommunication circuits for different modulation techniquesAM, FM, PMthe use of an

oscillator is must.

Oscillators are used as stable frequency sources in a variety of electronicapplications.

Oscillator circuits are used in computer peripherals, counters, timers,calculators, phase-locked loops, digital multi-metres, oscilloscopes,and numerous other applications.

APPLICATIONS OF OSCILLATORS:

CH 5 - Oscillator

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