View
407
Download
102
Category
Preview:
Citation preview
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Electronic DevicesNinth Edition
Floyd
Chapter 16
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Feedback Oscillators
Oscillators are electronic circuits that produce a periodic waveform with only the dc supply voltage as an input. In a feedback oscillator, a fraction of the output is returned with no net phase shift.
SummarySummary
VoutAv
Feedbackcircuit
Noninvertingamplifier
Vf
In phase
If the feedback circuit returns the signal in phase, a noninverting amplifier produces positive feedback.
VoutAv
Feedbackcircuit
Invertingamplifier
Vf
Out of phase
If the feedback circuit returns the signal out of phase, an inverting amplifier produces positive feedback.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Feedback Oscillators
Feedback oscillators require a small disturbance such as that generated by thermal noise to start oscillations. This initial voltage starts the feedback process and oscillations.
SummarySummary
t0
Acl > 1 Acl = 1
Computer simulations, such as Multisim, use digital signals, which do not have thermal noise. This often creates a problem for computer simulations of oscillators.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Wien-Bridge Oscillator
RC feedback is used in various lower frequency sine-wave oscillators. The text covers three: the Wien-bridge oscillator, the phase-shift oscillator, and the twin-T oscillator.
SummarySummary
The feedback circuit in a Wien-bridge uses a lead-lag circuit. When the R’s and C’s have equal values, the output will be ⅓ of the input at only one frequency and the phase shift at this frequency will be 0o.
R2
R1Vout
Vout
ffr
Vin
Vin13
C2
C1
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Wien-Bridge Oscillator
The basic Wien-bridge uses the lead-lag network to select a specific frequency that is amplified. The voltage-divider sets the gain to make up for the attenuation of the feedback network.
SummarySummary
The noninverting amplifier must have a gain of exactly 3.0 as set by R1 and R2 to make up for the attenuation. If it is too little, oscillations will not occur; if it is too much the sine wave will be clipped.
Vout
–
+
R4
R3
R2
R1
C1C2
Voltage-divider
Lead-lag network
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Wien-Bridge Oscillator
To produce the precise gain required, the Wien bridge needs some form of automatic gain control (AGC). One popular method is shown here and uses a JFET transistor.
SummarySummary
The key elements of the AGC circuit are highlighted in yellow. The diode charges C3 to the negative peak of the signal. This develops the gate bias voltage for the JFET that is related to the output level.
Rf
R3
R1
R2
C1
Q1
C2
–
+
Vout
C3R4
D1
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Wien-Bridge Oscillator
The JFET is operated in the ohmic region and can change its resistance rapidly if conditions change.
SummarySummary
Recall from Chapter 8 that a JFET acts as a variable resistor in the ohmic region. If the output increases, the bias tends to be larger, and the drain-source resistance increases (and vice-versa). In the Wien-bridge, the JFET drain-source resistance controls the gain of the op-amp and will compensate for any change to the output.
Ohmic region
VDS (V)
VG = 0 V
VG = 0.5 V-
VG = V- 1.0
VG = V- 1.5
ID
(mA)
00
1
1
2
2
3
3
4
4
5
5
6
7
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Wien-Bridge Oscillator
When the R’s and C’s in the feedback circuit are equal, the frequency of the bridge is given by
SummarySummary
Rf
R3
R1
R2
C1
Q1
C2
–
+
Vout
C3R4
D1
12πrf RC
10 k
1.0 k 10 k
680
680
4.7 nF
4.7 nF
1.0 F
What is fr for the Wien bridge?
12π
12π 680 4.7 nF
rf RC
= 48.9 kHz
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Phase-Shift Oscillator
The phase-shift oscillator uses three RC circuits in the feedback path that have a total phase shift of 180o at one frequency – for this reason an inverting amplifier is required for this circuit.
SummarySummary
–
+
Vout
R2
Rf
0 V
R1 R3
C3C2C1
Even with identical R’s and C’s, the phase shift in each RC circuit is slightly different because of loading effects. When all R’s and C’s are equal, the feedback attenuates the signal by a factor of 29.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Phase-Shift Oscillator
Conditions for oscillation with the phase-shift oscillator is that if all R’s and C’s are equal, the amplifier must have a gain of at least 29 to make up for the attenuation of the feedback circuit. This means that Rf /R3 ≥ 29.
SummarySummary
–
+
Vout
R2
Rf
0 V
R1 R3
C3C2C1
Under these conditions, the frequency of oscillation is given by
12π 6rf RC
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Phase-Shift Oscillator
Multisim can simulate the phase-shift oscillator, but has difficulty starting. In the Multisim file for Example 16-3, a switch is provided to provide a voltage spike to start oscillations. This is not needed in the actual circuit.
SummarySummary
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Phase-Shift Oscillator
SummarySummary
Design a phase-shift oscillator for a frequency of 800 Hz. The capacitors are to be 10 nF.
Start by solving for the resistors needed in the feedback circuit:
1 1
2π 6 2π 6 800 Hz 10 nFr
Rf C
8.12 k (Use 8.2 k.)
