<P> In a BJT circuit, the BJT has a different current - voltage (I - V) 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 . </P> <P> Semiconductor circuits typically have both DC and AC currents in them, with a source of DC current to bias the nonlinear semiconductor to the correct operating point, and the AC signal superimposed on the DC . Load lines can be used separately for both DC and AC analysis . The DC load line is the load line of the DC equivalent circuit, defined by reducing the reactive components to zero (replacing capacitors by open circuits and inductors by short circuits). It is used to determine the correct DC operating point, often called the Q point . </P> <P> Once a DC operating point is defined by the DC load line, an AC load line can be drawn through the Q point . The AC load line is a straight line with a slope equal to the AC impedance facing the nonlinear device, which is in general different from the DC resistance . The ratio of AC voltage to current in the device is defined by this line . Because the impedance of the reactive components will vary with frequency, the slope of the AC load line depends on the frequency of the applied signal . So there are many AC load lines, that vary from the DC load line (at low frequency) to a limiting AC load line, all having a common intersection at the DC operating point . This limiting load line, generally referred to as the AC load line, is the load line of the circuit at "infinite frequency", and can be found by replacing capacitors with short circuits, and inductors with open circuits . </P> <P> The load line diagram at right is for a resistive load in a common emitter circuit . The load line shows how the collector load resistor (R) constrains the circuit voltage and current . The diagram also plots the transistor's collector current I versus collector voltage V for different values of base current I . The intersections of the load line with the transistor characteristic curves represent the circuit - constrained values of I and V at different base currents . </P>

How to draw dc load line of a transistor