Review of BJT

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					                     EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)



BJT and MOSFET Revisited (Part 1) – Bipolar
Junction Transistor
BJT Characteristics and Properties

Circuit Symbols and Conventions

    npn bipolar transistor                                 pnp bipolar transistor




    Figure 1: npn and pnp transistor (a) block diagrams and (b) circuit symbols.

Basic Principle of Operation

“The voltage between two terminals (B-E) controls the current through the
third terminal (C).”

Modes of Operation



                                             Figure 2: Bias conditions for the four
                                             modes of operation of an npn
                                             transistor.




Forward-active region              for amplifier circuits.



Dr. Ungku Anisa, UNITEN, 2007                                                         1
                          EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)


Current-voltage Relationships in the Forward-active Region




Figure 3: Common-emitter circuits: (a) with npn transistor; (b) pnp transistor and
(c) with a pnp transistor biased with positive voltage source.


                       npn                                        pnp
             iC  I S exp v BE VT                     iC  I S exp v EB VT 
               i      I                                   i      I
          iE  C  S exp vBE VT                    iE  C  S exp vEB VT 
                                                                  
                 iC       IS                                iC       IS
          iB                 exp vBE VT          iB                 exp vEB VT 
                                                                  
                                      For both transistors
                      i E  iC  iB                              iC   i B
                                                                    
                  iE  1   iB                      iC  iE        iB
                                                                   1  
                                                               
                                                                
                          1                                 1   


Note:
  o I S = saturation current (strongly dependant on device and
      temperature)
  o VT = Thermal voltage
  o β = common – emitter current gain
  o α = common – base current gain




Dr. Ungku Anisa, UNITEN, 2007                                                             2
                      EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)


BJT Operating Curves
                                                                    Forward-active mode
                 iC (mA)
                                   Forward-
                                   active mode
                                                 Saturation
             Cut-off mode




                                         vBE (V)
  Figure 4: INPUT – OUTPUT           iC vs vBE                Figure 5: OUTPUT       iC vs vCE

Early Effect

For VCE > VBE (on), the output curve has a finite slope due to base–width
modulation (reduction of effective base width with increasing collector-
base reverse bias).

 Early voltage (stated
 as positive quantity).
 Typical VA values are
 in the range
 50 < VA < 300V.




Figure 6: Current-voltage characteristics for the common-emitter circuit, showing
Early voltage and finite output resistance, ro, of the transistor.

This effect is included in the collector current equation:

                            iC  I S exp v BE VT 1  vCE V A 


Dr. Ungku Anisa, UNITEN, 2007                                                                3
                     EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)


                                                    1
                         i                                VA
Output resistance, ro                                 
                            C

                         vCE
                                   vbe  const 
                                                
                                                             IC

where IC = quiescent collector current when vBE is constant and vCE is small
compared to VA.

DC Analysis Of Bipolar Transistor Circuits

   o dc biasing of linear amplifiers is required to obtain forward-active
     mode of operation

 Step 1:        Assume transistor is biased in the forward-active mode,
                i.e. VBE = VBE (on), IB > 0 and IC = βIB.

 Step 2:        Analyse the “linear” circuit with this assumption.
                  o Perform KVL on B-E (or E-B) loop to find IB.
                  o Calculate IC and IE from IB.
                  o Perform KVL on C-E (or E-C) loop to find VCE.

 Step 3:        Evaluate the resulting state of the transistor. If the initial
                assumed parameters and VCE > VCE (sat) are true, then the
                initial assumption is correct. However,
                    o if IB < 0, then the transistor is probably cut off, and
                    o if VCE < 0, the transistor is likely to be biased in
                        saturation.

 Step 4:        If the initial assumption is proven incorrect, then a new
                assumption must be made and the new “linear” circuit must
                be analysed. Step 3 must then be repeated.




Dr. Ungku Anisa, UNITEN, 2007                                                        4
                       EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)


Example: DC Analysis of Common-emitter Circuit
Calculate the IB, IC, IE and VCE. (Note: VBE (on) = 0.7V and β = 200)




Figure 7: Common-emitter circuit.




Load Line of Common-emitter Circuit
                      VBB VBE
  Input load line: I B   
                       RB    RB
                      V     V
  C-E load line: I C  CC  CE
                       RC   RC




Figure 8: (a) Input load line (from KVL on B-E (or E-B) loop and (b) common-
emitter transistor characteristics and collector-emitter (C-E) load line showing Q-
point for the previous example.


Dr. Ungku Anisa, UNITEN, 2007                                                          5
                                    EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)


      Voltage Transfer Characteristics

              o output voltage vs input voltage
              o used to visualise the operation of a circuit or the state of a transistor

      Example:
                                      Assume:                 Assume:
                                      VBE (on) = 0.7V         VEB (on) = 0.7V
                                      β = 120                 β = 80
                                      VCE(sat) = 0.2V         VEC (sat) = 0.2V
                                      VA = ∞                  VA = ∞



 o Vi ≤ 0.7V, Qn = cut off:                                o 4.3 ≤ Vi ≤ 5V, Qp = cut off:
         IB = IC = 0, Vo = V+ = 5V                                 IB = IC = 0, Vo = 0V

 o Vi > 0.7V, Qn = turned on (forward-active):             o Vi < 4.3V, Qp = turned on (forward-active):

             V i  0 .7                 Vi  0.7                5  0.7   Vi                    5  0.7   Vi 
    IB                 , I C  I B                      IB                       , I C  I B                   
                 RB                        RB                            RB                                 RB        
   and                                                       and
                                       Vi  0.7 RC                                   5  0.7   Vi 
            Vo  V   I C RC  5                                 Vo  I C RC  RC                    
                                    RB                                                        RB        
   This eq. is valid for 0.2 ≤ Vo < 5V.                      This eq. is valid for 0 ≤ Vo < 4.8V.

