Introduction to BJT Amplifier - Portal

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					Introduction to BJT
     Amplifier

   BJT (Review)
     Still remember about BJT?




The emitter current (iE) is the sum of the collector current (iC) and the base
current (iB)

                          iB << iE and iC            OTHER PRAMETERS &
                                                     EQUATIONS?
                        BJT
n Basic structure and schematic symbol




                   approximate
                    equivalents




                     transistor
                      symbols




     npn type                     pnp type
Refresh..
n Common-emitter
  current gain, β
   n   Range: 50 < β < 300
n Common-base current
  gain, α
   n   Range: always slightly
       less than 1
n The current relationship
  between these 2
  parameters are as
  follows:
Refresh..
n BJT as amplifying device
  n B-E junction is forward-biased
  n B-C junction is reverse-biased
BIASING OF BJT
n Remember…! for normal operation


  Ä   emitter-base junction is always forward-
      biased
                  AND

  Ä   collector-base junction is always reverse-
      biased
FORWARD BIASING E/B JUNCTION
REVERSE BIASING C/B JUNCTION
BIASING NPN TRANSISTOR
  Common-Emitter Circuit




(a) with an npn transistor
(b) with a pnp transistor
(c) with a pnp transistor biased with a positive voltage source
DC Analysis
- Common-Emitter Circuit
                           Transistor current-
                           voltage characteristics
                           of the common-emitter
                           circuit
    DC Analysis
    - Common-Emitter Circuit




Common-emitter circuit with an   Common-emitter dc equivalent
npn transistor                   circuit, with piecewise linear
                                 parameters
   DC Analysis
   - Common-Emitter Circuit




                                             Usually VBE(on) = 0.7 V




Common-emitter dc equivalent circuit


Look for calculation examples in Neamen (Chapter 3), Example 3.3 & 3.4
   DC Analysis
   - Load Line & Modes of Operation




        Figure A


Base on Figure A, using KVL
around B-E loop:
                              Base-emitter junction characteristics
                              and the input load line
   DC Analysis
   - Load Line & Modes of Operation
                                             Base on Figure A, 2 end
                                             points of the load line are
                                             found by setting IC = 0


                                             So, VCE = VCC = 10 V
                                             When VCE = 0,
                                             IC = VCC/RC = 5 mA

                                             IBQ is the value from the
                                             previous slide = 15 µA
                                             So, ICQ = βIBQ
                                             If β = 200,
                                             ICQ = 3000 µA = 3 mA


Common- emitter transistor characteristics   So, VEQ = 4 V
and the collector-emitter load line
   BJT as an Amplifier
• Amplification of a small ac
  voltage by placing the ac
  signal source in the base
  circuit
• Vin is superimposed on the
  DC bias voltage VBB by
  connecting them in series
  with base resistor RB:


• Small changes in the base
  current circuit causes large
  changes in collector current
  circuit
                                 E ND

				
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posted:4/9/2014
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