Bipolar Junction Transistors (BJT) - PowerPoint by zug10789

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									                                      September 10, 2007
EE 359 Electronic Circuits




       Bipolar Junction Transistors
                 (BJT)
              Small Signal Analysis
           Graphical Analysis / Biasing
         Amplifier, Switch and Logic Appl.

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                                             September 10, 2007
EE 359 Electronic Circuits

                       HW # 2
                   HW 2 due Friday
                   3.52, 3.72, 3.66 , 3.98




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EE 359 Electronic Circuits
         NPN Transistor Amplifier
  Example                              • NPN
  •Quiescent point
                                             VC  VCC  2.3x3  3.1V
     VBB  VBE 3  0.7
IB                        0.023mA
                                                        VCC

         RBB        100                                        10V

                   3  0.7
      I C  I B           2.3mA           3.0k ohm     R2

                    100
                                        R1                Q1
                                V1                        1DEAL_BJT_NPN
                                       100kohm
                                3V




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                                    September 10, 2007
EE 359 Electronic Circuits
                Small Signal Analysis

     VT      25mV
re                     10.8
     I E (2.3 / 0.99)mA

     IC   2.3mA
gm             92mA / V
     VT   25mV

               100
r                 1.09k
       gm        92



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EE 359 Electronic Circuits
                 BJT as Amplifier
  BJT as a voltage-controlled   BJT as a current-
current source ( a              controlled current source
transconductance amplifier)     (a current amplifier).




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EE 359 Electronic Circuits
                     Small Signal




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EE 359 Electronic Circuits
          Small Signal Analysis
 • Employ either hybrid-p model.
 • Using the first model
 • BJT as Amplifier

                                           Dependent
                                           Current Source
                            R1
                                                                     C
                           100kohm
                                       B
             V1                                         I1      R3
                                                       92mMho   3.0kohm   XMM1
                           VBE           R2
                                     1.1kohm
             1V 1Hz 0Deg

                                       E
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EE 359 Electronic Circuits
               Signal Waveforms




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EE 359 Electronic Circuits
       PNP Transistor Amplifier
  Example
  • Voltage Gain
  • Signal Waveforms
  • Capacitor couples
    input signal vi to
    emitter
  • DC bias with V+ & V-



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EE 359 Electronic Circuits
                        DC Analysis

  • Find operating pt. Q
             10  VE 10  0.7
      IE                     0.93mA
               RE      10
  • Let =100 and a=0.99
       I C  0.99 I E  0.92mA
       VC  10  I C RC  5.4V
  • The transistor is active
  • Max. signal swing
    depends on bias voltage

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EE 359 Electronic Circuits
            Small Signal Analysis

  • Replace BJT with T
    equivalent ckt.
  • Why? Base is gnded.
    More convenient than
    hybrid p
   a= 0.99
    re=25mV/0.93mA= 27 




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EE 359 Electronic Circuits
           Small Signal Equiv Ckt

  • VO/Vi
    =0.99x5k/27=183
  • Allowable signal
    magnitude?
  • But veb = vi For small
    signal limit to 10mV.
  Then, vc=1.833V


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EE 359 Electronic Circuits
               Graphical Analysis

  • Find DC bias point
  • Set vi=0 and draw load
    line to determine dc
    bias point IB (similar
    to diode ckts)




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EE 359 Electronic Circuits
           Graphical Construction

  • Load line has a slope
    of –1/RB
  • iB vs vBE from forward
    biased diode eqns

 Graphical construction for the
 determination of the dc base
 current



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EE 359 Electronic Circuits
                 Collector Current
   Graphical construction for determining the dc collector
   current IC and the collector-to-emmiter voltage




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EE 359 Electronic Circuits
   Small Signal Graphical Analysis

  • Signal is superimposed on
    DC voltage VBB
  • Corresponding to each
    instantaneous value of
    VBB + vi(t) draw a load
    line
  • Intersection of the iB -vBE
    curve with the load lines
  • Amplitude vi(t) small so
    ib linear

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EE 359 Electronic Circuits
                Collector Currrent

  • Corresponding to each
    instantaneous value of
    VCE + vce(t) operating
    point will be on the
    load line
  • Amplitude vi(t) small
    so ic linear



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EE 359 Electronic Circuits
       Bias Point vs Signal Swing
                             • Bias-point location limits
                               allowable signal swing
                             •    Load-line A results in bias
                                 point QA with a
                                 corresponding VCE which is
                                 too close to VCC and thus
                                 limits the positive swing of
                                 vCE.
                             • At the other extreme, load-
                               line B results in an operating
                               point too close to the
                               saturation region, thus
                               limiting the negative swing of
                               vCE.
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                                                   September 10, 2007
EE 359 Electronic Circuits
     Basic Single Stage Amplifiers




  Common-emitter amplifier with a resistance Re in the emitter.
    (a) Circuit. (b) Equivalent circuit with the BJT replaced
    with its T model (c) The circuit in (b) with ro eliminated.
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EE 359 Electronic Circuits

                 Common Base Amp




  The common-base amplifier. (a) Circuit. (b) Equivalent
  circuit obtained by replacing the BJT with its T model.
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                                                    September 10, 2007
EE 359 Electronic Circuits
               Common Collector




  The common-collector or emitter-follower amplifier. (a)
  Circuit. (b) Equivalent circuit obtained by replacing the BJT
  with its T model.
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EE 359 Electronic Circuits




   (c) The circuit redrawn to show that ro is in parallel with RL.
    (d) Circuit for determining Ro.

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EE 359 Electronic Circuits
       General Large Signal Model




    An npn resistor and its Ebers-Moll (EM) model. The scale or
    saturation currents of diodes DE (EBJ) and DC (CBJ) are
    indicated in parentheses.
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EE 359 Electronic Circuits




    The transport model of the npn BJT. This model is exactly
    equivalent to the Ebers-Moll model
    Saturation currents of the diodes in parentheses
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EE 359 Electronic Circuits

               BJT Digital Logic


Basic BJT digital logic
inverter.




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EE 359 Electronic Circuits


•voltage transfer
characteristic of the
inverter circuit
•RB = 10 k , RC = 1 k ,
  = 50, and VCC = 5V.




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EE 359 Electronic Circuits
                   Saturation Region




  The minority-carrier concentration in the base of a saturated transistor is
  represented by line (c). (b) The minority-carrier charge stored in the base
  can de divided into two components: That in blue produces the gradient
  that gives rise to the diffusion current across the base, and that in gray
  results in driving the transistor deeper into saturation.
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EE 359 Electronic Circuits




      The ic-vcb or common-base characteristics of an npn transistor. Note that
      in the active region there is a slight dependence of iC on the value of vCB.
      The result is a finite output resistance that decreases as the current level in
      the device is increased.
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EE 359 Electronic Circuits
      Common Base Characteristic

The hybrid-p model,
including the resistance
r , which models the
effect of vc on ib.




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EE 359 Electronic Circuits
           Common-emitter characteristics.




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                             September 10, 2007
EE 359 Electronic Circuits
     Common Emitter in Saturation
             Region




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