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I'm reading a datasheet (https://www.mouser.com/datasheet/2/348/qs6z5-774191.pdf) of a BJT and noticed that there is a chart titled "Ground emitter propagation characteristics". I can see that this chart plots Vbe vs Ic. I was wondering what's purpose of this chart? Isn't Ic/Ie controlled by base current (Ib) in BJTs? When is this chart normally used? Simply googling this doesn't find anything useful.

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  • \$\begingroup\$ I believe it just helps you see the base-emitter voltage for a variety of temperatures and collector currents. You can readily see the temperature variation is on the order of \$-2\:\text{mV}\$ per Kelvin degree, which is just a confirmation of what most of us keep in our heads about BJTs. Nothing unusual there. Also, you can readily see the effect of the base and emitter bulk resistances taking effect from about \$500\:\text{mA}\$ to about \$1\:\text{A}\$. Looks like about \$200\:\text{m}\Omega\$ at the emitter pin, just eyeing it. \$\endgroup\$
    – jonk
    Apr 25, 2022 at 4:14

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Isn't Ic/Ie controlled by base current (Ib) in BJTs?

No. It is a voltage controlled device, just like the FET, but suffers high "input leakage current" if you will.

Both the base current and the collector current have a rather ideal exponential dependence on the base voltage. As a result, Ic and Ib are related by a constant factor for some range of base voltage. This relation gives rise to the common mind model of Ic being proportional to Ib.

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    \$\begingroup\$ Some forum members may forgive me, but I cannot suppress the following remark: It is unbelievable - but in many technical books you can still read that the base current would "control" the collector current. But this is a pure assertion without any proof - derived from the misinterpretation of the relation Ib=Ic/B. But it should be clear and logical that - even from an energy point of view - it is impossible that a tiny current should be able to directly "control" a current that is larger up to a factor of 500. \$\endgroup\$
    – LvW
    Apr 25, 2022 at 8:07
  • \$\begingroup\$ As simple as this: Voltage -> field -> charges flows -> current ? \$\endgroup\$
    – Antonio51
    Apr 25, 2022 at 9:13

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