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I am confused about this datasheet for a transistor I am using. From my understanding, most transistors have a VBE of about 0.7v but the max on this one is 1.2v which I thought would destroy most transistors. Does VBE(ON) represent something different than VBE?

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    \$\begingroup\$ The Vbe is 1.2V when Ic=300mA, for a lower Ic (e.g. 10mA) Vbe should be 0.7V \$\endgroup\$
    – FedeWar
    Jul 20, 2021 at 7:06
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    \$\begingroup\$ Note that in this case, "max" value in the datasheet does NOT mean that the thing would be destroyed if it exceeded the value. That's what "Absolute maximum ratings" section is for. Here, it's just manufacturer guaranteeing how "good" the device is. In other words, it says "In the worst scenario, the VBE will be max 1.2V, but usually it will be even lower." \$\endgroup\$
    – akwky
    Jul 20, 2021 at 7:19

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Look at datasheet figure 5.

It's typically 0.7 at 10mA of collector current, but if there is need to drive more collector current, you need more base current, and then the Vbe voltage is higher.

And due to manufacturing tolerances, the datasheet guarantees, that when there is 300mA of collector current, the Vbe will never be higher than 1.2V. Typically, it will be about 0.85V according to figure 5.

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  • \$\begingroup\$ Sir How can we say more base current is when base emitter voltage is high is there some relation ? \$\endgroup\$
    – Orion_Pax
    Apr 14, 2022 at 2:24
  • \$\begingroup\$ Cant we just increase the supoorting voltage being applied with it $V_{BB}$ ? \$\endgroup\$
    – Orion_Pax
    Apr 14, 2022 at 2:26
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For a silicon diode junction, the average rule of thumb is that it will be 0.7V at some nominal current. This also applies to simple small signal silicon npn transistors. But this is just a rule of thumb. Your actual values will vary based on the part.

Specifically, for that transistor, the VBE(ON) (not saturation but just on) at 300 mA Ice and 1V Vce has a max of 1.2V to be considered within spec. Some transistors from that model and manufacturers will be all the way at 1.2V in that specific situation. Many will not. Like anything else, the values given tend to be nominal or in ranges and will vary based on temperature, voltage, looking at it funny, etc. Manufacturing tolerances will mean not all pieces of a product will have the same exact values.

If you look at the graph for figure 5 On Voltages the nominal value for Vbe is more like right under 0.9V at 300mA than it will be 1.2V. 1.2V is worst normal case. When designing with these parts you can either design to average values or design to worst case. There are trade offs to both ideas. If you are designing for precision current control to the nanoamp, you probably don't want to use a part with such loose tolerances. But for a general purpose led, the difference between running the transistor at 0.7V Vbe and 1.2Vbe probably isn't too much of a concern.

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  • \$\begingroup\$ Does VBE(ON) include the saturation region or does it only refer to the active region? \$\endgroup\$
    – Atl15
    Jul 20, 2021 at 7:27
  • \$\begingroup\$ @atl15 on means on. Saturation is SAT. Look at graph figure 5, they have individual plot lines and conditions. In saturation your Ice is going to be significantly larger compared to Ice for just on. \$\endgroup\$
    – Passerby
    Jul 20, 2021 at 7:38

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