Operating Point

Operating Point

The operating point is a point where the transistor can be operated efficiently. A line that is drawn with the values V_CC (when I_C = 0) and I_C (when V_CE = 0) is called the dc load line. The dc load line superimposed on the output characteristics of a transistor is used to learn the operating point of the transistor as shown in Figure 9.35.

Points P, Q, R in Figure 9.35 are called Q points or quiescent points which determine the operating point or the working point of a transistor. If the operating point is chosen at the middle of the dc load line (point Q), the transistor can effectively work as an amplifier. The operating point determines the maximum signal that can be obtained without being distorted.

For a transistor to work as a open switch, the Q point can be chosen at the cut-off region and to work as a closed switch, the Q point can be chosen in the saturation region.

EXAMPLE: 9.8

The current gain of a common emitter transistor circuit shown in figure is 120. Draw the dc load line and mark the Q point on it. (V_BE to be ignored).

Solution

β = 120