 o When Vo = 0.2V, Qn goes into saturation o When Vo = 4.8V, Qp goes into saturation and
   and Vi is found from:                     Vi is found from:
                 120Vi  0.7 5                               5  0.7   Vi 
       0.2  5                   Vi  1.9V     4.8  808
                      150                                                         Vi  2.8V
                                                                     RB        

    V+


                                                              V+ – VCE (sat)




VCE (sat)


                    VBE (on)                                                                       VCC – VBE (on)

      Dr. Ungku Anisa, UNITEN, 2007                                                                             6
                     EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)


AC Analysis Of Bipolar Transistor Circuits

   o A BJT linear amplifier magnifies an ac input signal and produces an
     output signal that is larger in magnitude and directly proportional to
     the input. Therefore, ac analysis of transistor circuits is required.
   o Superposition theory applies, i.e. perform dc and ac analysis
     separately.

 Step 1:        Analyse the circuit with only dc sources present. This will
                give the dc or quiescent solution. The transistor must be
                biased in the forward-active region in order to produce a
                linear amplifier.

 Step 2:        Replace each element in the circuit with its small-signal
                model. The small-signal hybrid- model applies for the
                transistor.

                To draw the small-signal model of the amplifier circuit:
                   o Start with the three terminals of the transistor.
                   o Then sketch the hybrid- equivalent circuit between
                      these terminals.
                   o Connect the small-signal model of the remaining
                      circuit elements to the transistor terminals.

 Step 3:        Analyse the small-signal equivalent circuit, setting the dc
                source components equal to zero, to produce the response of
                the circuit to time-varying input signals only.




Dr. Ungku Anisa, UNITEN, 2007                                                        7
                     EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)


Small-signal Hybrid- Equivalent Circuit of BJT




Figure 9: Small-signal model of the npn BJT containing (a) voltage controlled
current source and (b) current controlled current source.

                                                1
                                 i                V
    diffusion resistance, r   B                 T
                                 v BE
                                          Q  pt 
                                                    I CQ

                              i                I CQ
    transconductance, g m   C                
                              v BE
                                        Q  pt 
                                                 VT

                                          i                   
    ac common-emitter current gain,    C                      g m r
                                          i B
                                                        Q  pt 
                                                                

                                                                                1
                                                      i                               VA
    small-signal transistor output resistance, ro   C                            
                                                      vCE
                                                                    Q  pt 
                                                                                        I CQ


Note: The small-signal model of a pnp BJT is the same as in Figure 9
but with all ac voltage polarities and current directions reversed. All the
parameter equations stated above still apply for the pnp transistor.




Dr. Ungku Anisa, UNITEN, 2007                                                                   8
                     EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)


Basic Single Stage BJT Amplifiers

Common-emitter (CE) Amplifier




Figure 10: CE amplifier circuit.             Figure 11: Small-signal equivalent circuit.

Common-emitter (CE) Amplifier with Emitter Degeneration (ro = )




Figure 12: CE with emitter
degeneration amplifier circuit.              Figure 13: Small-signal equivalent circuit.

                                                        CE Amplifier with Emitter
                        Basic CE Amplifier
                                                        Degeneration (and ro = )
Input resistance,
                                  R1 R2 r                  R1 R2 r  1   RE 
Ri
Output                                                                 RC
                                   RC ro
resistance, Ro
                                         Ri                         RC      RC
                     Av   g m RC ro 
                                        R R             Av              
Voltage gain, Av
                                         i  s 
                                                                   1   RE RE
Current gain, Ai               Ai                                 Ai   


Dr. Ungku Anisa, UNITEN, 2007                                                          9
                      EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)


Common-collector (CC) Amplifier a.k.a. Emitter Follower




Figure 14: CC amplifier circuit.             Figure 15: Small-signal equivalent circuit.

Common-base (CB) Amplifier (and ro = )




  Figure 16: CB amplifier circuit.                Figure 17: Small-signal equivalent circuit.


                                                                      CB Amplifier
                           CC Amplifier
                                                                       (and ro = )
Input                                                                    r   1
resistance,            R1 R2 r  1   RE                              
Ri                                                                      1  gm
Output                       r
resistance,                          RE ro                                    RC
Ro                          1 

Voltage                 1   RE ro   Ri                        R R  r                       
              Av                                    1   Av  g m  C L                  R E RS 
gain, Av             r  1   RE ro   Ri  Rs 
                                                    
                                                                      R  1  
                                                                        S 
                                                                                                      
                                                                                                      
                                                                               
                                                                      Ai                1,
Current                      Ai  1                                      1   
gain, Ai
                                                                    as RE  , RL  0


Dr. Ungku Anisa, UNITEN, 2007                                                            10
                     EEEB273/EEEB314 Electronics II - BJT and MOSFET Revisited (Part 1)


Characteristics of Basic Single Stage BJT Amplifiers

           Input         Output                Voltage           Current
                                                                                  Application
       resistance, Ri resistance, Ro           gain, Av          gain, Ai
CE                                               High                                Power
          Moderate              High                               High
                                              (inverted)                            amplifier
CC                                                                                   Voltage
            High                Low         Almost unity           High
                                                                                     buffer
CB                                           High (non-                              Current
            Low                 High                               Unity
                                             inverting)                              buffer




Dr. Ungku Anisa, UNITEN, 2007                                                       11

				
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