The load line diagram at right is for a transistor connected in a common emitter circuit. It shows the collector current in the transistor IC versus collector voltage VCE for different values of base current Ibase. The load line represents a particular value of collector load resistor (RC). The intersections of the load line with the transistor characteristic curve represent the different values of IC and VCE at different base currents.
The point on the load line where it intersects the collector current axis is referred to as saturation point.[2] At this point, the transistor current is maximum and voltage across collector is minimum, for a given load. For this circuit, IC-SAT= VCC/RC.[3]
The cutoff point is the point where the load line intersects with the collector voltage axis. Here the transistor current is minimum (approximately zero) and emitter is grounded. Hence VCE-CUTOFF=Vcc.
The operating point of the circuit in this configuration is generally designed to be in the active region, approximately between middle of the load line and close to saturation point. In this region, the collector current is proportional to the base current, and hence useful for amplifier applications. a load line is normally drawn on Ic-Vce characteristics curves for the transistor used in amplifier circuit.