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I have to do a lab report which requires me to know the value of the base emitter voltage of the BJT in the forward active region. Last week in lab I measured a lot of data points for different Ib Ic and Vce. From this I determined my beta to be about 208. Now for the report I need to determine a value for the voltage drop from the base to the emitter (also I am using an NPN BJT). I am not so sure how to do this (or even if it can be done given my data set). I tried solving a system of equations relating Ic to Is and Vbe however I quickly realized that these equations could not be solved for Vbe. Any advice for how I can go about it? I also have values for beta forced and Va but I don't think these will help. The directions in my lab are not very clear so I am unsure if I even need to find Vbe or if I can just use the standard value of 0.7 volts.

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  • \$\begingroup\$ Were you using a known base resistor, and a known voltage supply? \$\endgroup\$ – CL. Nov 21 '15 at 16:28
  • \$\begingroup\$ I don't think so. If I knew that the problem would be much easier. \$\endgroup\$ – chasep255 Nov 21 '15 at 16:29
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    \$\begingroup\$ There is no such thing as Vbe ON. \$\endgroup\$ – Andy aka Nov 21 '15 at 16:39
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Unless your data is very precise, and includes values very close to VCE=0, you won't have enough information to determine VBE precisely.

Generally the VBE will be around 0.65 V for most transistors while operated at 'normal' collector currents. If you operated at 1/100 of this value, VBE would only change by about 0.12 V. If you operated at 50 C hotter than room temperature, the VBE would also only change by less than 100 mV. So if you don't need a value more precise than that, I'd suggest using 0.65 V as a good estimate.

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