Chapter11.ppt

7/18/2019 Chapter11.ppt

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McGraw-Hill 2008 The McGraw-Hill Companies Inc. All right

ElectronicsElectronicsPrinciples & ApplicationsPrinciples & Applications

Seventh EditionSeventh Edition

Chapter 11

Oscillators

(student version)

Charles A. Schuler



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Oscillator CharacteristicsRC Circuits

LC Circuits

Crystal CircuitsRelaxation OscillatorsUndesired OscillationsTroubleshootin

!irect !iital Synthesis

"#TRO!UCT"O#


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!ear Student$

This presentation is arraned in se%ents. &ach se%ent

is preceded by a ConceptPreviewslide and is 'olloed by a

ConceptReviewslide. hen you reach a ConceptReviewslide* you can return to the beinnin o' that se%ent by

clic+in on the Repeat Se%ent button. This ill allo you

to vie that se%ent aain* i' you ant to.


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Concept Preview

Oscillators convert dc to ac. Oscillators use positive feedback.

An amplifier will oscillate if it has positivefeedback and has more gain than loss in the

feedback path. Sinusoidal oscillators have positive feedback at

only one frequency.

A leadlag network produces a phase shift of !degrees at only one frequency.


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McGraw-Hill 2008 The McGraw-Hill Companies Inc. All rightSo%e possible output ave'or%s

Oscillator

Oscillators convert dc to ac.

ac out

dc in


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,in ,outA

- eedbac+

,outA

-eedbac+

An a%pli'ier ith neative 'eedbac+.

This a%pli'ier haspositive'eedbac+.

"t oscillates i' A / -.

Recall$ A 0 openloop ain and - 0 'eedbac+ 'raction


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,outA

- eedbac+

Sinusoidal oscillators have positive

'eedbac+ at only one 're2uency.

This can be acco%plished ith RC or LC netor+s.

're2uency

phase

3 45

5

45

'R

in

out

leadla

'R

inout


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Oscillator basics 2ui6

Oscillators convert dc to 7777777.

ac

"n order 'or an oscillator to or+* the 'eedbac+

%ust be 7777777777. positive

An oscillator can8t start unless ain (A) is

77777777 than 'eedbac+ 'raction (-). reater

Sine ave oscillators have the correct 'eedbac+phase at one 77777777777. 're2uency

The phase shi't o' an RC leadla netor+

at 'Ris 7777777777777. 5o


9/51McGraw-Hill 2008 The McGraw-Hill Companies Inc. All right

Concept "eview Oscillators convert dc to ac.

Oscillators use positive feedback.

An amplifier will oscillate if it has positivefeedback and has more gain than loss in thefeedback path.

Sinusoidal oscillators have positive feedback atonly one frequency.

A leadlag network produces a phase shift of !

degrees at only one frequency.

Repeat Se%ent



Concept Preview

#he $ien bridge oscillator can produce a lowdistortion sine wave output.

A $ien bridge oscillator operates at the resonantfrequency of its leadlag network.

#he gain of some oscillator circuits must bereduced after oscillations begin to avoid clipping.

Since common emitter amplifiers produce a phaseinversion% a second phase inversion is required for

positive feedback. "C networks can provide a &'! degree phase shift

at the desired frequency of oscillation.



ien bride oscillator

Only 'Rarrives at the 3input in phase.

leadla

in

out

R

C

C

R

2

RC

1'R0



in

out

The 'eedbac+ 'raction at 'Rin this circuit is onethird$

- 0in

out0

1

9

A%ust be / 9 'or oscillations to start. A'ter that* A%ust be reduced to avoid drivin the op a%p to ,SAT.

R:

:R1

R1

A 0 1 3R:

R1

One solution is a positive

te%perature coe''icient

device here to decrease ain.



A'ter the

oscillations

start* thela%p heats

to reduce

ain and

clippin.

R

,out

C

RL

:R1Tunsten

la%p

C R

R1

,out

ti%e



;1is an #channel



hen co%%one%itter a%pli'iers are used as

oscillators* the 'eedbac+ circuit %ust provide

a 1>5o

phase shi't to %a+e the circuit oscillate.

A

-Outo'phase

1>5o

1>5o

1>5o3 1>5

o0 9?5

o 0 5

o

"nphase



R-

RL

,CC

CCC

R R

eedbac+

1 :

9

9 RC netor+s provide a total phase shi't o' 1>5o

.

A phaseshi't oscillator based

on a co%%one%itter a%pli'ier



RC oscillator 2ui6

A properly desined ien bride oscillator

provides a 77777777 ave'or%. sine

The 'eedbac+ 'raction in a ien bride

oscillator is 77777777. 5.999

A tunsten la%p has a 777777777 te%perature

coe''icient. positive

The 'eedbac+ circuit in a co%%one%itter

oscillator provides 7777777 o' phase shi't. 1>5o

A phase shi't oscillator uses three RC sections

to provide a total shi't o' 777777. 1>5o



Concept "eview #he $ien bridge oscillator can produce a low

distortion sine wave output. A $ien bridge oscillator operates at the resonant

frequency of its leadlag network.

