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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
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ElectronicsElectronics
Principles & ApplicationsPrinciples & ApplicationsEighth EditionEighth Edition
Chapter 11Oscillators
Charles A. Schuler
©2013
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• Oscillator Characteristics• RC Circuits• LC Circuits• Crystal Circuits• Relaxation Oscillators• Undesired Oscillations• Troubleshooting• Direct Digital Synthesis
INTRODUCTION
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Some possible output waveforms
Oscillator
Oscillators convert dc to ac.
ac outdc in
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Vin VoutA
B Feedback
VoutA
B Feedback
An amplifier with negative feedback.
This amplifier has positive feedback.It oscillates if A > B.
Recall: A = open-loop gain and B = feedback fraction
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VoutA
B Feedback
Sinusoidal oscillators have positive feedback at only one frequency.
This can be accomplished with RC or LC networks.
frequency
ph
ase
+ 90
0
- 90
fR
in
out
lead-lag
fR
inout
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Oscillator basics quiz
Oscillators convert dc to _______.ac
In order for an oscillator to work, the feedbackmust be __________. positive
An oscillator can’t start unless gain (A) is________ than feedback fraction (B). greater
Sine wave oscillators have the correct feedbackphase at one ___________. frequency
The phase shift of an RC lead-lag networkat fR is _____________. 0o
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Wien bridge oscillator
Only fR arrives at the + input in phase.
lead-lag
in
out
R
C
C
R
RC
1fR =
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in
out
The feedback fraction at fR in this circuit is one-third:
B = inout =
1
3
A must be > 3 for oscillations to start. After that, Amust be reduced to avoid driving the op amp to VSAT.
R2 2R1
R1
A = 1 +R2
R1
One solution is a positivetemperature coefficient
device here to decrease gain.
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After theoscillationsstart, the
lamp heats up which increases
its resistanceto reducegain andclipping.
R
Vout
C
RL
2R1
Tungstenlamp
C R
R1
Vout
time
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Q1 is an N-channel JFET.After oscillations start, theoutput signal is rectifiedand the negative voltage is applied to the JFET’sgate. This increases its D-Sresistance which decreasesthe gain of the op amp.
Q1D
S G
Notice that the clippingsubsides as Q1 reducesthe loop gain.
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When common-emitter amplifiers are used asoscillators, the feedback circuit must providea 180o phase shift to make the circuit oscillate.
A
BOut-of-phase
180o
180o
180o + 180o = 360o = 0o
In-phase
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RB
RL
VCC
CCC
R R
Feedback
1 2
3
3 RC networks provide a total phase shift of 180o.
A phase-shift oscillator based on a common-emitter amplifier
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A phase-shift oscillator based on an inverting operational amplifier
The gain must be high enough to start the oscillator, but then clipping often results which causes distortion. These circuits often use gain reduction techniques after the oscillations begin.
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A phase-shift oscillator using two op-amps and gain limiting for
low distortion.
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RC oscillator quiz
A properly designed Wien bridge oscillatorprovides a ________ waveform. sine
The feedback fraction in a Wien bridgeoscillator is ________. 0.333
A tungsten lamp has a _________ temperaturecoefficient. positive
The feedback circuit in a common-emitteroscillator provides _______ of phase shift. 180o
A phase shift oscillator uses three RC sectionsto provide a total shift of ______. 180o
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+VCC
+VCC
The Hartley oscillator is LC controlled.
feedbacktank circuit
The supply tap is a signal ground. There is a 180o phase shift
across the tank.
180o
0o
signalground
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+VCC
+VCC
2 LC
1fR =
LC
The output frequency is equal to the resonant frequency.
L is the value for the entire coil.
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+VCC
This is called a Colpitts oscillator.
The capacitiveleg of the tank
is tapped.
feedback
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+VCC Note that the amplifierconfiguration is common-base.
The emitter is theinput and the collector
is the output. Thefeedback circuit
returns some of thecollector signal tothe input with no
phase shift.
signal ground
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+VCC
L CEQ
2 LCEQ
1fR =
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Quartz crystal Slab cut fromcrystal
Electrodesand leads
Schematicsymbol
Quartz is a piezoelectric material.
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Quartz crystals replace LC tanks when frequency accuracy is important.
Quartz disc Rear metalelectrode
Front metalelectrode
Contact pinsEquivalentcircuit
CP
CS
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Crystalequivalent
circuit
The equivalent R is verysmall and the Q is often
several thousand.
R
High-Q tuned circuits are notedfor narrow bandwidth and thistranslates to frequency stability.
The equivalent circuit also predicts two resonant
frequencies: series and parallel.A given oscillator circuit is
designed to use one or the other.
