Transistor Stabilisation

The collector current in a transistor changes rapidly when (i) the temperature changes,

(ii) the transistor is replaced by another of the same type. This is due to the inherent variations of transistor parameters. When the temperature changes or the transistor is replaced, the operating point (i.e. zero signal I_C and V_CE) also changes. However, for faithful amplification, it is essential that operating point remains fixed. This necessitates to make the operating point independent of these variations. This is known as stabilisation.

The process of making operating point independent of temperature changes or variations in transistor parameters is known as stabilisation.

Once stabilisation is done, the zero signal I_C and V_CE become independent of temperature variations or replacement of transistor i.e. the operating point is fixed. A good biasing circuit always ensures the stabilisation of operating point.

Need for stabilisation. Stabilisation of the operating point is necessary due to the following reasons :

(i) Temperature dependence of IC

(ii) Individual variations

(iii) Thermal runaway

(i) Temperature dependence of I_C.

The collector current I_C for CE circuit is given by:

I_C = β.I_B + I_CEO = β. I_B + ( β + 1) I_CBO

The collector leakage current I_CBO is greatly influenced (especially in germanium transistor) by tem-perature changes. A rise of 10°C doubles the collector leakage current which may be as high as 0.2 mA for low powered germanium transistors. As biasing conditions in such transistors are generally so set that zero signal I_C = 1mA, therefore, the change in I_C due to temperature variations cannot be tolerated. This necessitates to stabilise the operating point i.e. to hold I_C constant inspite of temperature variations.

(ii) Individual variations.

The value of β and V_BE are not exactly the same for any two transistors even 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 stabilise the operating point i.e. to hold I_C constant irrespective of individual variations in transistor parameters.

(iii) Thermal runaway.

The collector current for a CE configuration is given by :

I_C = β.I_B + ( β+ 1) I_CBO

The collector leakage current I_CBO is strongly dependent on temperature. The flow of collector current produces heat within the transistor. This raises the transistor temperature and if no stabilisation is done, the collector leakage current I_CBO also increases. It is clear from exp. (i) that if I_CBO increases, the collector current I_C increases by ( β+ 1) I_CBO. The increased I_C will raise the temperature of the transistor, which in turn will cause I_CBO to increase. This effect is cumulative and in a matter of seconds, the collector current may become very large, causing the transistor to burn out.

The self-destruction of an unstabilised 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 stabilised i.e. IC is kept constant. In practice, this is done by causing I_B to decrease automatically with temperature increase by circuit modification. Then decrease in βI_B will compensate for the increase in ( β+ 1) I_CBO, keeping IC nearly constant. In fact, this is what is always aimed at while building and designing a biasing circuit.

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