Definition and explanation of Stabilization and Need of Stabilization

Stabilization

When the temperature changes or the transistor is replaced the operating point also changes. However for faithful amplification, the operating point must be remains fixed. This necessity to make the operating point independent of these variations. The process of making operating point independent of temperature changes or variations in transistor parameters is known as stabilization.

Thus for maintaining the operating point independent of these variations, the following techniques are used. They are

1.       Stabilization technique

2.       Compensation technique

Need of Stabilization

The following are the factors that effect the stability of the operating point.

1.       Temperature dependence of I_C

2.       Change of V_BE and β due to replacement of transistor

3.       Thermal runway

1.       Temperature dependence of I_C:      The collector current I_C is given by

I_C  =  βI_B + (β + 1) I_CBO

The collector leakage current I_CBO doubles for every rise in 10^o C for germanium and 6^o C for silicon. Therefore change in temperature, changes the collector current. This necessitates to stabilize the operating point i.e. to hold I_C constant inspite of temperature variations.

2.       Changes of V_BE and β due to Replacement of Transistor: The values of V_BE and β of transistors are varied even when they are manufactured by the same firm and one of the same type. Further, V_BE itself decreases when temperature increases. When a transistor is replaced by another of the same type, these variations change the operating point. This necessitates to stabilize the operation point.

3.       Thermal runaway: The collector leakage current I_CBO increase with rise in temperature. The flow of collector current produces heat within the transistor. This raises the transistor temperature and I_CBO. From the expression (6) it is clear that the collector current I_C also increases. This increased I_C will raises the temperature of the transistor, which in turn will cause increase in I_CBO. This effect is cumulative and the collector current may become very large, causing the transistor to burn out. The self-destruction of an unstabilized transistor is known as thermal runaway. In order to avoid thermal runaway and consequent destruction of transistor, it is very essential that operating point is stabilized.