Unit 2 - Oscillators by Mehzabeen

7/25/2019 Unit 2 - Oscillators by Mehzabeen

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Oscill

ators

S.M.Mehzabeen

1

www.Vidyarthiplus.com



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Oscillators By S.M.Mehzabeen 2

Objectives

Define the basic oscillator circuit

Identify elements of feedback in the oscillator

Identify the conditions for oscillation to occur

Identify input and output characteristics

Identify common uses of the oscillator




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Word Wall

Oscillation: an effect that repeatedly and regularly

fluctuates about a mean value

Oscillator: circuit that produces oscillation

Characteristics: frequency, amplitude, distortion,

wave-shape, stability




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OscillatorIn our daily life

Digital watches, Invertors, Radios , T.V, Computers, Fans, MetalDetectors, Electronic Bells and lots more

Pendulum of a clock.

If you push on a pendulum to start it swinging, it will oscillate at

some frequency - it will swing back and forth a certain number

of times per second.

The length of the pendulum controls the frequency.

In pendulum potential energy is converted in kinetic energy




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Overview of the Oscillator

One of the basic building blocks of electronics

Input is a direct current (DC) power source

Output is alternating current (AC)

Can generate sub-audible frequencies or very highfrequencies

Most oscillators are amplifiers with feedback




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Amplifiers as Oscillators?

Most amplifiers will oscillate when conditions are correct

Example: Too high of a volume on public address system

= loud noise and squeals that are the result of acoustic

waves traveling from the speakers to the microphone

The result is oscillation




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Feedback and Amplifier GainConditions for Oscillation

Feedback alone does not promise oscillation

There is always some loss in the feedback circuit

Amplifier gain must be greater than this loss

Feedback must be in-phase




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In-phase Feedback

In-phase feedback is also called regenerative feedback

or positive feedback

When the original amplifier input and output signals are not

in-phase, the feedback circuit is used to reverse the phase




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Input Characteristics

Steady source of direct current (DC)

In many applications, the DC source requires a filter




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Output Characteristics

Amplitude

Frequency

Waveform type

Stability

On some oscillators, the capability to change frequency




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Oscillator Amplitude

Usually determined by the gain available from the

amplifier

Supply voltage and circuit losses affect amplitude




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Oscillator Frequency

Frequency of operation is normally determined by the

feedback circuit

Common feedback circuits used to determine oscillator

frequency include:

crystals

resistor and capacitor networks (RC)

coil and capacitor networks (LC) {tank circuit}




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Waveform Type

Generally, determined by:

Feedback circuitry

Output filter circuitry

May be sinusoidal (sine wave), square wave, or triangular

wave

Amplifier gain, or

Changes to input voltage




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Oscillator Stability

Sometimes referred to as a stable oscillator

Source of a signal with consistent amplitude

Source of a signal with consistent frequency




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Ability to Change Frequency

Oscillators sometimes have the ability to change

frequencies

Crystal oscillator frequency is controlled by changing the

crystal

Crystals are usually cut from quartz to generate a

specified frequency when operating




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Typical Uses of the Oscillator

Radio and

television stations

require oscillators

to develop the

basic signal to

transmit their

information



Vid hi l


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Cell phones, electronic

keyboards, and remote

controls use oscillators to

produce the required

frequencies for operation



Vid thi l


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Digital devices such as

computers, watches,

calculators, and iPods all

require oscillators to

generate the rectangular

waveform required for

operation



Vid thi l


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Variable oscillators, known

as signal generators, are

used to generate

frequencies and waveforms

needed for troubleshooting

and the testing of electronic

equipment



Vid thi l


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Oscillators By S.M.Mehzabeen

Need of an Oscillator

An oscillator circuit is capable of producing ac voltage of

desired frequency and waveshape.

21

To test performance of electronic circuits, it is called

signal generator.

It can produce square, pulse, triangular, or sawtooth

waveshape.



www Vidyarthiplus com


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High frequency oscillator are used in broadcasting.

Microwave oven uses an oscillator.

Used for induction heating and dielectric heating.


Oscillators are circuitsthat generate periodic

signals.





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An oscillator converts DC power from power supply to AC

signals power spontaneously without the need for an AC

input source (Note: Amplifiers convert DC power into AC

output power only if an external AC input signal is present.)


There are several approaches to design of oscillator circuits.

The approach to be discussed is related to the feedback

using amplifiers.





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A frequency-selective feedback path around an

amplifier is placed to return part of the output signal

to the amplifier input, which results in a circuit called

a linear oscillator that produces an approximately

sinusoidal output.


Under proper conditions, the signal returned by the feedback

network has exactly the correct amplitude and phase needed to

sustain the output signal.





