4_DC Biasing BJTs

download 4_DC Biasing BJTs

of 31

Transcript of 4_DC Biasing BJTs

  • 8/10/2019 4_DC Biasing BJTs

    1/31

    4_DC Biasing BJTs 1 of 31

    4_DC Biasing BJTs

    4.1

    Introduction4.2

    Operating Point

    Active or Linear Region Operation

    Base Emitter junction is forward biasedBase Collector junction is reverse biased

    Good operating point

    Cutoff Region Operation

    Base Emitter junction is reverse biased

    Saturation Region Operation

    Base Emitter junction is forward biasedBase Collector junction is forward biased

  • 8/10/2019 4_DC Biasing BJTs

    2/31

    4_DC Biasing BJTs 2 of 31

    4.3

    Fixed-Bias Circuit

    -VCE ICRC+ VCC= 0

    VCE= VCC ICRC

    Base Loop (voltage drops)

    -VCC+ IBRB+ VBE= 0

    IB= (VCC VBE)/RB

  • 8/10/2019 4_DC Biasing BJTs

    3/31

    4_DC Biasing BJTs 3 of 31

    Fixed_base_current_load_line.obj simulation

    Q1

    Q2N2222

    V1

    0Vdc

    I1

    0.5mAdc

    0

    R1

    40

    V2

    10Vdc

    I

    I

    V_V1

    0V 1V 2V 3V 4V 5V 6V 7V 8V 9V 10VIC(Q1) - I ( R1)

    0A

    200mA

    400mA

  • 8/10/2019 4_DC Biasing BJTs

    4/31

    4_DC Biasing BJTs 4 of 31

    Example 4.1

    Work through finding the base and collector currents.

  • 8/10/2019 4_DC Biasing BJTs

    5/31

    4_DC Biasing BJTs 5 of 31

    Saturation Region

    ICsat= (VCC- 0)/RCand in active region,

    IC= IB or IB= ICmax/

    In saturationIBis greater than ICmax/

    There is

    Saturation 0.2V is

    better approximation,

    but use 0V for now.

  • 8/10/2019 4_DC Biasing BJTs

    6/31

    4_DC Biasing BJTs 6 of 31

    Bjt_resistor_bias.obj simulation

    Q1

    Q2N2222

    V2

    10Vdc

    0

    R1

    150k

    V1

    5Vdc

    R2

    150k

    V3

    5Vdc

    Q2

    Q2N2222

    V4

    10Vdc

    0

    R3

    1k

    Run simulation and note voltages and currents.

  • 8/10/2019 4_DC Biasing BJTs

    7/31

    4_DC Biasing BJTs 7 of 31

    Saturation ....continued

    Modify Example 4.1

    Find ICmaxFind RBminimum where transistor is on the edge of saturation.

    Reduce RBfurther and find base and collector currents.

  • 8/10/2019 4_DC Biasing BJTs

    8/31

    4_DC Biasing BJTs 8 of 31

    Load-Line Analysis

    Maximum collector-emitter

    voltage occurs when ICequalszero.

    No drop across RC.

    Maximum current is

    ICsat= (VCC- 0)/RC

    Collector-emitter volageequlas zero.

  • 8/10/2019 4_DC Biasing BJTs

    9/31

    4_DC Biasing BJTs 9 of 31

    BJT_load_line.obj simulation

    Q1

    Q2N2222

    V1

    0Vdc

    I1

    0Adc

    0

    R1

    100

    V2

    10Vdc

    I

    I

    V_V1

    0V 1V 2V 3V 4V 5V 6V 7V 8V 9V 10VIC(Q1) - I ( R1)

    0A

    100mA

    200mA

  • 8/10/2019 4_DC Biasing BJTs

    10/31

    4_DC Biasing BJTs 10 of 31

  • 8/10/2019 4_DC Biasing BJTs

    11/31

    4_DC Biasing BJTs 11 of 31

  • 8/10/2019 4_DC Biasing BJTs

    12/31

    4_DC Biasing BJTs 12 of 31

    4.4 Emitter-Stabilized Bias Circuit

    Find the base current. Use Kirchhoffs voltage around the base-emitter loop.

  • 8/10/2019 4_DC Biasing BJTs

    13/31

    4_DC Biasing BJTs 13 of 31

    Show that,

    IC= (V

    CC V

    BE)

    RB+ (+ 1)REand when,

    RB

  • 8/10/2019 4_DC Biasing BJTs

    14/31

    4_DC Biasing BJTs 14 of 31

    Use the base current equation to find the part of the input resistance due to the emitter resistor.