#he gain of some oscillator circuits must bereduced after oscillations begin to avoid clipping.

Since common emitter amplifiers produce a phaseinversion% a second phase inversion is required for

positive feedback.

"C networks can provide a &'! degree phase shiftat the desired frequency of oscillation.

Repeat Se%ent



Concept Preview

"( oscillators often use )C tank circuits to controlthe frequency of oscillation. #he tank circuits aretapped to control the amount of feedback.

*artley oscillators use tapped coils while Colpitts

oscillators use capacitive taps. Common emitter oscillators require a &'! degree

phase shift across their tank circuits.

+uart, is a pie,oelectric material. $hen it

vibrates% it produces an electrical signal. +uart, crystals can replace tank circuits and

provide e-ceptional frequency stability.



3,CC

3,CC

The @artley oscillator is LC controlled.

'eedbac+tan+ circuit

The supply tap is a

sinal round. There

is a 1>5ophase shi't

across the tan+.

1>5o

5o

sinal

round



3,CC

3,CC

: LC

1'R 0

L

C

The output 're2uency is e2ual to the resonant 're2uency.

L is the value 'or the entire coil.



3,CC

This is called a Colpitts oscillator.

The capacitive

le o' the tan+

is tapped.

'eedbac+



3,CC #ote that the a%pli'ier

con'iuration is

co%%onbase.

The e%itter is theinput and the collector

is the output. The

'eedbac+ circuit

returns so%e o' the

collector sinal tothe input ith no

phase shi't.

sinal round



3,CC

L C&;

:

LC&;

1'R 0



;uart6 crystal Slab cut 'ro%

crystal

&lectrodes

and leads

Sche%atic

sy%bol

;uart6 is a pie6oelectric %aterial.



;uart6 crystals replace LC tan+s hen

're2uency accuracy is i%portant.

;uart6 discRear %etal

electrode

ront %etal

electrode

Contact pins&2uivalent

circuit

C

CS



Crystal

e2uivalent

circuit

The e2uivalent R is very

s%all and the ; is o'ten

several thousand.

R

@ih; tuned circuits are noted

'or narro bandidth and this

translates to 're2uency stability.

The e2uivalent circuit also

predicts tworesonant're2uencies$ seriesand parallel.

A iven oscillator circuit is

desined to use one or the other.

CS

C


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Crystals

The 'unda%ental 're2uency (series

resonance) is controlled by the 2uart6

slab or 2uart6 dis+ thic+ness.

@iher %ultiples o' the 'unda%ental are

called overtones.

The electrode capacitance creates a

parallel resonant 're2uency hich is

slihtly hiher.

Typical 're2uency accuracy is %easured

in parts per %illion (pp%).


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3,CC Crystal oscillator circuit

R-:

R-1

RC

R&

C:

C1

C&

vout

Btal

Replaces the

tan+ circuit


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ac+aed oscillators contain a 2uart6 crystal and theoscillator circuitry in a sealed %etal can.


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@ih're2uency oscillator 2ui6

A @artley oscillator has a tapped 7777777

in its tan+ circuit. coil

hen the capacitive le is tapped* the circuit

%iht be called 77777777. Colpitts

A 2uart6 crystal is a solidstate replace%ent

'or the 77777777 circuit. tan+

Crystals are %ore stable than LC tan+s due

to their very hih 77777777. ;

@iher %ultiples o' a crystal8s resonant

're2uency are called 77777777. overtones


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Concept "eview "( oscillators often use )C tank circuits to control

the frequency of oscillation. #he tank circuits aretapped to control the amount of feedback.

*artley oscillators use tapped coils while Colpittsoscillators use capacitive taps.

Common emitter oscillators require a &'! degreephase shift across their tank circuits.

+uart, is a pie,oelectric material. $hen itvibrates% it produces an electrical signal.

+uart, crystals can replace tank circuits andprovide e-ceptional frequency stability.

Repeat Se%ent


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Concept Preview

"ela-ation oscillators are controlled by "C timeconstants.

ni/unction transistors have a relatively highresistance from emitter to base & before they fire.

A 0# rela-ation oscillator produces twowaveforms1 e-ponential sawtooth and pulse.

#he operating frequency of a 0# oscillator isappro-imately equal to the reciprocal of its "C

time constant. Astable multivibrators are also "C controlled and

provide a rectangular output.


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So far% we have learned that1

Oscillators can be "C controlled by using

phaseshifts.

Oscillators can be )C controlled by using

resonance. Oscillators can be crystal controlled by using

resonance or overtones.

#here is another "C type called rela-ationoscillators. #hese are time-constant controlled.


