Download Chapter 11 - Oscillators

11-1
Electronics
Principles & Applications
Eighth Edition
Charles A. Schuler
Chapter 11
Oscillators
©2013
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INTRODUCTION
11-2
• Oscillator Characteristics
• RC Circuits
• LC Circuits
• Crystal Circuits
• Relaxation Oscillators
• Undesired Oscillations
• Troubleshooting
• Direct Digital Synthesis
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Oscillators convert dc to ac.
Oscillator
11-3
ac out
dc in
Some possible output waveforms
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Vin
11-4
Vout
A
An amplifier with negative feedback.
B
Feedback
Recall: A = open-loop gain and B = feedback fraction
Vout
A
This amplifier has positive feedback.
It oscillates if A > B.
B
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Feedback
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Vout
A
Sinusoidal oscillators have positive
feedback at only one frequency.
B
out
in
11-5
fR
in Feedback
out
lead-lag
phase
+ 90o
0o
- 90o
fR
frequency
This can be accomplished with RC or LC networks.
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Oscillator basics quiz
11-6
Oscillators convert dc to _______.
ac
In order for an oscillator to work, the feedback
must be __________.
positive
An oscillator can’t start unless gain (A) is
________ than feedback fraction (B). greater
Sine wave oscillators have the correct feedback
phase at one ___________.
frequency
The phase shift of an RC lead-lag network
at fR is _____________.
0o
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Wien bridge oscillator
11-7
Only fR arrives at the + input in phase.
in
R
lead-lag
C
out
R
C
fR =
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1
2pRC
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The feedback fraction at fR in this circuit is one-third:
11-8
A must be > 3 for oscillations to start. After that, A
must be reduced to avoid driving the op amp to VSAT.
in
R2 @ 2R1
out
R2
A=1+
R1
in
B = out =
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1
3
R1
One solution is a positive
temperature coefficient
device here to decrease gain.
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After the
oscillations
Vout start, the
lamp heats up
which increases
its resistance
to reduce
gain and
clipping.
11-9
RL
2R1
R1
Tungsten
lamp
C
R
R
C
Vout
time
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11-10
Notice that the clipping
subsides as Q1 reduces
the loop gain.
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D
Q1
S
G
Q1 is an N-channel JFET.
After oscillations start, the
output signal is rectified
and the negative voltage
is applied to the JFET’s
gate. This increases its D-S
resistance which decreases
the gain of the op amp.
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11-11
When common-emitter amplifiers are used as
oscillators, the feedback circuit must provide
o
a 180 phase shift to make the circuit oscillate.
A
180
o
o
o
o
180 + 180 = 360 = 0
In-phase
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Bo
180
o
Out-of-phase
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A phase-shift oscillator based
on a common-emitter amplifier
RL
Feedback
3
C
R
1
C
11-12
2
C
VCC
RB
R
o
3 RC networks provide a total phase shift of 180 .
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11-13
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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11-14
A phase-shift oscillator using two
op-amps and gain limiting for
low distortion.
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RC oscillator quiz
11-15
A properly designed Wien bridge oscillator
provides a ________ waveform. sine
The feedback fraction in a Wien bridge
oscillator is ________.
0.333
A tungsten lamp has a _________ temperature
coefficient.
positive
The feedback circuit in a common-emitter
oscillator provides _______ of phase shift. 180o
A phase shift oscillator uses three RC sections
to provide a total shift of ______. 180o
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The supply tap is a
signal ground. There
o
is a 180 phase shift
across the tank.
11-16
+VCC
180o
0
o
+VCC
signal
ground
tank circuit
feedback
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The Hartley oscillator is LC controlled.
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11-17
The output frequency is equal to the resonant frequency.
+VCC
L
C
+VCC
fR =
1
2p LC
L is the value for the entire coil.
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This is called a Colpitts oscillator.
+VCC
11-18
The capacitive
leg of the tank
is tapped.
feedback
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11-19
+VCC
Note that the amplifier
configuration is
common-base.
signal ground
The emitter is the
input and the collector
is the output. The
feedback circuit
returns some of the
collector signal to
the input with no
phase shift.
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11-20
+VCC
fR =
1
2p LCEQ
L CEQ
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Quartz is a piezoelectric material.
Quartz crystal
Slab cut from
crystal
11-21
Schematic
symbol
Electrodes
and leads
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Quartz crystals replace LC tanks when
frequency accuracy is important.
