Stabilization and Thermal Runaway in the Transistor

  • The process of making the operating point independent on temperature changes and variation of other transistor parameters is called as Stabilization.

The operating point of the transistor changes when
  • Temperature changes
  • Transistor is replaced by another transistor of similar type

Stabilize the operating point
  • The operating point of the transistor is made stable or fixed by if the transistor operating parameter does not depend on temperature and another transistor.

Necessary for stabilization
The operating point of the transistor changes due to
( 1 ) Temperature changes

        IC = βIB + ICEO
            = βIB + ( 1 + β ) ICBO
IC = Collector current
β = Current amplification factor in common emitter configuration
  • The collector leakage current depends upon rise in the temperature. 
  • A small change in the temperatures also changes in collector current as a result operating point changes therefore the operating point is made stable in spite of temperature changes.

( 2 ) Individual transistor parameter changes
  • The value of current amplification factor β and base to emitter voltage VBE for any two same rating transistors is not exactly same in spite of they are made of similar material. 
  • As the temperature increases, the VBE decreases. 
  • When transistor is replaced by another transistor of same type, the operating point changes therefore the operating point should make stable in spite of collector current changes / transistor is replaced by another one.

Thermal runaway

        IC = βIB + ( 1 + β ) ICBO……….( 1 )
  • The collector leakage current ICBO depends upon temperature and it produces heat in the transistor. 
  • This will result in collector leakage current further increases. 
  • As the collector leakage current ICBO increases, the collector current also increases [ From equation ( 1 ) ].  
  • The increase in collector current further increases temperature and this process cumulative result collector current increases such a extent that it may burn out transistor.

The self destruction or burn out of transistor due to rise in temperature is called as thermal runaway.
How to avoid thermal runaway?
  • The transistor is protected by thermal runaway if its operating point makes stable by keeping collector current constant.

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