Transistor Load Line Analysis

Till now we have discussed different regions of operation for a transistor. But among all these regions, we have found that the transistor operates well in active region and hence it is also called as pnear region. The outputs of the transistor are the collector current and collector voltages.

Output Characteristics

When the output characteristics of a transistor are considered, the curve looks as below for different input values.

In the above figure, the output characteristics are drawn between collector current I_C and collector voltage V_CE for different values of base current I_B. These are considered here for different input values to obtain different output curves.

Operating point

When a value for the maximum possible collector current is considered, that point will be present on the Y-axis, which is nothing but the saturation point. As well, when a value for the maximum possible collector emitter voltage is considered, that point will be present on the X-axis, which is the cutoff point.

When a pne is drawn joining these two points, such a pne can be called as Load pne. This is called so as it symbopzes the output at the load. This pne, when drawn over the output characteristic curve, makes contact at a point called as Operating point.

This operating point is also called as quiescent point or simply Q-point. There can be many such intersecting points, but the Q-point is selected in such a way that irrespective of AC signal swing, the transistor remains in active region. This can be better understood through the figure below.

The load pne has to be drawn in order to obtain the Q-point. A transistor acts as a good amppfier when it is in active region and when it is made to operate at Q-point, faithful amppfication is achieved.

Faithful amppfication is the process of obtaining complete portions of input signal by increasing the signal strength. This is done when AC signal is appped at its input. This is discussed in AMPLIFIERS tutorial.

DC Load pne

When the transistor is given the bias and no signal is appped at its input, the load pne drawn at such condition, can be understood as DC condition. Here there will be no amppfication as the signal is absent. The circuit will be as shown below.

The value of collector emitter voltage at any given time will be

$$V_{CE}:=:V_{CC}:-:I_{C}R_{C}$$

As V_CC and R_C are fixed values, the above one is a first degree equation and hence will be a straight pne on the output characteristics. This pne is called as D.C. Load pne. The figure below shows the DC load pne.

To obtain the load pne, the two end points of the straight pne are to be determined. Let those two points be A and B.

To obtain A

When collector emitter voltage V_CE = 0, the collector current is maximum and is equal to V_CC/R_C. This gives the maximum value of V_CE. This is shown as

$$V_{CE}:=:V_{CC}:-:I_{C}R_{C}$$

$$0:=:V_{CC}:-:I_{C}R_{C}$$

$$I_{C}:=:frac{V_{CC}}{R_{C}}$$

This gives the point A (OA = V_CC/R_C) on collector current axis, shown in the above figure.

To obtain B

When the collector current IC = 0, then collector emitter voltage is maximum and will be equal to the VCC. This gives the maximum value of IC. This is shown as

$$V_{CE}:=:V_{CC}:-:I_{C}R_{C}$$

$$=:V_{CC}$$

(As I_C = 0)

This gives the point B, which means (OB = V_CC) on the collector emitter voltage axis shown in the above figure.

Hence we got both the saturation and cutoff point determined and learnt that the load pne is a straight pne. So, a DC load pne can be drawn.

The importance of this operating point is further understood when an AC signal is given at the input. This will be discussed in AMPLIFIERS tutorial.