DC and AC Load Lines
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 closed circuits). It is used to determine the correct DC operating point, often called the Q point.
Once a DC operating point is defined by the DC load line, an AC load line with, in general, a different slope can be drawn, through the DC operating point, to calculate the AC output. 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 be replacing capacitors with short circuits, and inductors with open circuits.
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