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Q&A

Small signal analysis of BJT in saturation region

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The small signal analysis of a BJT when the BJT is in amplification mode:

is easy.However what happens when the BJT is in saturation mode?

Can we replace the dependent current source with a resistor for the small signal analysis in saturation region with the value of the resistor being the inverse of the slope Ic vs Vce?(yellow line).

I have searched the Internet a lot and couldnt find what I am saying in the post.Is it wrong to assume that the CE of a BJT when the BJT is in saturation can be replaced by a resistor?

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There is a contradiction (1 comment)
A BJT doesn't have a CE junction (2 comments)

1 answer

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A resistor is not a good model for a saturated BJT. According to your graphs it is, but those are simplified. There isn't really a single slope for all base currents as your graph shows. Even if you can approximate some region of interest with a fixed slope, that slope will vary quite a bit between parts and with temperature. Of course that's true of the other parts of the graph too.

In any case, if the series load is a resistor, then you know operation is always along the red line regardless of what the transistor does.

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