Load line (electronics)
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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 resistor which shares a current with the device in question. Since both currents are the same, the operating point of the circuit will be at the intersection of the curve with the load line.[1]
In the simple case of a diode shown, there is a single voltage across the diode and a single current through it. The load line represents the current in the resistor. When VD = VDD, there will be no voltage across the resistor, so the current will be 0. If VD = 0, the current will be at its maximum.
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.
[edit] Load lines for common configurations
[edit] 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.
[edit] References
- ^ Adel Sedra, Kenneth Smith. Microelectronic Circuits, 5th ed.
