Load Line (electronics)

Load Line (electronics)

A load line is used in graphical analysis of nonlinear electronic circuits, representing the constraint other parts of the circuit place on a non-linear device, like a diode or transistor. It is usually drawn on a graph of the current vs the voltage in the nonlinear device, called the device's characteristic curve. A load line, usually a straight line, represents the response of a linear circuit connected to the nonlinear device in question. The operating point(s) of the circuit are the points where the characteristic curve and the load line intersect; at these points the current and voltage parameters of both parts of the circuit match.

The example at right shows how a load line is used to determine the current and voltage in a simple diode circuit. The nonlinear diode is in series with a linear circuit consisting of a resistor and a voltage source. The graph, representing voltage across the diode V_D versus current I through the diode, is an exponential curve. The load line (diagonal line) represents the relationship between current and voltage in the linear part of the circuit. Since the current going through the three elements in series must be the same, and the voltage at the connection of the resistor and diode must be the same, the operating point of the circuit will be at the intersection of the curve with the load line.

In a BJT circuit, the BJT has a different current-voltage (I_C-V_CE) 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.