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How can I calculate the junction-to-ambient thermal resistance, Rth(j-a), of a small-signal transistor?

The junction-to-ambient thermal resistance, Rth(j-a), of a small-signal transistor can be calculated from its collector power dissipation as follows:

 

Equation: Rth(j-a) = (Tj Max - Ta) / PC MAX

 

  • Rth(j-a): Junction-to-ambient thermal resistance
  • Tj Max: Maximum junction temperature
  • Ta: Ambient temperature (temperature condition of PC Max)
  • PC Max: Maximum collector power dissipation
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