CS
CP
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Crystals• The fundamental frequency (series
resonance) is controlled by the quartz slab or quartz disk thickness.
• Higher multiples of the fundamental are called overtones.
• The electrode capacitance creates a parallel resonant frequency which is slightly higher.
• Typical frequency accuracy is measured in parts per million (ppm).
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+VCCCrystal oscillator circuit
RB2
RB1
RFC
RE
C2
C1
CE
vout
Xtal
Replaces thetank circuit
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Packaged oscillators contain a quartz crystal and theoscillator circuitry in a sealed metal can.
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High-frequency oscillator quiz
A Hartley oscillator has a tapped _______ in its tank circuit. coil
When the capacitive leg is tapped, the circuitmight be called ________. Colpitts
A quartz crystal is a solid-state replacementfor the ________ circuit. tank
Crystals are more stable than LC tanks dueto their very high ________. Q
Higher multiples of a crystal’s resonantfrequency are called ________. overtones
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So far, we have learned that:
• Oscillators can be RC controlled by using phase-shifts.
• Oscillators can be LC controlled by using resonance.
• Oscillators can be crystal controlled by using resonance or overtones.
• There is another RC type called relaxation oscillators. These are time-constant controlled.
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Base 2
Base 1
Emitter
RECALL that a unijunction transistor fires when its emitter voltage reaches VP.
VP
Emitter current
Em
itte
r vo
ltag
e
Then, the emitter voltagedrops due to its negativeresistance characteristic.
UJTs can be used inrelaxation oscillators.
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+VBB
R
C
A UJT relaxation oscillatorprovides two waveforms.
RC f RC
Exponential sawtooth
Pulse
VP
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+VBB
R3
C
A programmable unijunction transistor (PUT) is a better choice.
43
4
RR
R
η =
R1
R2 R4
PUT
VP = 0.7 V + ηVBB
VP
(η is called the intrinsic standoff ratio)
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= 0.69RC
= 0.69 x 47 k x 3.3 nF
= 0.107 ms
t = 2 = 0.214 ms
f = 1/t = 4.67 kHz
This multivibrator is also RC controlled.
0 V
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Non-sinusoidal op-amp oscillators can also use RC time constants for frequency control, as shown in this circuit.
Exponential sawtooth, which is sometimes treated as a triangle.
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Undesired oscillations:
make amplifiers useless.
Why is this a problem?
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OutputR
C
Parasitic capacitancescombine with resistances
to form un-wanted lag networks.
R
C
R
C
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R
C
This can lead tounwanted oscillations
since the feedbackbecomes positive
at some higher frequency.
It’s the equivalent of a phase-shift oscillator.
Total Lag = 180o
R
C
R
C
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R
CR
C
R
C
However,if the gain is less
than unity at thatfrequency, the
amplifier will not oscillate.
There is always some frequency where feedback becomes positive.
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100 k10 k1 10 100 1k 1M0
20
80
40
60
100
120
Frequency in Hz
Gain in dB
The typical op amp has this characteristic:
Break frequency setby a dominant (intentional)
internal lag circuit.
The gain isless than unity
before combinedlags total 180o
of phase shift.
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Methods of preventing oscillation:
• Reduce the feedback with bypass circuits, shields, and careful circuit layout.
• Cancel feedback with a second path … this is called neutralization.
• Reduce the gain for frequencies where the feedback becomes positive … this is called frequency compensation.
• Reduce the total phase shift … this is called phase compensation.
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Oscillator troubleshooting:
• No output: supply voltage; component failure; oscillator is overloaded.
• Reduced output: low supply voltage; bias; component defect; loading.
• Frequency instability: supply voltage; poor connection or contact; temperature; RC, LC, or crystal.
• Frequency error: supply voltage; loading; RC, LC, or crystal.
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Phaseaccumulator
Sine lookuptable
DAC LPF
Clock
Direct digital synthesizer
Frequency tuningword (binary)
(also called a numerically controlled oscillator)
The tuning word changes the phase increment value.
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30o phaserotation
45o phaserotation
NOTE: Increasing the phase increment increases the frequency.
Access thesine table every 30o
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Direct digital synthesizer using the AD9850 IC
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Oscillator wrap-up quiz
Relaxation oscillators are controlled by RC__________ __________. time constants
Negative feedback becomes positive at somefrequency due to _______ ______. RC lags
Gain rolloff to prevent oscillation is called____________ compensation. frequency
Direct digital synthesizers are also called_____ _____ oscillators. numerically controlled
Direct digital synthesizers use a sine____________ table. lookup
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REVIEW
• Oscillator Characteristics• RC Circuits• LC Circuits• Crystal Circuits• Relaxation Oscillators• Undesired Oscillations• Troubleshooting• Direct Digital Synthesis