electronic principles - pccspot.pcc.edu/~wlara/eet222/slides/23.pdf · title: microsoft powerpoint...
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MALVINO & BATES
SEVENTH EDITION
Electronic
PRINCIPLES
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Oscillators
ChapterChapter2323
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Topics Covered in Chapter 23
• Theory of sinusoidal oscillation
• The Wien-bridge oscillator
• Other RC oscillators
• The Colpitts oscillator
• Other LC oscillators
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Topics Covered in Chapter 23
(Continued)
• Quartz crystals
• The 555 timer
• Astable operation of the 555 timer
• 555 circuits
• The phase-locked loop
• Function generator ICs
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voutAV
B
Sinusoidal oscillation requires
both the correct phase and loop gain.
AVB < 1
AVB = 1
AVB > 1
φφφφ = 0° = positive feedback..
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Sinusoidal oscillators
• The starting signal is thermal noise.
• AVB > 1 at startup (AVB is the loop gain).
• The feedback network determines B and
the phase of the feedback.
• Only one frequency arrives at the input
as an in-phase signal (positive feedback).
• Either AV or B is eventually decreased so
that AVB = 1.
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Wien-bridge oscillator
• Used for low to moderate frequencies (5
Hz to 1 MHz)
• Produces an almost perfect sine wave
• Nonlinear resistance is used to decrease
loop gain to 1
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R
vout
C
Wien-Bridge oscillator circuit
RL2R’
Tungsten
lamp
C R
R’
2ππππRC
1fr =
vout t
Resistance of lamp
increases until
equilibrium is reached
R’
.
.. .
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Other RC oscillators
• Twin-T uses an amplifier and RC circuits
to produce the required loop gain and
phase shift
• Twin-T works well at only one frequency
• Phase shift uses an amplifier and RC
circuits
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Colpitts oscillator
• LC oscillators are preferred for
frequencies between 1 MHz to 500 MHz
• One of the most widely used oscillators
• Recognized by the capacitive voltage
divider
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+VCCColpitts oscillator circuit
2ππππ LC
1fr =
R2
R1
RFC
RE
C2
C1
CE
L
C3
vout
C1 C2
C1 +C2
C =
C1
C2
B =
C2
C1
AVmin =
..
.
.
.
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Other LC oscillators
• Armstrong uses a transformer to produce
a feedback signal
• Hartley uses an inductive voltage divider
to produce a feedback signal
• Clapp has a small series capacitor in the
inductive branch of the resonant circuit
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+VCCColpitts CB oscillator
2ππππ LC
1fr =
R2
R1
RFC
RE
C2
C1
LC3
C1 C2
C1 +C2
C =C1
C1 +C2
B =
C1
AVmin =C1 +C2
RL
C4.
.
.
. ..
.
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+VCCHartley oscillator
2ππππ LC
1fr =
R2
R1
RFC
R3
C L1
vout
L = L1 + L2
L2
L1
B =
L1
L2
AVmin =
L2
.
.
.
.
. .
.
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Quartz crystal Slab cut from
crystal
Electrodes
and leads
Schematic
symbol
Crystal-controlled oscillators are used
when frequency stability is important.
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+VCCCrystal oscillator
R2
R1
RFC
R3
C2
C1
CE
vout
Xtal
.
.
.
.
.
..
.
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Crystals
• The fundamental frequency (series
resonance) is controlled by the slab
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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The 555 timer
• Contains two comparators, an RS flip-
flop, and an npn transistor
• Has an upper trip point (UTP) and a
lower trip point (LTP)
• In the monostable mode, the input
triggers must fall below the LTP to start
the action
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Monostable operation of the 555 timer IC
555 vout
+VCC
T
8
3
1
2
Wvout
T T = trigger
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Astable operation of the 555 timer IC
555 vout
+VCC
8
3
1
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5 kΩΩΩΩ
R
S Q
Q5 kΩΩΩΩ
5 kΩΩΩΩ
Gnd1
Out
3
+VCC8Discharge
Threshold
Control
Trigger
7
6
5
2
555
Reset4
UTP2VCC /3
LTPVCC /3
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RC RC
RB RB
RS RR
+VCC
S R
A discrete RS flip-flop
S
R
Q
Q
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1
3
+VCC
7
6
2
8
555 IC configured for monostable operation
R
C W
W = 1.1RC
. .
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Astable operation of the 555
• Produces a rectangular output whose
duty cycle can be set between 50 and 100
percent
• A control voltage is sometimes used to
determine the frequency
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1
3
+VCC
7
6
2
8
555 IC configured for astable operation
R2
CW
W = 0.693(R1 + R2)CR1
TT = 0.693(R1 + 2R2)C
R1 + 2R2
R1 + R2D =
.
.
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1
3
+VCC
7
6
2
8
555 voltage-controlled oscillator
R2
C
R1
W = -(R1 + R2)C lnVCC - Vcon
VCC - 0.5Vcon
W + 0.693R2C
1f =
5
Vcon applied to pin 5
+Vcon
+Vcon /2
.
.
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555 circuits
• Time delays
• Alarms
• Ramp outputs
• Pulse-width modulators
• Pulse-position modulators
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+VCC
8
R
6 5
4
3
2 1
7
C
555
Pulse-width modulation with the 555 timer IC
Clock in
Modulating
signal in
A
B
PWM
out
UTP = 2VCC /3 + vmod
W = -RC ln 1 -UTP
VCC( )
W
T
T
The output frequency is
established by the input clock.
. .
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Pulse width is variable
+VCC
8
R1
6 5
4
3
2 1
7
C
555
Pulse-position modulation with the 555 timer IC
Modulating
signal in
A
B
PPM
outR2Space is constant
The leading edge of each pulse
is a function of the modulation.
Space = 0.693R2C
.
.
.
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Phase-locked loops
• It is possible to phase-lock an oscillator to
a signal by using a phase detector and
negative feedback.
• PLLs can be used to remove noise from a
signal.
• PLLs can be used to demodulate an FM
signal.
• PLLs are available as monolithic ICs.
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Function generator ICs
• Ability to produce sine, square, triangle, pulse, and sawtooth waveforms
• By connecting external resistors and capacitors, output waveforms can be made to vary in frequency and amplitude
• AM/FM, voltage-to-frequency conversion, and FSK can also be performed