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Chapter 14 Feedback and Oscillator Circuits

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Page 1: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Chapter 14Feedback and Oscillator Circuits

Page 2: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Feedback ConceptsFeedback ConceptsFeedback ConceptsFeedback ConceptsFeedback ConceptsFeedback ConceptsFeedback ConceptsFeedback Concepts

The effects of negative feedback on an amplifier:

Disadvantage Disadvantage • Lower gain

AdvantagesAdvantages

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

AdvantagesAdvantages• Higher input impedance• More stable gain• Improved frequency response• Lower output impedance• Reduced noise• More linear operation

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Page 3: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Feedback Connection TypesFeedback Connection TypesFeedback Connection TypesFeedback Connection TypesFeedback Connection TypesFeedback Connection TypesFeedback Connection TypesFeedback Connection Types

• Voltage-series feedback• Voltage-shunt feedback• Current-series feedback• Current-shunt feedback

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

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Page 4: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

VoltageVoltageVoltageVoltageVoltageVoltageVoltageVoltage--------Series FeedbackSeries FeedbackSeries FeedbackSeries FeedbackSeries FeedbackSeries FeedbackSeries FeedbackSeries Feedback

For voltage-series feedback, the output voltage is fed back in series to the input.

The feedback gain is given by:

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

2

21f R

RRβ

1A

++++====≅≅≅≅

44

Page 5: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

VoltageVoltageVoltageVoltageVoltageVoltageVoltageVoltage--------Shunt FeedbackShunt FeedbackShunt FeedbackShunt FeedbackShunt FeedbackShunt FeedbackShunt FeedbackShunt Feedback

For a voltage-shunt feedback amplifier, the output voltage is fed back in parallel with the input.

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

i

of R

RA −−−−====

The feedback gain is given by

55

Page 6: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

For a current-series feedback amplifier, a portion of the output current is fed back in series with the input.

CurrentCurrentCurrentCurrentCurrentCurrentCurrentCurrent--------Series FeedbackSeries FeedbackSeries FeedbackSeries FeedbackSeries FeedbackSeries FeedbackSeries FeedbackSeries Feedback

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

Efeie

fe

Eie

feE

iefe

s

of Rhh

h

Rh

h)R(1

hh

βA1A

V

IA

++++

−−−−≅≅≅≅

++++

−−−−−−−−++++

−−−−====

++++========

To determine the feedback gain:

66

Page 7: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

CurrentCurrentCurrentCurrentCurrentCurrentCurrentCurrent--------Shunt FeedbackShunt FeedbackShunt FeedbackShunt FeedbackShunt FeedbackShunt FeedbackShunt FeedbackShunt Feedback

For a current-shunt feedback amplifier, a portion of the output current is directed back in parallel with the input.

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

s

of I

IA ====

The feedback gain is given by:

77

Page 8: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Summary of Feedback EffectsSummary of Feedback EffectsSummary of Feedback EffectsSummary of Feedback EffectsSummary of Feedback EffectsSummary of Feedback EffectsSummary of Feedback EffectsSummary of Feedback Effects

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

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Page 9: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Frequency Distortion with FeedbackFrequency Distortion with FeedbackFrequency Distortion with FeedbackFrequency Distortion with FeedbackFrequency Distortion with FeedbackFrequency Distortion with FeedbackFrequency Distortion with FeedbackFrequency Distortion with Feedback

• If the feedback network is purely resistive, then the gain with feedback will be less dependent on frequency variations. In some cases the resistive feedback removes all dependence on frequency variations.

• If the feedback includes frequency dependent components

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

• If the feedback includes frequency dependent components (capacitors and inductors), then the frequency response of the amplifier will be affected.

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Page 10: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Noise and Nonlinear DistortionNoise and Nonlinear DistortionNoise and Nonlinear DistortionNoise and Nonlinear DistortionNoise and Nonlinear DistortionNoise and Nonlinear DistortionNoise and Nonlinear DistortionNoise and Nonlinear Distortion

• The feedback network reduces noise by cancellation. The phase of the feedback signal is often opposite the phase of the input signal.

