electronics principles & applications fifth edition chapter 11 oscillators ©1999...
TRANSCRIPT
![Page 1: Electronics Principles & Applications Fifth Edition Chapter 11 Oscillators ©1999 Glencoe/McGraw-Hill Charles A. Schuler](https://reader036.vdocuments.us/reader036/viewer/2022062301/56649ed35503460f94be29c0/html5/thumbnails/1.jpg)
ElectronicsElectronics
Principles & ApplicationsPrinciples & ApplicationsFifth EditionFifth Edition
Chapter 11Oscillators
©1999 Glencoe/McGraw-Hill
Charles A. Schuler
![Page 2: Electronics Principles & Applications Fifth Edition Chapter 11 Oscillators ©1999 Glencoe/McGraw-Hill Charles A. Schuler](https://reader036.vdocuments.us/reader036/viewer/2022062301/56649ed35503460f94be29c0/html5/thumbnails/2.jpg)
• 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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A 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 oscillation, but just barely 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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Here is how thepositive temperature
coefficient works:
R
Vout
C
RL
2R1
Tungstenlamp
C R
R1
Vout t
Resistance of lampincreases until
equilibrium is reached
R1
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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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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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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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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.
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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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