DC Load Line & Operating Point of CE Transistor

Output Characteristic of CE transistor

• The collector current for various emitter – collector voltage is known by drawing output characteristic of common emitter ( 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

      V_CE = V_CC – I_CR_C……… ( 1 ) 

      I_C = ( V_CC – V_CE ) / R_C

      I_C = V_CC / R_C – V_CE / R_C

      I_C = – V_CE / R_C + V_CC / R_C …………… ( 2 )

      Here Y axis = I_C

      X axis = V_CE

      Slope m = – 1 / R_C

      Constant C = V_CC / R_C

• The DC Load line represents locus of V_CE – I_C for given value of collector resistor R_C. 
• The end points of the load line is represented by

( 1 ) When collector current I_C = 0 in the equation ( 2 )

        0 = – V_CE / R_C + V_CC / R_C

         V_CE = V_CC

       This is second point B of the load line.

( 2 ) When collector to emitter voltage V_CE = 0 in the equation ( 2 )

       I_C = – V_CE / R_C + V_CC / R_C

      When V_CE = 0

       I_C = V_CC / R_C

       This is first point A of the load line.

• When collector current I_C = V_CC / R_C ( Maximum ), the voltage      V_CE = 0. Similarly when collector current I_C = 0, the V_CE = V_CC.

Operating Point ( Quiscent Point ( Q Point )

• The zero signal ( No AC signal ) value of I_C and V_CC 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 V_CC – I_C 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 I_C = OR

      Collector – Emitter voltage V_CE = OS

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