    Look at the loop equation written earlier

    VCC IBRB VBE- (+ 1)RE = 0

    Now draw the circuit that matches this equation,

    Note that the voltage drop across Re has not changed. The resistor Re is reflected back into the base input

    circuit and multiplied.

  • 8/10/2019 4_DC Biasing BJTs

    15/31

    4_DC Biasing BJTs 15 of 31

    Collector-emitter loop

    Go around the loop and show the voltage relationships including

    VC, VE, and VB

  • 8/10/2019 4_DC Biasing BJTs

    16/31

    4_DC Biasing BJTs 16 of 31

    R1

    20k

    R2

    10k

    Rc

    1k

    Re

    1k

    Vcc

    15Vdc

    Q1

    Q2N2222

    0

    R3

    100k

    Run simulation and note the voltages and currents.

  • 8/10/2019 4_DC Biasing BJTs

    17/31

    4_DC Biasing BJTs 17 of 31

    Emitter-stabilized bias circuit for Example 4.4.

    Find the currents and voltages for this circuit.

    Review the load line for the circuit.

    Write the collect-emitter loop equation again and show the min and max current conditions.

  • 8/10/2019 4_DC Biasing BJTs

    18/31

    4_DC Biasing BJTs 18 of 31

    4.5 Voltage-Divider Bias

    Show that by developing a Thevenin equivalent for the input the circuit becomes the same as the emitter-

    stabilized circuit.

  • 8/10/2019 4_DC Biasing BJTs

    19/31

    4_DC Biasing BJTs 19 of 31

  • 8/10/2019 4_DC Biasing BJTs

    20/31

    4_DC Biasing BJTs 20 of 31

    4.6 Beta-stabilized circuit for Example 4.7.

    Work the circuit.

  • 8/10/2019 4_DC Biasing BJTs

    21/31

    4_DC Biasing BJTs 21 of 31

    Common_base

    CB-1.obj simulation

    R1

    1k

    Rc

    1k

    Q1

    Q2N2222

    V15Vdc

    Vcc

    12Vdc

    0

    CB-2.obj simulation

    Rc

    1.5k

    Q1

    Q2N2222

    V1

    3Vdc

    Vcc15Vdc

    0

    Vee

    3.5Vdc

    Rc1

    2.5k

  • 8/10/2019 4_DC Biasing BJTs

    22/31

    4_DC Biasing BJTs 22 of 31

    Common Collector (Emitter Follower)

    CC-1.obj simulation

    RB

    1k

    Re

    1k

    Q1

    Q2N2222VB

    5Vdc

    Vcc

    12Vdc

    0

    V

    CC-2.obj simulation

    RB

    100k

    Re

    1k

    Q1

    Q2N2222VB

    5Vdc

    Vcc

    12Vdc

    0

    Re1

    1k

  • 8/10/2019 4_DC Biasing BJTs

    23/31

    4_DC Biasing BJTs 23 of 31

    DC bias circuit with voltage feedback.

  • 8/10/2019 4_DC Biasing BJTs

    24/31

    4_DC Biasing BJTs 24 of 31

  • 8/10/2019 4_DC Biasing BJTs

    25/31

  • 8/10/2019 4_DC Biasing BJTs

    26/31

    4_DC Biasing BJTs 26 of 31

    4.8 Design Operations

    Start with a set of transistor collector-emitter characteristics.Draw a load line.

    Determine the values of component values, voltages and currents.

    Perform for,

    Fixed bias circuit.

    Emitter-stabilized circuit.

    Voltage-divider circuit.

  • 8/10/2019 4_DC Biasing BJTs

    27/31

    4_DC Biasing BJTs 27 of 31

    4.9 Transistor Switching Networks

    4 DC Bi i BJT 28 f 31

  • 8/10/2019 4_DC Biasing BJTs

    28/31

    4_DC Biasing BJTs 28 of 31

    4 DC Bi i BJT 29 f 31

  • 8/10/2019 4_DC Biasing BJTs

    29/31

    4_DC Biasing BJTs 29 of 31

    4.11 PNP Transistors

    Perform for,

    Fixed bias circuit.

    Emitter-stabilized circuit.

    4 DC Biasing BJTs 30 of 31

  • 8/10/2019 4_DC Biasing BJTs

    30/31

    4_DC Biasing BJTs 30 of 31

    4.12 Bias Stabilization

    Shift in dc bias point (Q-point) due to change in

    temperature: 100 degrees C

    4 DC Biasing BJTs 31 of 31

  • 8/10/2019 4_DC Biasing BJTs

    31/31

    4_DC Biasing BJTs 31 of 31

    4.13 Practical Applications

    Relay driver

    Transistor switch

    Constant current source - CB and CE configurations.