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Types of Oscillators

Sinusoidal or non-sinusoidal.

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An oscillator generating square wave or a pulse

train is called multivibrator:

1. Bistable multivibrator (Flip-Flop Circuit).

2. Monostable multivibrator.

3. Astable multivibrator (Free-running).





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Types of Oscillators

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Depending upon type of feedback, we have

1. Tuned Circuit (LC) oscillators.

2. RC oscillators, and

3. Crystal oscillators.





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The gain with positive feedback is given as

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Using Positive Feedback

By making 1 A

= 0, or

A

= 1, we get gain as infinity

This condition (A = 1) is known as Barkhausen Criterion of oscillations

It means you get output without any input !




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What happens to the output ?

There are three possibilities.

How is it Possible ?





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Basic principles for oscillation An oscillator is an amplifier with positive feedback.

(1)f

VsVeV

(2)oVfV

(3)oVsVAfVsVAeAVoV





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oVsVAfVsVAeAVoV

oAsAVoV sAVoVA1

Basic principles for oscillation

The closed loop gain is:

A1A

sV

oV

fA





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Basic principles for oscillation

In general Aand are functions of frequency and thus

may be written as;

ssA1sA

s

sV

oVs

fA

is known as loop gain

ssA





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(1) IfA < 1,we get decayingof dampedoscillations.


y p



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(2) IfA > 1,we get growing oscillations.


y p



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(3) IfA = 1,we get sustained oscillations. In this

case, the circuit supplies its own input signal.


y p



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Wherefrom comes the starting voltage ?

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Each resistor is a noise generator.

The feedback network is a resonant circuit giving

maximum feedback voltage at frequency f0, providing

phase shift of 0 only at this frequency.

The initial loop gainA> 1.

The oscillations build up only at this frequency.

After the desired output is reached,Areduces to unity.


y p



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Tank Circuit

37

LCparallel circuit is called

tank circuit.

Once excited, it oscillates

at




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The energy keeps oscillating between electric

potential energyand magnetic filed energy.

Tank Circuit




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Oscillators are the circuits which coverts DC

Voltage from battery to AC Voltage

Without excitation input signal

A simple example

If you charge up the capacitor with

a battery and then insert theinductor into the circuit, here's

what will happen



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General Form of LC Oscillators

Z=z1II hie=

ZL=z +z3II z2

Z L= II z2

Z L= + II



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General Form of LC Oscillators

Av=1

=1

General Equation of LC Oscillator:

hie(z1+z2+z3) +z1z2(1+hfe)z1z3=0



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Hartley Oscillator

52



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Circuit Description

6.RFC-Radio frequency Choke(isolation b/w A.C & D.C)

5.CC1 & CC2 are the coupling capacitors.

Reactance value very high for high

frequencies.(open circuit)

Reactance value zero for D.C.Conditions.




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Derivation of frequency of oscillation

Z1=jwL1+jwM

z2= jwL2 +jwM

z3=






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hie(jwL1+jwM+ jwL2 +jwM- )+

jwhie(L1+ L2+2M ) (W2



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D i i f f f ill i



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Equating Real part = 0



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Ci c it Desc iption



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Circuit Description

2.R1 and R2 biasing resistors.

Amplifier stage:

1.Transistor Active device in Common emitter

Configuration.

3.RE-Biasing emitter resistance

4.CE-Emitter bypass capacitors.


Ci c it Desc iption



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Circuit Description



Reactance value very high for high frequencies.(open

circuit)




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DERIVATION OF FREQUENCY OF OSCILLATION



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hie( + = 0

j hie( + -wL ) +(



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Equating Real part = 0

= = w2c2L

hfe= - 1

= =c2L 1+hfe=

hfe=




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Solution :



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Solution :



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Circuit Description



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Circuit Description



Reactance value very high for high frequencies.(open

circuit)






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Provides improved stability

Permits capacitive tuning of the oscillator if C3 is variable.

3

1

2

1

Lcfr

)323121

321(2

1

cccccccccL

fr

21

21

3 cc

cc

c


ADVANTAGES of Colpitts oscillation



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ADVANTAGES of Colpittsoscillation

The frequency is stable and accurate.

The good frequency stability.

The stray capacitances have no effect on C3which

decides the frequency.

Keeping C3 variable, frequency can be varied in the

desired range.



