Chapter_8 Power Amplifier

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Transcript of Chapter_8 Power Amplifier

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    SJTU Zhou Lingling 1

    Chapter 8

    Oscillator and

    Power Amplifier

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    SJTU Zhou Lingling 2

    Outline

    Oscillator

    Power amplifier

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    SJTU Zhou Lingling 3

    Oscillator

    Basic principles of sinusoidal oscillator.

    The Wien-bridge oscillator

    The phase shift oscillator

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    SJTU Zhou Lingling 4

    Basic Principles of Sinusoidal

    Oscillator

    The oscillator feedback loop

    The basic structure of a sinusoidal oscillator. A positive-feedback loop is formed by an

    amplifier and a frequency-selective network.

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    SJTU Zhou Lingling 6

    Basic Principles of Sinusoidal

    Oscillator

    Nonlinear amplitude control

    To ensure that oscillations will start, the A is

    slightly greater than unity.

    As the power supply is turned on, oscillation

    will grown in amplitude.

    When the amplitude reaches the desired level,

    the nonlinear network comes into action andcause the A to exactly unity.

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    SJTU Zhou Lingling 8

    A Popular Limiter Circuit for

    Amplitude Control

    Transfer characteristic of the limiter circuit;

    When Rfis removed, the limiter turns into a comparator with the

    characteristic shown.

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    SJTU Zhou Lingling 9

    Oscillator Circuits

    Op Amp-RC Oscillator Circuits

    The Wien-Bridge Oscillator

    The phase-Shift Oscillator

    LC-Tuned Oscillator Colpitts oscillator

    Hareley oscillator

    Crystal Oscillator

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    SJTU Zhou Lingling 10

    The Wien-Bridge Oscillator

    A Wien-bridge oscillator without amplitude stabilization.

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    SJTU Zhou Lingling 11

    The Wien-Bridge Oscillator

    The loop gain transfer function

    Oscillating frequency

    To obtain sustained oscillation

    sCRsCR

    RRsL

    13

    11)( 2

    !

    RC1

    0 ![

    21

    2 !R

    R

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    SJTU Zhou Lingling 12

    The Wien-Bridge Oscillator

    A Wien-bridge oscillator with a limiter used for amplitude control.

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    SJTU Zhou Lingling 13

    The Phase-Shift Oscillator

    The circuit consists of a negative-gain amplifier and three-section RC ladder

    network.Oscillating frequency is the one that the phase shift of the RC network is 1800

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    SJTU Zhou Lingling 14

    The Phase-Shift Oscillator

    A practical phase-shift oscillator with a limiter for amplitude stabilization.

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    SJTU Zhou Lingling 15

    The LC-Tuned oscillator

    Colpitts Oscillator

    A parallel LC resonator

    connected between collector and

    base.

    Feedback is achieved by way ofa capacitive divider

    Oscillating frequency is

    determined by the resonance

    frequency.

    )(121

    210

    CC

    CCL

    ![

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    SJTU Zhou Lingling 16

    The LC-Tuned oscillator

    Hartley Oscillator

    A parallel LC resonator

    connected between collector and

    base.

    Feedback is achieved by way ofan inductive divider.

    Oscillating frequency is

    determined by the resonance

    frequency.

    )(121

    210

    CC

    CCL

    ![

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    SJTU Zhou Lingling 17

    Crystal Oscillators

    A piezoelectric crystal. (a) Circuit symbol. (b) Equivalent circuit.

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    SJTU Zhou Lingling 18

    Crystal Oscillators

    Crystal reactance versus

    frequency (neglecting the small

    resistance r, ).

    A series resonance at

    A parallel resonance atss LC1![

    )(1ps

    psp

    CC

    CCL

    ![

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    SJTU Zhou Lingling 19

    Crystal Oscillators

    A Pierce crystal oscillator utilizing a CMOS inverter as an amplifier.

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    SJTU Zhou Lingling 20

    Power Amplifier

    Small-signal approximation and models

    either are not applicable or must be used

    with care.

    Deliver the power to the load in efficient

    manner.

    Power dissipation is as low as possible.

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    SJTU Zhou Lingling 21

    Classification of Power Amplifier

    Power amplifiers are classified according to

    the collector current waveform that results

    when an input signal is applied.

    Conducting angle.

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    SJTU Zhou Lingling 22

    Classification of Power Amplifier

    Collector current waveforms for transistors operating

    in (a) class A, (b) class B

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    SJTU Zhou Lingling 23

    Classification of Power Amplifier

    class AB class C

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    SJTU Zhou Lingling 24

    Class B Output Stage

    A class B output

    stage.

    Complementary

    circuits.

    Push-pull operation

    Maximum power-

    conversion efficiencyis 78.5%

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    SJTU Zhou Lingling 25

    Transfer Characteristic

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    SJTU Zhou Lingling 26

    Crossover Distortion

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    SJTU Zhou Lingling 27

    Power Dissipation

    The load power

    Maximum load power

    L

    oL

    R

    VP

    2

    2

    1!

    L

    CC

    VVL

    oL

    R

    V

    R

    VP

    CCo 2

    2

    12

    2

    max !!

    !

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    SJTU Zhou Lingling 28

    Power Dissipation

    Total supply power

    Maximum total supply power

    CC

    L

    os V

    R

    VP

    2

    T

    !

    L

    CC

    VV

    CC

    L

    os

    R

    VV

    R

    VP

    CCo

    2

    max

    22

    TT

    !!

    !

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    SJTU Zhou Lingling 29

    Power Dissipation

    Power-conversion efficiency

    Maximum power-conversion efficiency

    CC

    o

    V

    V

    4

    TL !

    %5.78

    4 max !!

    ! CCo VVCC

    o

    V

    VTL

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    SJTU Zhou Lingling 30

    Power Dissipation

    Power dissipation

    Maximum Power dissipation

    L

    oCC

    L

    oD

    R

    VV

    R

    VP

    2

    2

    12!

    T

    max2

    2

    2

    2

    maxmax

    2.02

    2

    12

    L

    L

    CC

    VVL

    oCC

    L

    oDPDN

    PR

    V

    R

    VV

    R

    VPP

    CCo

    !!

    !!

    !

    T

    TT

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    SJTU Zhou Lingling 31

    Class AB Output Stage

    A bias voltage VBB is applied between the bases ofQ and QP, giving rise to a bias

    currentIQ . Thus, for small vI, both transistors conduct and crossover distortion is

    almost completely eliminated.

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    SJTU Zhou Lingling 32

    A Class AB Output Stage Utilizing

    Diodes for Biasing

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    SJTU Zhou Lingling 33

    A Class AB Output Stage Utilizing

    A VBEMultiplier for Biasing