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After 40+ years you would think I would know this, somehow it never came up, but for the life of me I don't see it.

Typical transistors specs list a plethora of values but one that always seems to be missing is \$I_{b_{MAX}}\$. That is, the maximum current you can pass through the base emitter junction.

Obviously you can't exceed the power rating across the base-emitter diode forward drop, so 626mW/0.7V ~ 900mA, but I have a feeling there is a fusing number for the bonding wire. Perhaps it is the same as the max collector current.. 200mA.

What number is the right one, if any?

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    \$\begingroup\$ I have seen it listed but I’m racking me brain to think on what. It was to do with a question on this site so maybe bc547, 2n3904 or maybe 2n2222..... \$\endgroup\$ – Andy aka Feb 1 '18 at 19:32
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    \$\begingroup\$ It would be related to secondary breakdown due to current crowding, but that's normally spec in the SOA plot for IC-VC. It would be less than IC max \$\endgroup\$ – sstobbe Feb 1 '18 at 19:56
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    \$\begingroup\$ Related: electronics.stackexchange.com/questions/158522/… \$\endgroup\$ – vofa Feb 1 '18 at 19:57
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    \$\begingroup\$ Found one on an ST BD139 datasheet: 0.5A. \$\endgroup\$ – Oldfart Feb 1 '18 at 19:59
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    \$\begingroup\$ Generally it is shown in power transistors' (BD139, BD243, 2N3055 etc) datasheets. But I cannot say the same thing for general purpose ones. \$\endgroup\$ – Rohat Kılıç Feb 1 '18 at 20:13
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Aside from the answers here the NXP BC847 lists a maximum Ibm as 100mA for a <= 1ms pulse.

enter image description here

It is less than the maximum collector current under the same conditions.

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  • \$\begingroup\$ Yup seen those, but no indication of continuous. I was toying with an idea to use the BE as a diode in a current detection circuit for a different question , but the current gets a tad large. \$\endgroup\$ – Trevor_G Feb 1 '18 at 20:34
  • \$\begingroup\$ Maybe 50% if Ic is a good ball-park number in general though. \$\endgroup\$ – Trevor_G Feb 1 '18 at 20:36
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    \$\begingroup\$ @Trevor_G Depends on how brave you are. 10mA is mentioned in the testing of Vbe. \$\endgroup\$ – Spehro Pefhany Feb 1 '18 at 20:44
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The assembly house will use the same size bond wire for all 3 leads. Unless the base metallization from bond-pad into the base region has narrow metal, I'd use the same # as emitter or collector. Some old datasheets would, with pride, show die photos.

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