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Basic characteristic of the transistor
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Basic characteristic of the transistor

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  • 1. Basic Characteristic of The Transistor
    M. K. Laliwala
    Assistant Professor
  • 2. Basic Characteristic of The TransistorObjectives:
    • Forward Current Transfer Ratio, CB
    • 3. AC Current Gain
    • 4. Relation Between α and β
    • 5. Collector Cutoff Current,ICBO
    • Forward Current Transfer Ratio, CB
    The graph of Ic-VCB for different values of IE is horizontal
    lines. We also know, Ic=IB + IE, but in this case IB is very small
    therefor, , Ic= IE.
    D.C. Current Gain α or αdc or hfE
    α = αdC =hfE =Ic/IE
    Where Ic is collector current due dc current IE
  • 6. AC Current Gain
    Defined as ratio ΔIc/ ΔIE and denoted by αac or hfb
    Ability of the device to transfer
    current from low resistance input
    co the high resistance output
    i.etransfer resistor=resistance
    From the above graph, for VCB =10V, we have IE =4mA, Ic=3.92mA
    and when IE=6mA, Ic=5.88mA
    Thus αac= hfb=ΔIc/ΔIE =(5.88-3.92)/(6-4)
    =0.98 <1
    Thus in CB ampifier current gain Ai is less than one, however, current in the output flows through large load resistance, it gives high voltage gain Av and therefore power gain Ap.
  • 7. Relation Between α and β
    Let us consider
    IE=IB+Ic------------------------------------(1)
    α=Ic/IE ____________(2)
    β=Ic/IB____________(3)
    Substituting IB=IE-Ic from (1) to (3)
    we get, β=Ic/( IE-Ic)
    Dividing numerator and denominator by IE
    β=Ic/IE/( IE-Ic)/IE, Substituting α from (2) we get,
    β= α/1-α
    Similarly, we get for α=β/1-β
  • 8. Collector Cutoff Current,ICBO
    The reverse current that is flowing through the base-collector junction, when the emitter is open is called collector cutoff current ICBO
    Collector Cutoff Current,ICBO
    In CB, we have Ic=IB+IE
    but IB is very small, therefore Ic=IE
    and α=Ic/IE, therefore Ic=αIE
    Including collector cutoff current ICBO
    we get, Ic= αIE+ ICBO
    but, ICBO=ICO
    TherforeIc= αIE+ ICO___________________(4)
    Equation (4) gives collector cutoff equation for CB amplifier
  • 9. Similarly collector cutoff equation for CE amplifier is Ic=βIB+ICEO
    Here,ICEO=(β+1)ICO
    Therefore, Ic=βIB+(β+1)ICO_________(5)
    Comparing (4) and (5), we can say in CE amplifier leakage current is β times larger than CB.
    Since ICO is temperature dependent, when temperature changes large change in collector current occurs.

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