Please explain why operating point is fixed in the centre of action region of transistor?

 

The Q point is not always in the center of the active region of the transistor. Its position depends on the application. However, most of the times its in the center as to have the maximum swing (variation) of the output signal. Also, the Q point is fixed in a voltage divider bias circuit, in other configurations like the common emitter bias circuit it varies with temperature and β.

 

I agree with mik3. The Q-point is most often set at the midpoint of the output voltage range so that the output signal will be able to achieve the maximum peak to peak voltage level without prematurely clipping of the output signal.

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