Bipolar Junction Transistors

3(a). Common Emitter transistor configuration

  • In CE configuration emitter is made common to both input and output or the input is between the base and emitter and output is between emitter and collector.
  • Figure below shows an transistor circuit using npn transistor in CE mode

  • Input section of the circuit contains base emitter base junction .Thus two input variables are
    1. IB ,the base current
    2. VBE ,the voltage in the input section
  • Similarly output section of the circuit contains CE junction .Thus two output varibel are
    1. IC ,the collector current
    2. VCE ,the collector emitter voltage
  • Transistor characterstics are the curves that relate transistor currents and voltage .There are two types of characterstics curves
    1. Input characterstics :- relates input current with the input voltage for a given output voltage
    2. Output characterstics :- relates output current with output voltage for a given input current
  • We now know what are transistor characterstics and now we shall discuss the characterstics curves for a npn transistor in CE configuration (connection known in fig 3)

i) Input characterstics:-

  • To study input characterstics of transistors in CE configuration a curve is plotted against base current IB and base emitter voltage VBE keeping collector emitter voltage VCE fixed
  • Input characterstics relate IB and VBE for different values of VCE
  • Graph below shows the input characterstics of npn transisator which are similar to those of a forward biased pn junction FIG
  • Change in output voltage VCE does not result in lrage deviation of the curves
  • To keep transistor in active state ,it is required to keep VCE large enough to make CB junction reverse biased
  • Dynamic resistance of the transistor at a given VCE is ri=ΔVBE/ΔIB for VCE=constant ---(3) input resistnace ri is defined as the ratio of change in base emitter voltage (ΔVBE) to the resulting change in base current (ΔIB) at constant CE voltage(VCE)
  • Value of ri can vary from few hundered to few thousand ohms

ii) Output characterstics:-

  • These curves relate output current IC to the voltage between collector and emmitter for various value sof the input current IB
  • Output characterstics are shown below in figure 5 FIG
  • In active region ,IC increase slowly as VCE incraeses
  • From output chracterstics we can determine dynamic output resistnace whihc is defined as the ratio of the change in CE voltgae (ΔVCE) to the change in collector current (ΔIC) at a constant base current IB Thus, ro=ΔVCE/ΔIC for IB=constant ---(4)
  • current amplification factor (Β) is defined as the ratio of change in collector current to the change in base current at constant CE voltage => Βac=ΔIC/ΔIB at VCE=constant ---(5) Βac is also known as ac current gain
  • Simple ratio of IC and IB is known as DC current gain.Thus Βdc=IC/IB ---(6) From fig (5) it is clear that for constant IB ,current IC increases with VCE. This indicates that Βdc incraese with VCE
  • When VCE drops below VBE,the CB junction becomes forward baised and IC decraese rapidly with VCE is
  • When both junctions are FB ,transistors works in saturation region in this region IC no longer depends on IB
  • Inactive region collector region is Βdc times greater than IB.This shows that small input current IB produces large output current IC

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