Transistors by nyut545e2


   Fundamentals
       What transistors do
       How to analyse transistor circuits
       Small and large signals
   Common-Emitter Amplifier
       Review of analysis and design
    The Bipolar Junction Transistor

   BJT is a current amplifier
       The collector current is controlled by a much smaller base
       The sum of the collector and base currents flow into or out
        of the emitter
   Base-emitter junction looks a lot like a PN junction
         Operating Regions - Cut Off
                         If the base current is zero, the
                          collector current is also zero
                         It doesn’t matter how big the collector-
                          emitter voltage, VCE, is
                         i.e. collector-emitter junction looks like
             IC  0       an open circuit
                         In this state, the transistor is in the
IB  0                    cut-off region
      Operating Regions - Active
                   Base current flows and controls the
                    larger collector current
                   Collector current is proportional to the
                    base current
                   Transistor is in the active region
      IB           Operation can be summarised by two
                                     VBE 
                       I C  I S exp        I C  I B
                                      VT 

                       VT  kT / q  25 mV 
   Operating Regions - Saturation
                           Collector current rises in proportion to
                            the base current
                           As collector current rises, resistor
                            voltage rises and collector-emitter
                   VS       voltage falls
            IC            When VCE  0, it can’t go any lower
                            and the collector current cannot get
                            any higher
                           The transistor is saturated
     1 VS                  Collector-emitter junction looks like a
IB                         short circuit
      R
   BJT amplifiers work by controlling the
    collector current by the base-emitter voltage
   This is only possible in the active region
   Cut-off and saturation regions correspond to
    the transistor turning fully ‘off’ or ‘on’ like a
   In the active region, the transistor is only
    partly ‘on’ and the current can be controlled
          Small Signals
                                   We want circuits with a linear
                                    response but real transistors
                                    aren’t linear

              iv                  If the range of voltages/currents
                                    is kept small, response is
                       DI = i
   I                                approximately linear
                    DV = v
                                   Average (or quiescent) levels
                                    are denoted by capital letters
                V                  Small variations (i.e. signals!)
                      Voltage       are denoted by lower case
Small Signal Collector Current

                VBE 
  I C  I S exp        iC  vBE
                 VT 
Mutual Conductance
                   IC and VBE are exponentially
                   iC and vBE, on the other hand,
                    are approximately linearly
                   The constant of
                    proportionality, gm, is known
                    as the mutual conductance
                   It isn’t a real conductance,
                    but it is the ratio between a
  iC  g mvBE       current and a voltage
Estimating gm
                    The small signal behaviour is
                     estimated by a tangent to the
                     exponential IC-VBE curve
                    gm is, therefore, simply the
                     gradient of the curve
                          dI C   d         VBE 
                    gm           I S exp  
                         dVBE dVBE          VT 
                        IS   VBE 
                        exp  
                        VT    VT 
  iC  g mvBE           I
                        C
                  Assume that the transistor is biased in the
                  active region somehow…
                    iC  g mvBE
                  Collector voltage, VC, is related to IC by Ohm’s
       RC         law
            IC     VC  VS  I C RC

                  Small signal ratio between collector voltage
                  and collector current is:
                    vC dVC
                              RC
                    i C dI C
                        vC vC iC
                  So:               RC g m
                        vBE iC vBE
Simple Common-Emitter Amplifier
              IB provides a d.c. base current to
               bias the transistor in the active
              CIN couples the input voltage,
               removing the d.c. base bias voltage
              CIN is a short circuit to a.c. signals…
              …but an open circuit to the d.c.
               bias current
              vBE is, therefore, equal to vIN
           vBE  vIN
           iC  g mvBE  g mvIN
           VOUT  VS  I C RC
           vOUT dVOUT
                       RC
            iC   dI C
           vOUT vOUT iC
                           RC g m
            vIN   iC   vIN
             Gain is proportional to gm which is,
              in turn, proportional to IC
             In this circuit,
                I C  I B
             Unfortunately,  has a very wide
             The gain is, therefore, not
Reliable Biasing
                   I E  I B  IC
                   I C  I B
                    IC  I E
                      Collector current is set
                       accurately regardless of 
                      CE ensures that the whole of
                       the a.c. input voltage is still
                       dropped across VBE
                      RB provides the d.c. base
                       bias current
                      Usually, the current source is
                       approximated by a resistor
Practical Amplifier
                     To analyse the circuit:
                         Determine quiescent
                         Calculate mutual
                         Calculate small signal
                              Voltage Gain
                              Input Impedance
                              Output Impedance
                              Cut-off frequency
The Story so Far
   Small signal analysis is used to simplify calculations
    by ‘linearising’ the non-linear response of the
   Using mutual conductance, gain calculations are now
    only a couple of lines of equations
   Careful choice of the biasing network leads to reliable
   Next time – practical amplifier calculations, input &
    output impedances and capacitor calculations

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