11-22
Rear metal
electrode
Quartz disc
Front metal
electrode
CP
CS
Equivalent
circuit
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Contact pins
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The equivalent R is very
small and the Q is often
several thousand.
Crystal
equivalent
circuit
11-23
High-Q tuned circuits are noted
for narrow bandwidth and this
translates to frequency stability.
CP
R
CS
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The equivalent circuit also
predicts two resonant
frequencies: series and parallel.
A given oscillator circuit is
designed to use one or the other.
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Crystals
11-24
• 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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Crystal oscillator circuit
+VCC
11-25
RFC
RB1
vout
C1
RB2
C2
RE
Xtal
CE
Replaces the
tank circuit
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11-26
Packaged oscillators contain a quartz crystal and the
oscillator circuitry in a sealed metal can.
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High-frequency oscillator quiz
11-27
A Hartley oscillator has a tapped _______
in its tank circuit.
coil
When the capacitive leg is tapped, the circuit
might be called ________.
Colpitts
A quartz crystal is a solid-state replacement
for the ________ circuit.
tank
Crystals are more stable than LC tanks due
to their very high ________.
Q
Higher multiples of a crystal’s resonant
frequency are called ________. overtones
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So far, we have learned that:
11-28
• 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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11-29
Emitter voltage
RECALL that a unijunction transistor
fires when its emitter voltage reaches VP.
VP
Then, the emitter voltage
drops due to its negative
resistance characteristic.
Base 2
Emitter current
Emitter
UJTs can be used in
relaxation oscillators.
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Base 1
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11-30
A UJT relaxation oscillator
provides two waveforms.
t = RC
f @
+VBB
1
RC
R
Exponential sawtooth
VP
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C
Pulse
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A programmable unijunction
transistor (PUT) is a better choice.
+VBB
R4
η =
R3  R4
VP = 0.7 V + ηVBB
11-31
R1
VP
R3
PUT
C
R2
R4
(η is called the intrinsic standoff ratio)
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This multivibrator is also RC controlled.
11-32
t = 0.69RC
= 0.69 x 47 kW x 3.3 nF
= 0.107 ms
t = 2t = 0.214 ms
0V
f = 1/t = 4.67 kHz
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Non-sinusoidal op-amp
oscillators can also use
RC time constants for
frequency control, as
shown in this circuit.
11-33
Exponential sawtooth,
which is sometimes
treated as a triangle.
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11-34
Undesired oscillations:
make amplifiers useless.
Why is this a problem?
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Parasitic capacitances
combine with resistances
to form un-wanted
lag networks.
11-35
R
R
C
R
Output
C
C
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It’s the equivalent of a phase-shift oscillator.
11-36
Total Lag = 180
o
R
R
C
R
C
C
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This can lead to
unwanted oscillations
since the feedback
becomes positive
at some higher frequency.
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There is always some frequency
where feedback becomes positive.
11-37
R
R
C
R
C
C
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However,
if the gain is less
than unity at that
frequency, the
amplifier will not oscillate.
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The typical op amp has this characteristic:
120
100
11-38
Break frequency set
by a dominant (intentional)
internal lag circuit.
The gain is
less than unity
before combined
lags total 180o
of phase shift.
80
60
Gain in dB
40
20
0
1
10 100 1k 10 k 100 k 1M
Frequency in Hz
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Methods of preventing
oscillation:
11-39
• 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:
11-40
• 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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Direct digital synthesizer
(also called a numerically controlled oscillator)
11-41
Phase
accumulator
Sine lookup
table
DAC
LPF
Clock
Frequency tuning
word (binary)
The tuning word changes the phase increment value.
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o
30 phase
rotation
Access the
sine table
o
every 30
11-42
NOTE: Increasing the phase increment increases the frequency.
o
45 phase
rotation
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11-43
Direct digital synthesizer using the AD9850 IC
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Oscillator wrap-up quiz
11-44
Relaxation oscillators are controlled by RC
__________ __________.
time constants
Negative feedback becomes positive at some
frequency 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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11-45
REVIEW
• Oscillator Characteristics
• RC Circuits
• LC Circuits
• Crystal Circuits
• Relaxation Oscillators
• Undesired Oscillations
• Troubleshooting
• Direct Digital Synthesis
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