Rf = 29R = 238 k. –
+
Vout
R2
Rf
R1 R3
C3C2C1
Calculate the feedback resistor needed:
10 nF 10 nF 10 nF
8.2 k 8.2 k 8.2 k
238 k
The following slide shows the Multisim check.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Phase-Shift Oscillator
SummarySummary
Because the Rf is not precise, you will see the output “grow” in Multisim. In actual circuits, you can use a potentiometer to adjust a precise gain, but the circuit will be sensitive to temperature change.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Phase-Shift Oscillator
SummarySummary
You can also use back-to back zener diodes to limit the output. The output is limited to about 7 Vpp with 1N4372A (3.0 V) zeners.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Twin-T Oscillator
The basic twin-T oscillator combines a low-pass and high-pass filter to form a notch filter at the oscillation frequency.
SummarySummary
R2
R1
Vout
Low-pass
High-pass
Twin-T filter
–
+
An excellent notch filter can be formed by using R’s and C’s related by a factor of 2 as shown here.
R R
R/2
C C
2C
With this relationship, the oscillation frequency is approximately
12πrf RC
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Twin-T Oscillator
Two improvements to the basic circuit are shown here – adding the parallel diodes and R6 significantly reduces distortion by attenuating harmonics. The potentiometer adds output amplitude adjustment.
SummarySummary
The frequency is a little higher than the predicted value of 1.94 kHz. With ±15 V power supplies, the measured values are:f = 2.28 kHz @2.0 Vpp
Amplitude = 0 to 27 VppR2
R
R
1B
1A
Vout
–
+
1.0 nF 1.0 nF
2.0 nF
82 k 82 k
330 k
41 k
10 k
10 k
220
OutputAmplitude
1N914AD D1 2 and
741C
R3 R4
R6
R5
C1
C2 C3
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Colpitts Oscillator
LC feedback oscillators use resonant circuits in the feedback path. A popular LC oscillator is
SummarySummary
the Colpitts oscillator. It uses two series capacitors in the resonant circuit. The feedback voltage is developed across C1.
In
L
Vf Vout
AvVf Vout
IC1 C2
OutThe effect is that the tank circuit is “tapped”. Usually C1 is the larger capacitor because it develops the smaller voltage.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Colpitts Oscillator
SummarySummary
The resonant frequency is found by
L
Zin
C1 C2
Vout
T
12πrf LC
If Q > 10, this formula gives good results.
2
2T
112πr
QfQLC
Recall that the total capacitance of two series capacitors is the product-over-sum of the individual capacitors. Therefore,
1 2
1 2
1
2πrf
C CLC C
For Q < 10, a correction for Q is
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Hartley Oscillator
SummarySummary
The Hartley oscillator is similar to the Colpitts oscillator, except the resonant circuit consists of two series inductors (or a single tapped inductor) and a parallel capacitor. The frequency for Q > 10 is
In
AvVf Vout
Out L1 L2
COne advantage of a Hartley oscillator is that it can be tuned by using a variable capacitor in the resonant circuit.
T 1 2
1 12π 2π
rf L C L L C
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Crystal Oscillator
SummarySummary
Crystal oscillators are highly stable oscillators for demanding circuits such a radio transmitters. Crystals have very high Q.
The crystal acts as the resonant circuit for the modified Colpitts oscillator and stabilizes the oscillations. The capacitors still tap off a feedback signal to the CE amplifier.
R4
L
R2
R1
+VCC
XTAL
Vout
C4
C2C1
C3
Manufacturers prepare natural crystals (usually quartz) by mounting a very thin slab between metal electrodes. When a small ac voltage is applied, the crystal oscillates at its own resonant frequency.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Relaxation Oscillators
SummarySummary
Relaxation oscillators are characterized by an RC timing circuit and a device that periodically changes state.
–
+
R1
Vout
Integrator
–
+Comparator
R3
R2
C
The triangular wave oscillator is an example. For this circuit, the device that changes states is a comparator with hysteresis (Schmitt trigger). The RC timing device is an integrator. The comparator output can be used as a square wave output. The trigger points set the triangle’s peak-to-peak voltage:
3UTP
2max
RV VR
3LTP
2min
RV VR
-
A square wave can be taken as an output here.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Relaxation Oscillators
SummarySummary
For the triangular wave generator, the frequency is found from:2
1 3
14r
RfR C R
–
+
R1
Vout
Integrator
–
+Comparator
R3
R2
C
What is the frequency of the circuit shown here?
10 nF
82 k
22 k
10 k
2
1 3
14
1 22 k4 82 k 10 nF 10 k
rRf
R C R
= 671 Hz
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Relaxation Oscillators
SummarySummary
Normally, the triangle wave generator uses fast comparators to avoid slew rate problems. For non-critical applications, a 741 will work nicely for low frequencies (<2 kHz). The circuit here is one you can construct easily in lab. (The circuit is the same as Example 16-4 but with a larger C.)The waveforms are:
Both channels: 5 V/div 250 s/div
–
+
R1
V
V
out1
out2
–
+
R3
R2
C
33 k
10 k
10 k
0.1 F741
741
Square wave
Triangle wave
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Relaxation Oscillators
SummarySummary
A sawtooth VCO also uses an integrator to create the ramp portion of the waveform. In this case, when VC > VG + 0.7 V, the PUT fires and the capacitor discharges rapidly.In this circuit, the device that changes state is a PUT and the RC timing circuit is an integrator.
