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Let's say we've got our BJT operating in its saturation mode. Can we make (a rough) approximation now and make calculations for the circuit assuming that both Ube=0.7V and Ubc=0.7V (for an npn). Or do these values change in sat mode?

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If a NPN BJT is operating in the saturation mode, I usually take the following rough assumptions:

  • \$V_{be} = 0.7 \mbox{V}\$
  • \$V_{ce} = 0.2 \mbox{V}\$

This gives an slightly "assymetrical" \$V_{bc} = 0.5 \mbox{V}\$ because the transistor cannot become an "ideal" short.

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The definition of saturation is VBC > 0. Thus VCE is less than VBE.

Under normal conditions, VBE is around 0.7 V, and the transistor will operate reasonably predictability until VBC is over 0.3 or 0.4 V, meaning VCE is less than 0.3 or 0.4 V

These values all depend on temperature, transistor characteristics, and the current flowing in it.

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