Lecture 10 Bipolar Junction Transistor...

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  • Lecture 10

    Bipolar Junction Transistor (BJT)

    BJT 1-1

  • Outline Bipolar Junction Transistor (BJT) – Cont’d

     BJT configurations  DC biasing circuits and analysis

    BJT 1-2

  • Test Yourself

     Choose the more appropriate answer

    1) The β of a transistor is its (a) current gain (b) voltage gain (c) power gain (d) internal resistance

    BJT 1-3

  • BJT Configurations

    BJT 1-4

    The base is common to both input (emitter–base) and output (collector–base) of the transistor

    Common-Base configuration

  • BJT Configurations

    BJT 1-5

    This curve shows the relationship between of input current (IE) to input voltage (VBE) for three output voltage (VCB) levels

    Common-Base configuration

    Emitter (or input)


  • BJT Configurations

    BJT 1-6

    This graph demonstrates the output current (IC) to an output voltage (VCB) for various levels of input current (IE)

    Common-Base configuration Collector (or output)


  • BJT Configurations

    BJT 1-7

    • The emitter is common to both input (base-emitter) and output (collector- emitter)

    • The input is on the base and the output is on the collector

    Common-Emitter configuration

  • BJT Configurations

    BJT 1-8

    Common-Emitter configuration

    Collector (or output) Characteristics Base (or input) Characteristics

  • BJT Configurations

    BJT 1-9

    • The input is on the base and the output is on the emitter • The characteristics are similar to those of the common-

    emitter configuration, except the vertical axis is IE

    Common-Collector configuration

  • I-V characteristics of BJT

    BJT 1-10

    Load line

  • I-V characteristics of BJT (cont’d)

    BJT 1-11

  • Operating Limits for Each BJT Configuration

    BJT 1-12

    • VCE is at maximum and IC is at minimum (IC_min= ICEO) in the cutoff region • IC is at maximum and VCE is at minimum (VCE_min = VCEsat = VCEO) in the

    saturation region • The transistor operates in the active region between saturation and


  • Power Dissipation in BJT

     Common-base

     Common-emitter

     Common-collector

    BJT 1-13

    CCBCmax IVP 

    CCECmax IVP 

    ECECmax IVP 

  • Example (1)

    BJT 1-14

    Analyze the BJT circuit, shown in the figure, to get the values of: IB, IC, IE,α,VCE, VCB, VBE and max. power dissipated in the BJT

  • BJT Biasing

    Definition: The DC voltages applied to a transistor in

    order to turn it on so that it can amplify the AC signal

    BJT 1-17 No Bias

    Good Bias

  • BJT Circuits at DC

     The BJT operation mode depends on the voltages at EBJ and BCJ

     The I-V characteristics are strongly nonlinear

     Simplified models and classifications are needed to speed up the hand-calculation analysis

    BJT 1-18

  • DC analysis of BJT circuits

     Step 1: assume the operation mode

     Step 2: use the conditions or model for circuit analysis

     Step 3: verify the solution

     Step 4: repeat the above steps with another assumption if necessary

    BJT 1-19

  • DC Biasing Circuits

     Fixed-bias circuit

     Emitter-stabilized bias circuit

     Voltage divider bias circuit

    DC bias with voltage feedback circuit

    BJT 1-20

  • Fixed-bias circuit

    BJT 1-21

    Base-Emitter Loop

    From Kirchhoff’s voltage law:

    +VCC – IBRB – VBE = 0

    Solving for base current:


    BECC B


    VV I

     

    Collector-Emitter Loop

     


    VV II


    BECC BCCollector current:

    From Kirchhoff’s voltage law: CCCCCE RIVV 

  • Example (2)

    BJT 1-22

    Design the following circuit so that Ic = 2 mA and Vc= 5 V. For this particular transistor, β =100 and VBE=0.7 V

  • Solution  To design the circuit, we need to determine

    values of RC and RE We assume BJT works in active mode

    Ic = 2 mA = (15- Vc)/ RC  then RC = 5 kΩ VB = 0 V, VBE = 0.7 V

    VE = VB – VBE = 0 – 0.7 = - 0.7 V

    RE = (VE -(-15))/ IE IE = (β +1)/ β Ic = 2.02 mA RE = 14.3/2.02 = 7.07 kΩ

    BJT 1-23

  • BJT 1-24

    Lecture Summary

    Covered material  Bipolar Junction Transistor (BJT) – Cont’d

     BJT configurations  DC biasing and analysis

    Material to be covered next lecture

     Continue BJT  DC biasing and analysis

    • Examples

     BJT switching time