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DC Load Line vs AC Load Line-Difference between DC Load Line and AC Load Line

This page on DC Load Line vs AC Load Line describes basic difference between DC Load Line and AC Load Line.

DC Load Line

DC Load Line

The DC Load Line is the locus of all possible operating point at which BJT remains in active region.

➨If base current IB < Icsat/β then transistor operates in active region.

➨If IB > Icsat/β , then it operates in saturation region.

➨Collector current Ic = Vcc-Vce/Rc as depicted in the figure-1. At saturation, Vce = Vces

Vce = Vcc at cutoff when Ic = 0 , Maximum value of current Icmax = Vcc/Rc
This is the ideal case. The Ic vs Vce curve along with load line is shown in the figure-1.

AC Load Line

AC Load Line

Figure-2 depicts AC Load line curve between Ic and Vce. It is a CE amplifier where in,
Icq = Quiescent collector current
Vceq = Quiescent collector-emitter voltage

AC load resistance as observed by collector is expressed as follows,
rc = RC || RL

AC saturation current (Icsat) = Icq + Vceq/rc

AC cutoff voltage, Vce(cut) = Vceo + Ico*rc

SUMMARY:
DC Load Line: For CE transistors, it is straight line which joins VCE = VCC on the voltage axis to the point VCC/RC on the current axis. This line is on output characteristic curve (RC is the resistor in collector circuit).

AC Load Line: It is straight line through the quiescent operating point having slope corresponding to AC load resistance Rac = RC | | RL.

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