Lect 12 Feb 24

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    Lecture 12-3

    BE Diode Characteristic

    We can effectively use a simplified model for the diode if we know theapproximate operating range of the BE diode characteristic

    0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

    0

    1

    2

    3

    mA

    IE

    Q1Default

    + 0VVBE

    0.000 pA

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    Lecture 12-4

    BE Diode Characteristic

    Note that VON changes if were analyzing an order of magnitude less

    current

    So how do we know what the real VON is?

    Q1Default

    + 0VVBE

    0.000 pA

    0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

    0.0

    0.1

    0.2

    0.3

    mA

    IE

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    Lecture 12-5

    Simplified BJT Circuit Analysis

    Assuming VBE is 0.78 volts, we can approximate this circuit solution by hand

    analysis

    Q1

    RB100E3

    +2VVIN

    RC

    1E3

    + 5VVCC

    IS=1e-16

    100=

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    Lecture 12-6

    Simplified BJT Circuit Analysis

    What happens as RC is decreased?

    Will it remain in the active region?

    Q1

    RB100E3

    +2VVIN

    RC500

    + 5VVCC

    IS=1e-16 100=

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    Lecture 12-7

    Simplified BJT Circuit Analysis

    What happens as RC is increased?

    Will it remain in the active region?

    Q1

    RB100E3

    +2VVIN

    RC5000

    + 5VVCC

    IS=1e-16 100=

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    Lecture 12-8

    Saturation

    When both the EBJ and CBJ are forward biased, the transistor is no longer inthe active region, but it is in the saturation region of operation

    We can easily solve for the maximum iC that we can have before we reach

    saturationfor this circuit

    Q1

    RB

    +VIN

    RC

    + 5VVCC

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    Lecture 12-9

    Saturation

    With both diodes forward biased, the collector-to-emitter voltage, vCE,saturates toward a constant value

    _

    VBC

    +

    _

    VBE

    +

    ~0.4 volts

    ~0.7-0.9 volts

    VBC

    VBE

    _

    +

    _

    + _

    +

    vCEsat

    0.3volts

    0 1 2 3 4 5

    -1

    0

    1

    2

    mA

    IC

    VCE

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    Lecture 12-

    Saturation

    In saturation, increasing iC shows little increase in iB. Why?

    0.0 0.2 0.4 0.6 0.8 1.0

    -1

    0

    1

    2

    mA

    IC

    VCE

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    Lecture 12-

    Regions of Operation

    The complete i-v characteristic is:

    0 1 2 3 4 5

    -1

    1

    3

    mA

    VCE

    IB=1A

    IB=5A

    IB=10A

    IB=15A

    IB=20A

    IC

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    Lecture 12-

    Regions of Operation

    VCE

    IB=1A

    IB=5A

    IB=10A

    IB=15A

    IB=20A

    0.0 0.2 0.4 0.6 0.8 1.0

    1

    3

    mA saturation active

    cut-off

    IC

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    Lecture 12-

    Temperature Variations

    The collector current vs. the base-emitter voltage follows a diodecharacteristic, which like a diode, is temperature dependent

    0.6 0.7 0.8

    0

    2

    4mA

    VBE

    T=32CT=27C

    T=22C

    Q1

    Default

    + 0VVBE

    R4

    1E3

    + 15VVCC

    Does this value of RC significantly impact the values for iC in this example?

    IC

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    Lecture 12-

    Temperature Variations

    In saturation, the collector current no longer increases with increasing VBE.

    Why not?

    VBE

    T=32CT=27C

    T=22C

    Q1

    Default

    + 0VVBE

    R4

    100E3

    + 15VVCC

    0.6 0.7 0.8

    0.0

    0.1

    0.2

    mA

    IC

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    Lecture 12-

    Base Width Variation

    n-typep-typen-typeCE

    BVBE VCB

    x

    In the active region, iC does vary somewhat with VCB (hence RC in our

    previous examples) due to the variation it causes in the base width.

    Effective base width, W*, decreases with increasing VCB

    What do you expect would happen to iC as W* decreases?

    W*

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    Lecture 12-

    Early Voltage

    The IC vs. VCE curves in the active region have a finite slope to them due tothis iC dependence on VCB

    Early showed that these slopes all converge to one negative voltage point

    VCE

    IB

    =1A

    IB=5A

    IB=10A

    IB=15A

    IB=20A

    -20 -10 0 10

    -1

    1

    3

    mA VAF=20 in SPICE(VA in the book)

    -VA

    IC

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    Lecture 12-

    Early Voltage

    The finite slope in the active region due to decreasing base width can beapproximated by

    ic

    Ise

    vbe

    VT

    1

    vce

    VA

    -------+

    =

    This means that the output resistance between the collector and emitter is notinfinite --- very important for analog design

    vce

    iC

    go

    0=

    0 1 2 3 4 5 6

    -1

    1

    3

    iC (mA)

    at some fixedib point

    vce

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    Lecture 12-

    Early Voltage

    The output conductance, or resistance, at a fixed ib point represents the slopeof the line tangent to that point on the curve:

    ic

    Ise

    vbe

    VT

    1

    vce

    VA

    -------+

    =

    Generally not considered for dc bias point calculations, but ro can have a

    significant impact on a transistor amplifier gain

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    Lecture 12-

    Early Voltage

    ie

    ie

    Is

    ----e

    vbe

    VT

    =

    E

    B

    C

    ib

    The equivalent circuit models can be modified accordingly:

    ro

    VAiC

    -------=

    ib

    E

    B

    C

    ib

    Is

    ----e

    vbe

    VT

    = ro

    VAiC

    -------=

    or

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    Lecture 12-

    dc Bias Point Calculations

    ro is generally not considered for hand calculations of dc bias point -- why?

    For hand calculations: use VBE=0.7 and assume that the transistor is in the

    active region; Later verify that your assumptions were correct.

    4V

    10V

    3.3k

    RC

    Whats the maximum value that RC can be without reaching saturation? Assume =100.

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    Lecture 12-

    dc Bias Point Calculationsdc Bias Point Calculations

    4V

    10V

    3.3k

    RC

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    Lecture 12-

    What value of RC saturates the transistor?

    10V

    2k

    RC

    100=

    -10V

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    Lecture 12-

    dc Bias Point Calculations

    What value of VCC saturates the transistor for this same circuit?

    10V

    2k

    1k

    100=

    VCC