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-ase :

-ase 1

&%itter

R&CALL that a uniunction transistor

'ires hen its e%itter voltae reaches ,.

,

&%itter current

&%itter

voltae

Then* the e%itter voltae

drops due to its neative

resistance characteristic.

U


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3,--

R

C

A U


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0 5.?4RC

0 5.?4 x DE +

x 9.9 n

0 5.15E %s

t 0 :0 5.:1D %s

' 0 1Ft 0 D.?E +@6

This %ultivibrator is also RC controlled.

5 ,


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Concept "eview "ela-ation oscillators are controlled by "C time

constants. ni/unction transistors have a relatively high

resistance from emitter to base & before they fire.

A 0# rela-ation oscillator produces two

waveforms1 e-ponential sawtooth and pulse.

#he operating frequency of a 0# oscillator isappro-imately equal to the reciprocal of its "Ctime constant.

Astable multivibrators are also "C controlled andprovide a rectangular output.

Repeat Se%ent


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Concept Preview

Amplifiers provide gain but should not oscillate. Parasitic "C lag networks make negative feedback

positive at some frequency. 2f there is gain at thatfrequency% an amplifier will be unstable.

(requency compensation stabili,es feedbackamplifiers by decreasing the gain at thosefrequencies where the feedback becomes positive.

3ypassing% shielding% neutrali,ation% and phase

compensation are other ways to ensure stability. 4irect digital synthesis is a method to generate

many% highly accurate% frequencies.


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Undesired oscillations$

%a+e a%pli'iers useless.hy is this a proble%G

arasitic capacitances


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OutputR

C

arasitic capacitances

co%bine ith resistances

to 'or% unanted

lag networks.

R

C

R

C

"t8 th i l t ' h hi't ill t


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R

C

This can lead tounwanted oscillations

since the 'eedbac+

beco%es positive

at so%e hiher 're2uency.

"t8s the e2uivalent o' a phaseshi't oscillator.

Total La 0 1>5

o

R

C

R

C


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R

CR

C

R

C

@oever*i' the ain is less

than unity at that

're2uency* the

a%pli'ier will not oscillate.

There is alaysso%e 're2uencyhere 'eedbac+ beco%es positive.


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155 +15 +1 15 155 1+ 1H5

:5

>5

D5

?5

155

1:5

re2uency in @6

=ain in d-

The typical op a%p has this characteristic$

-rea+ 're2uency set

by a do%inant (intentional)internal la circuit.

The ain is

less than unitybe'ore co%bined

las total 1>5o

o' phase shi't.

H th d ' ti


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Hethods o' preventin

oscillation$

"educe the feedback with bypass circuits% shields%

and careful circuit layout.

Cancel feedback with a second path 5 this is called

neutrali,ation. "educe the gain for frequencies where the feedback

becomes positive 5 this is called frequency

compensation.

"educe the total phase shift 5 this is calledphase

compensation.


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Oscillator troubleshootin$

#o output$ supply voltaeI co%ponent 'ailureIoscillator is overloaded.

Reduced output$ lo supply voltaeI biasI

co%ponent de'ectI loadin. re2uency instability$ supply voltaeI poor

connection or contactI te%peratureI RC* LC* or

crystal.

re2uency error$ supply voltaeI loadinI RC*LC* or crystal.


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hase

accu%ulator

Sine loo+up

table!AC L

Cloc+

!irect diital synthesi6er

re2uency tunin

ord (binary)

(also called a nu%erically controlled oscillator)

The tunin ord chanes the phase incre%ent value.

Access the


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95ophase

rotation

DJophase

rotation

#OT&$ "ncreasin the phase incre%ent increases the 're2uency.

Access the

sine table

every 95o

O ill i


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Oscillator rapup 2ui6

Relaxation oscillators are controlled by RC

7777777777 7777777777. ti%e constants

#eative 'eedbac+ beco%es positive at so%e

're2uency due to 7777777 777777. RC las

=ain rollo'' to prevent oscillation is called

777777777777 co%pensation. 're2uency

!irect diital synthesi6ers are also called

77777 77777 oscillators. nu%erically controlled

!irect diital synthesi6ers use a sine

777777777777 table. loo+up

i


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Concept "eview Amplifiers provide gain but should not oscillate.

Parasitic "C lag networks make negative feedbackpositive at some frequency. 2f there is gain at thatfrequency% an amplifier will be unstable.

(requency compensation stabili,es feedback

amplifiers by decreasing the gain at thosefrequencies where the feedback becomes positive.

3ypassing% shielding% neutrali,ation% and phasecompensation are other ways to ensure stability.

4irect digital synthesis is a method to generatemany% highly accurate% frequencies.

Repeat Se%ent


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R&,"&

Oscillator CharacteristicsRC CircuitsLC Circuits

Crystal CircuitsRelaxation OscillatorsUndesired Oscillations

Troubleshootin!irect !iital Synthesis

Documents

Chapter11.ppt