electronic-devices-9th-edition-by-floyd pp16a

38
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Electronic Devices Ninth Edition Floyd Chapter 16

Transcript of electronic-devices-9th-edition-by-floyd pp16a

Page 1: electronic-devices-9th-edition-by-floyd pp16a

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th editionThomas L. Floyd

Electronic DevicesNinth Edition

Floyd

Chapter 16

Page 2: electronic-devices-9th-edition-by-floyd pp16a

© 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.

Page 3: electronic-devices-9th-edition-by-floyd pp16a

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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.

Page 4: electronic-devices-9th-edition-by-floyd pp16a

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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

Page 5: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 6: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 7: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 8: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 9: electronic-devices-9th-edition-by-floyd pp16a

© 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.

Page 10: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 11: electronic-devices-9th-edition-by-floyd pp16a

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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

Page 12: electronic-devices-9th-edition-by-floyd pp16a

© 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.

Page 13: electronic-devices-9th-edition-by-floyd pp16a

© 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.

Page 14: electronic-devices-9th-edition-by-floyd pp16a

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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.

Page 15: electronic-devices-9th-edition-by-floyd pp16a

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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

Page 16: electronic-devices-9th-edition-by-floyd pp16a

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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

Page 17: electronic-devices-9th-edition-by-floyd pp16a

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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.

Page 18: electronic-devices-9th-edition-by-floyd pp16a

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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

Page 19: electronic-devices-9th-edition-by-floyd pp16a

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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

Page 20: electronic-devices-9th-edition-by-floyd pp16a

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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.

Page 21: electronic-devices-9th-edition-by-floyd pp16a

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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.

Page 22: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 23: electronic-devices-9th-edition-by-floyd pp16a

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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

Page 24: electronic-devices-9th-edition-by-floyd pp16a

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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

-

Page 25: electronic-devices-9th-edition-by-floyd pp16a

© 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

-

Page 26: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 27: electronic-devices-9th-edition-by-floyd pp16a

© 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.

Page 28: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 29: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 30: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 31: electronic-devices-9th-edition-by-floyd pp16a

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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

Page 32: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 33: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 34: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 35: electronic-devices-9th-edition-by-floyd pp16a

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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

Page 36: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 37: electronic-devices-9th-edition-by-floyd pp16a

© 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

Page 38: electronic-devices-9th-edition-by-floyd pp16a

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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