–
+
Ri
–
+VIN
C
PUT
VG
+
–
Vp
Vout0 V
off
charge–
+
Ri
–
+VIN
C
PUT
VG
+
–
Vp
Vout0 V
discharge
–
+
Ri
–
+VIN
C
PUT
VG
+
–
Vp
Vout0 V
The frequency is found by:
IN
F
14 i p
Vf
R C V V
-
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Relaxation Oscillators
SummarySummary
Another relaxation oscillator that uses a Schmitt trigger is the basic square-wave oscillator. The trigger points are set by R2 and R3. The capacitor charges and discharges between these levels:
The period of the waveform is given by:
Vout
–
+R2
R3
R1
Vf
VC
C
31
2
22 ln 1
RT R C
R
3UTP
2 3max
RV V
R R
3LTP
2 3max
RV VR R
-
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The 555 Timer
SummarySummary
The 555 block diagram is shown.
RA5 k
5 k
5 k
+
–
+
–
(8)
VCC
(6)
(5)
(2)
(7)Discharge
Trigger
Controlvoltage
Threshold
RB
RC
Flip-flop
Outputbuffer
Output(3)
Dischargepath transistor
Lowercomparator
Uppercomparator
(1)
Gnd
(4)
Reset
RQ
S
Qd
RA
+
–
+
–
(8)
+VCC
(6)
(5)
(2)
(7)
RB
RC
Vout(3)
(1)(4)
+VCC
R2
Qd
Discharging
+–
Cha
rgin
g
R1
2 1 2 1
on off on
2 1 2 1
2 2 2
1 1VCC
13
VCC23
VC
555
RQ
S
Cext
Addition of a few external parts forms a versatile astable multivibrator.
The frequency and duty cycle are given by:
1 2
1.442r
ext
fR R C
1 2
1 2
Duty cycle 100%2
R RR R
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Selected Key TermsSelected Key Terms
Feedback oscillator
Relaxation oscillator
Positive feedback
An electronic circuit that operates with positive feedback and produces a time-varying output signal without an external input signal.
An electronic circuit that uses an RC timing circuit to generate a nonsinusoidal waveform without an external input signal.
The return of a portion of the output signal such that it reinforces and sustains the input signal.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
1. The Wien-bridge oscillator uses the network shown. If R’s and C’s are equal, the maximum Vout will be phase-shifted by
a. 0o
b. 90o
c. 180o
d. 270o
R2
R1VoutVin
C2
C1
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
2. The ohmic region of a JFET is the operating region for a JFET used in a Wien-bridge AGC circuit because the
a. resistance is constant
b. resistance depends on VDS
c. resistance depends on VG
d. resistance depends on ID
VDS (V)
VG = 0 V
VG = 0.5 V-
VG = V- 1.0
VG = V- 1.5
ID
(mA)
00
1
1
2
2
3
3
4
4
5
5
6
7
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
3. Assume the Wien-bridge oscillator shown is operating normally and the output voltage is a 6 Vpp sine wave. VG should be
a. 1 Vpp sine wave
b. 2 Vpp sine wave
c. +3 VDC
d. -2.3 VDC
Rf
R3
R1
R2
C1
Q1
C2
–
+
Vout
C3R4
D1VG
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
4. The twin-t oscillator uses the network shown. At the oscillator frequency, the output of this network is phase-shifted by
a. 0o
b. 90o
c. 180o
d. 270o
VoutVin
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
5. The overall frequency response characteristic of the twin-t oscillator network shown is that of a
a. low-pass filter
b. high-pass filter
c. band-pass filter
d. notch filter
VoutVin
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
6. An example of an LC feedback oscillator is a
a. phase-shift oscillator
b. relaxation oscillator
c. Colpitts oscillator
d. Wien-bridge oscillator
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
7. In a crystal oscillator, the crystal acts like a
a. band-pass filter
b. resonant circuit
c. notch filter
d. power source
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
8. The waveforms at Vout1 and Vout2 should be a
a. sawtooth wave and a sine waveb. triangle wave and a square wavec. sawtooth wave and a square wave
d. triangle wave and a sine wave
–
+
R1
V
V
out1
out2
–
+
R3
R2
C
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
9. The output waveform from this circuit should be a
a. sawtooth wave
b. triangle wave
c. square wave
d. sine wave –
+
Ri
–
+VIN
C
PUT
VG
+
–
Vp
Vout0 V
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
10. To make a basic astable multivibrator using a 555 timer, as a minimum you need
a. one resistor and one capacitor
b. one resistor and two capacitors
c. two resistors and one capacitor
d. two resistors and two capacitors
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
Answers:
1. a
2. c
3. d
4. c
5. d
6. c
7. b
8. b
9. a
10. c
Recommended