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					                                                           8/14/2010




         Bipolar Junction Transistors - I
                     (BJT-I)

                         BASICS




•A bipolar junction transistor (BJT) is a three terminal
device constructed of doped semiconductor material

•BJT is used in amplifying or switching applications, in
discrete circuits and in IC design, both in analog and
digital domain

•Bipolar Junction Transistors are so named because
their operation involves both electrons and holes
which are negative and positive charge carriers




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Physical Structure of PNP Transistor

                    P   J1      N      J2        P
                Emitter Base                 Collector
   Emitter      region region                region         Collector
    (E)                                                       (C)

             Emitter-Base               Collector-Base
             Junction                   Junction
                                Base
                                (B)


 Symbol                     C
                                            Arrow indicates the direction
                                            in which the current flows in
                B                           the emitter

                            E




Physical Structure of NPN Transistor

                    N   J1       P     J2        N
                Emitter Base                 Collector
   Emitter      region region                region         Collector
    (E)                                                       (C)

             Emitter-Base               Collector-Base
             Junction                   Junction
                                Base
                                (B)


 Symbol                     C
                                            Arrow indicates the direction
                                            in which the current flows in
                B                           the emitter

                            E




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NPN
             C                                                              PNP
                                                                                       C
 B                                                                            B
             E                                                                         E

                  Mode of Operation           B-E Junction         B-C Junction
                        Active               Forward Biased        Reverse Biased
                      Saturation             Forward Biased        Forward Biased
                        Cut-Off              Reverse Biased        Reverse Biased


 Amplifier: Signal voltage (or current) is amplified
            Output AC signal power is more than input AC signal power


 Switch:          At Saturation, very low resistance between C and E
                  At Cut-Off, No current flow from C to E




    Operation of a PNP Transistor in the Active mode

         C                         Forward Biased        Reverse Biased
B                             P                     N             P
         E
             IE                                              αIE                  IC   C
     E

                                                              ICO


                                                    IB
                         +     -                              +       -
                                               B                   VCB
                             VEB
     IE= IB+ IC                     α is the fraction of the total emitter
     IC = αIE+ ICO                  current which goes from the emitter,
                                    across the base, to the collector




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    Operation of an NPN Transistor in the Active mode

        C                               Forward Biased            Reverse Biased
B                            -     N                       P               N


                            
        E               -
                                                   Diffusing         Collected
                                                                                     αIE
                                   Injected
                                   electrons       electrons         electrons                  C
    E
            IE          IE                                                                 IC
                                                                         +
                                       Injected holes                     Drift Current
                                                                                     IC0
                                                   IB                Recombined
                                                 ++         + +      electrons


                                                      IB
                             -     +                                       -     +

                                 VEB                    B                      VCB
     IE= IB+ IC
     IC = αIE+ ICO




                 IC = αI E +I CO                                  I E =I B +IC
                 IC = α(I B +IC )+ICO
                 (1-α)I C =αI B +ICO
                         α        1
                 IC =       IB +     I CO
                        1-α      1-α

                                                α
                 Define                   β=
                                               1-α


                    I C  βI B  β  1I CO



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  Different Transistor configurations –

  A Transistor may be operated in three different
  configurations :

          Common Base (CB)

          Common Emitter (CE)

          Common Collector (CC)




Common Base (CB) Transistor Configuration

                    PNP                                 NPN

 IE                                  IC   IE                                  IC
           P         N     P                        N P              N    C
      E                          C             E

                IB       B                              IB   B
           + -            + -                       - +          -   +
          VEE               VCC                    VEE               VCC

          IE                IC                     IE                IC
 E                                   C    E                                    C

               IB                                       IB
                     B                                       B




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Common Emitter (CE) Transistor Configuration
                                      IC                                                                        IC
 PNP                             C
                                                                       NPN                                 C

              IB              P                                                          IB            N
                                                       -                                                                      +
                   B         N                                                                B        P
      -                                                    VCC                 +                                                  VCC
VBB                                                +               VBB                                                        -
                             P                                                                         N
      +                                                                        -
                                  E                                                                         E
                        IE                                                                        IE


                                      IC
                                                                                                                     IC
                                               C                                                                          C
                   IB                                                                        IB
              B
                                                                                   B
                         IE                                                                           IE

                                 E                                                                          E




Common Collector (CC) Transistor Configuration

 PNP                                 IE                                                                    IE
                                                                       NPN
                              E                                                                    E
              IB             P                                                     IB             N
                             N                     +                                              P                       -
                   B                                       VEE                          B                                     VEE
      -                                            -                   +
VBB                          P                                   VBB                                                      +
                                                                                                  N
      +                                                                -
                       IC        C                                                           IC       C



                                          IE                                                           IE
                                               E                                                                     E
                  IB                                                                IB
          B                                                                B

                        IC                                                                  IC

                                                                                                  C
                              C




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                                                                    IC
                                                                         C
                                                 IB
The Common Emitter (CE) configuration        B
is the most common configuration for                       IE
using a transistor.
                                                                E
We will discuss this in more detail in our
subsequent slides concentrating mainly
                                                           NPN
on the way in which this can be used as
an amplifier.
                                                                    IC
To keep things simple, we will mainly                                    C
look at circuits using an NPN transistor              IB
but the usage of a PNP transistor will be    B
very similar (just use voltage supplies of
opposite polarity!).                                       IE

                                                                E

                                                           PNP




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