Given the following circuit:
simulate this circuit – Schematic created using CircuitLab
How do I solve it for base, collector, and emitter voltages (VB, VC, VE)? Assuming the BJT is in active mode and the base-emitter voltage (VBE) is 0.7 V.
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2\$\begingroup\$ VB is really easily found as it's just a simple voltage divider and from there, you should be able to find the rest relatively easily. \$\endgroup\$– DomNov 3, 2015 at 2:35
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1\$\begingroup\$ @Dom, a 24k ohm divider with a 2 kohm load is not "just a simple divider", at least not for someone who is asking this kind of question. \$\endgroup\$– The PhotonNov 3, 2015 at 2:45
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1 Answer
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Assume the BJT is in active mode- start by ignoring the base current since transistor gain is high in active mode. You can check that assumption later, but since beta is not given you can typically safely assume it doesn't play into this homework problem.
Find Vb from the 10V and 60K/40K resistors. You then know Vb- Ve is related by Vbe.
Then find the emitter current. Since we're ignoring base current, we can then easily determine the collector current and thus collector voltage.
In a real case, the base loads the voltage divider a bit (maybe by 1/200 or 1/300 of the emitter current) so the emitter voltage might be 5-10% less, and the collector voltage a bit higher.
answered Nov 3, 2015 at 3:03