Load-Line Analysis in BJT Circuits

So far we have been studying BJT analysis dependent on the level of β over their correspondingoperating points (Q-point). In this discussion we will check out how a given circuit conditions can help in determining the possible range of operating points or Q-points and in establishing the actual Q-point.

What is Load Line Analysis

在任何电子系统中，在半导体器件上施加的负载通常会对操作点或装置的操作区域产生显着影响。

If an analysis is carried out through a graph drawing, we would be able to draw straight line across the characteristics of the device for establishing the applied load. The intersection of the load line with the characteristics of the device can be used for determining the point of operation or the Q-point of the device. This sort of an analysis is, for apparent reasons, known as load-line analysis.

How to Implement Load Line Analysis

The circuit shown in the following Fig 4.11(a) determines an output equation which provides a relationship between the variables IC and VCE as shown below:

VCE = VCC - ICRC (4.12)

Alternately, the output characteristics of the transistor as shown in the diagram (b) above also provide the relationship between the two variables IC and VCE.

This essentially helps us to get a circuit diagram based equation and a range of characteristics through a graphical representation which work with similar variables.

当同时满足它们的约束时，建立了两者的常见结果。

Alternatively this may be understood as solutions being achieved from two concurrent equations, where one is set up with the help of the circuit diagram, while the other from the BJT datasheet characteristics.

在图4.11B中，我们可以看到IC的特性IC与BCE的VCE，所以现在我们能够叠加由EQ（4.12）描述的直线上方的特性。

The easiest method of tracing Eq (4.12) over the characteristics could be executed by the rule which says that any straight line is determined by two distinct points.

通过选择IC马= 0,我们发现the horizontal axis becomes the line where one of the points takes its position.

Also by replacing IC = 0mA in Eq (4.12) we get:

This determines one of the points for the straight line, as indicated in fig 4.12 below:

Now if we choose VCE = 0V, this sets up the vertical axis as the line where our second point takes its position. With this situation, now we are able to find that IC can be evaluated by the following equation.

which can be clearly witnessed on Fig. 4.12.

通过连接由EQS确定的两点。（4.13）和（4.14），可以绘制由EQ 4.12建立的直线。

如图所示，这条线图4.12被识别为负载线since it is characterized by the load resistor RC.

By solving the established level of IB, the actual Q-point could be fixed as shown in Fig 4.12

If we vary the magnitude of IB by varying the RB value, we find the Q-point shifts towards upward or downward across the load line as depicted in Fig. 4.13.