Load line (electronics)

Load line (electronics)

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 to which the nonlinear device in question is connected to. 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. [Adel Sedra, Kenneth Smith. Microelectronic Circuits, 5th ed.]

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 effect the operating point of the curcuit.

It should be noted that the load line is used for "dc" analysis, and has no bearing on small-signal analysis once an operating point is identified.

Load lines for common configurations

Common-Emitter

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 load resistor (RC).

The point on the load line where it intersects the collector current axis is referred to as "saturation point". 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.

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.

References

* [http://electronics.indianetzone.com/1/analysis_of_dc_load_line.htm Indianetzone.com]
* [http://www.vias.org/feee/bjt_09.html Vias.org]


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