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 Question:

breakdown voltage in BJT
  • vasa

    How breakdown voltage of common base configuration is higher than that of common emitter configuration??

    in common base, break down is more or less similar to PN junction breakdown. please explain how breakdown occurs in common emitter configuration??? I read because of transistor amplification, common emitter Breakdown has less value than PN junction breakdown. more explanation needed to understand clearly.

  •  Answers:

  • Alfred Centauri

    For the common-base configuration, the base is common ("grounded") and the collector-base breakdown voltage is determined with the emitter open so, as you write, the breakdown voltage is essentially that of a PN junction.

    For the common-emitter configuration, the emitter is common and the collector-emitter breakdown voltage is determined with the base open. In this mode, the base width modulation and impact ionization effects enhance the avalanche breakdown of the collector-base junction leading to a lower breakdown voltage.

    See, for example, this.

  • johnfound

    In common-emitter schematic, the reverse CB current flows through the open BE junction and is amplified by h21e factor. This way the collector current raises h21e times faster than in common-base configuration and the avalanche breakdown will happen on lower voltages.