A load line is used in graphic analysis of circuits, representing the constraint other parts of the circuit place on a non-linear device, like a diode or transistor. A load line represents the response of a linear circuit connected to the nonlinear device in question. The operating point is where the parameters of the nonlinear device and the parameters of the linear circuit match, according to how they are connected while still adhering to their internal systems.[1]
In the example on the right, the nonlinear diode is placed in series with a linear circuit consisting of a resistor and a voltage source. The load line represents the relationship between current and voltage in the linear part of the circuit while the exponential represents the relationship between current and voltage in the nonlinear device. Since the current going through three elements in series should be the same, the operating point of the circuit will be at the intersection of the exponential with the load line.
In a BJT circuit, the BJT has a different current-voltage (IC-VCE) characteristic depending on the base current. Placing a series of these curves on the graph shows how the base current will affect the operating point of the circuit.
The load line can be used for both dc and ac analysis. Once a dc operating point is defined by the dc load line an ac load line with, in general, a different slope intersects the dc operating point. In reality there are many ac load lines that vary from the dc load line to a limiting load line all having a common intersection at the dc operating point. This limiting load line is generally referred to as the ac load line and is the load line of the ac circuit (capacitors shorted, dc sources opened, etc...).
The given load line diagram is for the Common emitter configuration. The load line diagram illustrates all possible values of collector current (IC) and the collector voltage (VCE in this case) for a given (RC).
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.