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Franklin oscillator



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Franklin oscillator

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SERIES RESONATING CIRCUIT



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SERIES RESONATING CIRCUIT

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PRACTICAL FRANKLIN OSCILLATOR CIRCUIT



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PRACTICAL FRANKLIN OSCILLATOR CIRCUIT

81



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LsjRsYL

1cjYc

CjLsRs

LsjRsYT

222YCYLYT = Cj

LsjRs

1

CjLsRs

LsjRs

YT

222

CjLsRs

Lsj

LsRs

RsYT

222222





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Equate Imaginary Part to zero:

0222

C

LsRs

Ls

222LsRsc

Ls

222 Rsc

LsLs 2

22 1

Ls

Rs

LsC

2

21

Ls

Rs

LsC )

1(

2

12

2

Ls

Rs

LsCf





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Q

LsCf

2

1

2

2

Ls

Rs1

The impedance at resonance condition is

determined from the real part.

Yrzr

1

Rs

LsRs

Rs

LsRs

Yrzr

222221





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Q

Substituting value 2

CRs

Lszr )1(

2

22

LsRs

LsCRsLsRszr

RsCRs

LsRszr

CRs

Lszr



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ARMSTRONG OSCILLATOR



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TUNED COLLECTOR OSCILLATOR



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WHY?



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Hence Called Tuned Collector Oscillator.

The Tuned Circuit Is Placed In The Collector Of The

Transistor.


CIRCUIT DESCRIPTION



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Employs Transformer Action.

LC Oscillator.

Voltage Across The Secondary Used As A Feedback.

Primary Acts As A Inductor(L)

Biasing Point Selected By R1,R2,RE.



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Oscillators By S.M.Mehzabeen101


RC Oscillatorswww.Vidyarthiplus.com


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1. RC Phase shift Oscillator.

Three types :

2. Wein Bridge Oscillator.

3. Twin T Oscillator.


TRANSISTORIZED RC FEEDBACK NETWORK



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RC FEEDBACK NETWORK



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Derivation for RC phase shift oscillator.



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R1=R2=R

Let c1 =c2=c3=c

R3+(Rth II Rin)=R


Derivation for RC phase shift oscillator.



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Derivation for RC phase shift oscillator.www.Vidyarthiplus.com


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Solution :



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WIEN BRIDGE OSCILLATORwww.Vidyarthiplus.com


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sCR

R

sCRZR

ZRZ

sC

sRC

sCRZRZ

CCP

CS

1

111

11

11


WIEN BRIDGE OSCILLATORwww.Vidyarthiplus.com


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2

1

1

1

)1(

1

1

1

11

sCRsCR

sCRR

sCR

sC

R

sCR

ZZZZZ

SP

SP

sC

sRCZRZZ CS

1

2

Input Loading Output Loading


Amplifier GainVVV



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2

1

2

21

1

21

0

1

1

2

1

210

1

21

11

21

210

00

)1(

11

)1(

1

1

,0

1

,

sCRsCR

sCRR

R

RA

sosCRsCR

sCRRZwhere

R

RZ

I

V

V

VA

andZI

VISince

R

R

R

RR

V

V

soRRRVRIVVV

andRR

VIIusewe

V

VgetTo

I

V

V

V

I

VA

r

S

i

i

r

S

i

i

oi

o

i

S

i

iS

r



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Advantages and Disadvantages of wien bridge oscillator



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It uses both positive and

negative feedback, it

provides better stability

and high over all gain.

Advantages

More components used

costlier.

Disadvantages

Frequency can be easily

adjusted by varying R or C

It cant generate very high

frequency.

121


TWIN T OSCILLATORwww.Vidyarthiplus.com


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Crystal Oscillatorwww.Vidyarthiplus.com


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Piezoelectric effect.

The crystal is suitably cut and then mounted between

two metallic plates.

The fundamental frequency is given as



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Normally C is much smaller than C



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Normally, Csis much smaller than Cm.

Therefore, Cp is slightly less than Cs.

Hence, the frequency fpis 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.




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The fois between 411 kHz and 412 kHz.


Pierce Oscillator

If we assume that the current through C1 2 is larger than



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1

1

1

CjIvo

2

1

1

CjIvi

2

1

C

Cn

v

v

i

o

If we assume that the current through C1,2 is larger than

the collector current (high Q), then we see that the same

current flows through both capacitors. The voltage at the

input and output is therefore



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MODIFICATIONS FOR FREQUENCY STABILITY

Enclosing the circuit in a constant temperature chamber



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Enclosing the circuit in a constant temperature chamber.

Maintaining constant voltage by using the zener diodes.

Load effect is reduced by coupling the oscillator to the

load loosely

Circuit having high input impedance and low output

impedance.



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Hartley Oscillator.

Review



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y

Colpitts Oscillator.

RCPhase Shift Oscillator.

Wien Bridge Oscillator.

Crystal Oscillator.

Series and Parallel Resonance

Documents

Unit 2 - Oscillators by Mehzabeen