• Nonlinear distortion is also reduced simply because the gain is reduced. The amplifier is operating in midrange and not at the

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

reduced. The amplifier is operating in midrange and not at the extremes.

1010

Page 11: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Bandwidth with FeedbackBandwidth with FeedbackBandwidth with FeedbackBandwidth with FeedbackBandwidth with FeedbackBandwidth with FeedbackBandwidth with FeedbackBandwidth with Feedback

Feedback increases the bandwidth of an amplifier.

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1111

Page 12: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Gain Stability with FeedbackGain Stability with FeedbackGain Stability with FeedbackGain Stability with FeedbackGain Stability with FeedbackGain Stability with FeedbackGain Stability with FeedbackGain Stability with Feedback

Gain calculations with feedback are often based on external resistive elements in the circuit. By removing gain calculations from internal variations of ββββ and gm, the gain becomes more stable.

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

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Page 13: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Phase and Frequency Considerations Phase and Frequency Considerations Phase and Frequency Considerations Phase and Frequency Considerations Phase and Frequency Considerations Phase and Frequency Considerations Phase and Frequency Considerations Phase and Frequency Considerations

At higher frequencies the feedback signal may no longer be out of phase with the input. The feedback is thus positive and the amplifier, itself, becomes unstable and begins to

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

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Page 14: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Oscillator OperationOscillator OperationOscillator OperationOscillator OperationOscillator OperationOscillator OperationOscillator OperationOscillator Operation

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

The feedback signal must be positive.The feedback signal must be positive.

If the feedback signal is not positive or the gain is less than one, the oscillations dampens out.

The overall gain must equal oneThe overall gain must equal one(unity gain).

If the overall gain is greater than one, the oscillator eventually saturates.

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Page 15: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Types of Oscillator CircuitsTypes of Oscillator CircuitsTypes of Oscillator CircuitsTypes of Oscillator CircuitsTypes of Oscillator CircuitsTypes of Oscillator CircuitsTypes of Oscillator CircuitsTypes of Oscillator Circuits

PhasePhase--shift oscillatorshift oscillatorWien bridge oscillatorWien bridge oscillator

Tuned oscillator circuitsTuned oscillator circuitsCrystal oscillatorsCrystal oscillators

Unijunction oscillatorUnijunction oscillator

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

Unijunction oscillatorUnijunction oscillator

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Page 16: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

PhasePhasePhasePhasePhasePhasePhasePhase--------Shift OscillatorShift OscillatorShift OscillatorShift OscillatorShift OscillatorShift OscillatorShift OscillatorShift Oscillator

The amplifier must supply enough gain to compensate for losses. The overall gain must be unity.

The RC networks provide the necessary phase shift for a positive feedback.

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

The values of the RC components also determine the frequency of oscillation:

6RCπ2

1f ====

more…more…

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Page 17: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

PhasePhasePhasePhasePhasePhasePhasePhase--------Shift OscillatorShift OscillatorShift OscillatorShift OscillatorShift OscillatorShift OscillatorShift OscillatorShift Oscillator

The amplifier must supply enough gain to compensate for losses. The overall gain must be unity.

The RC networks provide the necessary phase shift for a positive feedback.

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

The values of the RC components also determine the frequency of oscillation:

RC62

1f

π

=

more…more…

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Page 18: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Wien Bridge OscillatorWien Bridge OscillatorWien Bridge OscillatorWien Bridge OscillatorWien Bridge OscillatorWien Bridge OscillatorWien Bridge OscillatorWien Bridge Oscillator

• The feedback resistors

The amplifier must supply enough gain to compensate for losses. The overall gain must be unity.

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

• The feedback resistors are R3 and R4.

• The phase-shift components are R1, C1and R2, C2.

