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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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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
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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
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Concept Preview
#he $ien bridge oscillator can produce a lowdistortion sine wave output.
A $ien bridge oscillator operates at the resonantfrequency of its leadlag network.
#he gain of some oscillator circuits must bereduced after oscillations begin to avoid clipping.
Since common emitter amplifiers produce a phaseinversion% a second phase inversion is required for
positive feedback. "C networks can provide a &'! degree phase shift
at the desired frequency of oscillation.
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ien bride oscillator
Only 'Rarrives at the 3input in phase.
leadla
in
out
R
C
C
R
2
RC
1'R0
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in
out
The 'eedbac+ 'raction at 'Rin this circuit is onethird$
- 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.
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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
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;1is an #channel
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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
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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
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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
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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
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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. $hen it
vibrates% it produces an electrical signal. +uart, crystals can replace tank circuits and
provide e-ceptional frequency stability.
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3,CC
3,CC
The @artley oscillator is LC controlled.
'eedbac+tan+ circuit
The supply tap is a
sinal round. There
is a 1>5ophase shi't
across the tan+.
1>5o
5o
sinal
round
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3,CC
3,CC
: LC
1'R 0
L
C
The output 're2uency is e2ual to the resonant 're2uency.
L is the value 'or the entire coil.
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3,CC
This is called a Colpitts oscillator.
The capacitive
le o' the tan+
is tapped.
'eedbac+
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3,CC #ote that the a%pli'ier
con'iuration is
co%%onbase.
The e%itter is theinput and the collector
is the output. The
'eedbac+ circuit
returns so%e o' the
collector sinal tothe input ith no
phase shi't.
sinal round
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3,CC
L C&;
:
LC&;
1'R 0
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;uart6 crystal Slab cut 'ro%
crystal
&lectrodes
and leads
Sche%atic
sy%bol
;uart6 is a pie6oelectric %aterial.
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;uart6 crystals replace LC tan+s hen
're2uency accuracy is i%portant.
;uart6 discRear %etal
electrode
ront %etal
electrode
Contact pins&2uivalent
circuit
C
CS
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Crystal
e2uivalent
circuit
The e2uivalent R is very
s%all and the ; is o'ten
several thousand.
R
@ih; tuned circuits are noted
'or narro bandidth and this
translates to 're2uency stability.
The e2uivalent circuit also
predicts tworesonant're2uencies$ 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