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Will a BJT transistor still be in saturation mode when exceeding 0.7V?
Wimberleytech:
--- Quote from: petert on February 05, 2020, 04:56:42 pm ---
So given the maximum rating of the continuous collector current of IC = 500mA, and assuming a gain range from 110 to 800, the maximum for IB would be between 500mA/110 and 500mA/800, which is from 4.55 mA down to 0.625 mA.
What I conclude from this though is not that IB couldn't be higher (no limit is specified in the datasheet and the base may be differently "fragile" from the collector, so a direct derivation based on gain might not apply), but that IC would go beyond its maximum ratings if IB exceeded the values computed above, and therefore it does not matter, much what the actual limit for IB is.
Is that correct?
--- End quote ---
There are problems here.
1) hfe is a small-signal parameter and you are using it as a large-signal one.
2) hfe range given is for Ic = 2mA. Thus your calculations do not apply.
3) dc current gain varies as a function of collector current.
You are getting a lot of good answers here in response to your questions. But, I suspect we are not getting to the root of your problem. It might be better if you present a circuit or problem you are trying to solve in the context of a BJT.
SilverSolder:
--- Quote from: petert on February 05, 2020, 04:56:42 pm ---
[...]
--- Quote from: SilverSolder on February 05, 2020, 02:38:16 pm ---As @Wimberleytech said, everything makes more sense if you think in terms of the current from B to E, and think of the Vbe voltage as a kind of side effect of that.
As a first approximation, you can think of the CE current as the BE current times the gain of the transistor.
--- End quote ---
So given the maximum rating of the continuous collector current of IC = 500mA, and assuming a gain range from 110 to 800, the maximum for IB would be between 500mA/110 and 500mA/800, which is from 4.55 mA down to 0.625 mA.
--- End quote ---
That is good thinking, yes.
--- Quote from: petert on February 05, 2020, 04:56:42 pm ---
What I conclude from this though is not that IB couldn't be higher (no limit is specified in the datasheet and the base may be differently "fragile" from the collector, so a direct derivation based on gain might not apply), but that IC would go beyond its maximum ratings if IB exceeded the values computed above, and therefore it does not matter, much what the actual limit for IB is.
Is that correct?
--- End quote ---
Most datasheets do not specify the max base current, probably because it is assumed that you would not normally try to push more current than needed to achieve the target CE current - typically 10% of the collector current (depending on the min gain, e.g. your calculations above).
There is one situation where it can make sense to "overdrive" the BE junction and push more current than that: when the transistor is saturated.
For example, look at the attached spec for the common 2N3906 transistor, page 6, figure 14. Here you can see how there is nothing much gained from increasing the base current beyond a certain point, once the transistor has saturated, but it might make sense to "squeeze it to the max" by driving it hard. (Another way of thinking of this, is that the gain drops quickly when saturation is reached, so you have to supply much more base current to get the last few drops out of the transistor.)
petert:
I found some relevant posts:
https://electronics.stackexchange.com/questions/158522/bjt-absolute-maximum-base-current
https://www.rohm.com/products/faq-search/faqId/231
https://electronics.stackexchange.com/questions/353511/missing-bjt-parameter-i-b-max
petert:
References regarding the original question:
https://electronics.stackexchange.com/questions/12477/how-do-bjt-transistors-work-in-a-saturated-state
https://electronics.stackexchange.com/questions/254391/terminology-for-bjt-regions-of-operation
https://electronics.stackexchange.com/questions/5336/i-dont-care-how-a-transistor-works-how-do-i-get-one-to-work
http://www.zen22142.zen.co.uk/Design/bjtsw.htm
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