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Page 19: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Tuned Oscillator CircuitsTuned Oscillator CircuitsTuned Oscillator CircuitsTuned Oscillator CircuitsTuned Oscillator CircuitsTuned Oscillator CircuitsTuned Oscillator CircuitsTuned Oscillator Circuits

Tuned oscillators use a parallel LC resonant circuit (LC tank) to provide the oscillations.

There are two common types:

ColpittsColpitts—The resonant circuit is an inductor and two capacitors.

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

HartleyHartley—The resonant circuit is a tapped inductor or two inductors and one capacitor.

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Page 20: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Colpitts Oscillator CircuitColpitts Oscillator CircuitColpitts Oscillator CircuitColpitts Oscillator CircuitColpitts Oscillator CircuitColpitts Oscillator CircuitColpitts Oscillator CircuitColpitts Oscillator Circuit

eqo

LCπ2

1f ====

The frequency of oscillation is determined by:

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

where:

21

21eq CC

CCC

++++====

2020

Page 21: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Hartley Oscillator CircuitHartley Oscillator CircuitHartley Oscillator CircuitHartley Oscillator CircuitHartley Oscillator CircuitHartley Oscillator CircuitHartley Oscillator CircuitHartley Oscillator Circuit

The frequency of oscillation is determined by:

CLπ2

1f

eqo ====

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

where:

M2LLL 21eq ++++++++====

2121

Page 22: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Crystal OscillatorsCrystal OscillatorsCrystal OscillatorsCrystal OscillatorsCrystal OscillatorsCrystal OscillatorsCrystal OscillatorsCrystal Oscillators

The crystal appears as a resonant circuit.

The crystal has two resonant frequencies:

Series resonant conditionSeries resonant condition• RLC determine the resonant frequency• The crystal has a low impedance

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

Parallel resonant conditionParallel resonant condition• RL and CM determine the resonant frequency• The crystal has a high impedance

The series and parallel resonant frequencies are very close, within 1% of each other.

2222

Page 23: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Series Resonant Crystal OscillatorSeries Resonant Crystal OscillatorSeries Resonant Crystal OscillatorSeries Resonant Crystal OscillatorSeries Resonant Crystal OscillatorSeries Resonant Crystal OscillatorSeries Resonant Crystal OscillatorSeries Resonant Crystal Oscillator

• RLC determine the resonant frequency

• The crystal has a low impedance

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

2323

Page 24: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Parallel Resonant Crystal OscillatorParallel Resonant Crystal OscillatorParallel Resonant Crystal OscillatorParallel Resonant Crystal OscillatorParallel Resonant Crystal OscillatorParallel Resonant Crystal OscillatorParallel Resonant Crystal OscillatorParallel Resonant Crystal Oscillator

• RL and CM determine the resonant frequency

• The crystal has a high impedance

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

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Page 25: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Unijunction OscillatorUnijunction OscillatorUnijunction OscillatorUnijunction OscillatorUnijunction OscillatorUnijunction OscillatorUnijunction OscillatorUnijunction Oscillator

[[[[ ]]]]η)(11lnCR1

fTT

o −−−−====

The output frequency is determined by:

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

Where ηηηη is a rating of the unijunction transistor with values between 0.4 and 0.6.

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Page 26: Chapter 14 Feedback and Oscillator Circuits - KMUTTwebstaff.kmutt.ac.th/~suwat.pat/material/ene212 ch 14 FB Osc... · Chapter 14 Feedback and Oscillator Circuits. Feedback Concepts

Unijunction Oscillator WaveformsUnijunction Oscillator WaveformsUnijunction Oscillator WaveformsUnijunction Oscillator WaveformsUnijunction Oscillator WaveformsUnijunction Oscillator WaveformsUnijunction Oscillator WaveformsUnijunction Oscillator WaveformsThe unijunction oscillator (or relaxation oscillator) produces a sawtooth waveform.

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

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