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I was trying to use S8050 in a schematic and thus was checking the datasheet. The datasheet says Vbe Sat. is 1.2V. Is it correct? As a learner I have seen most of the transistors have Vbe 0.7, I am little confused in calculating the base resistor.. Please help ... here is the datasheet..

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Figure 4 from the S8050 datasheet.

The datasheet shows that \$ V_{be} \$ is as you expected at modest currents. At higher currents it increases dramatically.

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Note that this is the max spec. The manufacturer is telling you this is the worst case (largest) Vbe you will ever encounter with this transistor for any combinations of valid values found anywhere else in the datasheet, and probably plus a little to cover their butts.

So if you feed it 50 mA of base current, 500 mA of collector current, and keep it at the coldest operational temperature allowed, Vbe is not going to exceed 1.2 V no matter which part you pick out of the bin, and even on a Tuesday with a full moon.

At a more normal operating point at a more normal temperature, you probably get pretty close to the 600 to 750 mV of most transistors.

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  • \$\begingroup\$ At the risk of being called by Nick Alexeev as "rep-whore", I was going to answer this question approximately as you did. But you beat me at that :-( \$\endgroup\$ – Ale..chenski Apr 13 '18 at 21:39
  • \$\begingroup\$ @AliChen When we all have the same thoughts as Olin...is that a good thing ? Olin seems poetic today (Friday, 13th). At the sight of a BJT, let's all give the moon a good howl. \$\endgroup\$ – glen_geek Apr 13 '18 at 21:52
  • \$\begingroup\$ Hmm, understood.. So if I use this transistor to drive a 5v 70Ohs relay and want to put a LED on the base (to notify if the base is being driven), then the total voltage drop will be .7V + 1.6V approx, right? Actually, here is the question I already asked :) electronics.stackexchange.com/questions/368407/… \$\endgroup\$ – Rakesh Mehta Apr 13 '18 at 21:52

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