26 January 2020

DC Load Line & Operating Point of CE Transistor


DC Load Line

  • Let us consider the NPN transistor. We will consider only DC conditions considering that NO Signal ( AC Supply ). 
  • The collector to emitter voltage
      VCE = VCC – ICRC……… ( 1 ) 
      IC = ( VCC – VCE ) / RC
      IC = VCC / RC – VCE / RC
      IC = – VCE / RC + VCC / RC …………… ( 2 )
      Here Y axis = IC
      X axis = VCE
      Slope m = – 1 / RC
      Constant C = VCC / RC

dc-load-line-of-transistor

  • The DC Load line represents locus of VCE – IC for given value of collector resistor RC
  • The end points of the load line is represented by

( 1 ) When collector current IC = 0 in the equation ( 2 )
        0 = – VCE / RC + VCC / RC
         VCE = VCC
       This is second point B of the load line.
( 2 ) When collector to emitter voltage VCE = 0 in the equation ( 2 )
       IC = – VCE / RC + VCC / RC
      When VCE = 0
       IC = VCC / RC
       This is first point A of the load line.
  • When collector current IC = VCC / RC ( Maximum ), the voltage      VCE = 0. Similarly when collector current IC = 0, the VCE = VCC.

operating-point-of-the-transistor

Operating Point ( Quiscent Point ( Q Point )

  • The zero signal ( No AC signal ) value of IC and VCC are known as operating point
  • When AC signal is applied, the operating point moves on the DC Load line. 
  • It is also called as Quiscent point because it is point which lies on VCC – IC when AC signal is not applied. 
  • Let us consider that base current is 10 µA when no signal is applied.
       At NO signal
      Collector current IC = OR
      Collector – Emitter voltage VCE = OS
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