## 26/01/2020

### DC Load Line & Operating Point of CE Transistor

• 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

• 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 ( 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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