Uses of DC Load line

The below figure shows the output characteristic curves for the transistor in CE mode. The DC load line is drawn on the output characteristic curves. 

Load line To draw load line, we have to find saturation current and the cutoff voltage. After plotting these values on the vertical and the horizontal axes, a line is drawn joining these two points, which represents DC load line. It represents all possible combinations of the collector current Ic and the collector voltage Vc (or Vce) for the given load resistorRc. 

Saturation point The point at which the load line intersects the characteristic curve near the collector current axis is referred to as the saturation point. At this point of time, the current through the transistor is maximum and the voltage across collector is minimum for a given value of load. Therefore saturation current for the fixed bias circuit, Ic (sat) =Vcc/Rc 

Cutoff point The point where the load line intersects the cutoff region of the collector curves is referred as the cutoff point (i.e. end of load line). At this point, collector current is approximately zero and emitter is grounded for fixed bias circuit.Therefore, Vce (cut) = Vc = Vcc

Operating point The "Q point" for a transistor amplifier circuit is the point along its operating region in a "quiescent ", where no input signal gets amplified 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. 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 circuit. 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. Common emitter load line. 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. 

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