The Stability of quiescent operating point:

Let us suppose that the transistor is replaced by another transistor of the same type. The bdc of the 2 transistors of same type may not be same. So, if bdc increases then for same I_B, output characteristic shifts upward. If the bdc decreases, output characteristic shifts downward. Since I_B is maintained constant, so the operating point shifts from Q to Q1 as shown in the figure. The new operating point can be totally unsatisfactory.

Thus, to maintain operating point stable, IB  should be allowed to change so as to maintain V_CE & I_C constant as b_dc changes.

Figure 

A second cause for the bias instability is a variation in the temperature. The reverse saturation current changes with the temperature. Particularly, I_CO  doubles for every 10^oC rise in the temperature. The collector current IC causes collector junction temperature to rise, which in turn increases the I_CO. As a result of this growth I_CO, I_C  will increase ( b_dc  I_B  + (1+ b _dc  ) I_CO  ) and continues. It can be possible that the process goes on and the ratings of transistors are exceeded. This increase in IC changes the characteristic and hence operating point.

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