5
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I'm having trouble understanding how much current flows through the base of a BJT. With a MOSFET the answer is easy: 0. How do I calculate the amount of base current in a BJT and am I doing it wrong if my circuit cares?

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    \$\begingroup\$ What kind of circuit? Do you want to saturate the transistor (logic circuit, on/off), or use it as an amplifier? \$\endgroup\$ – tcrosley Sep 29 '10 at 13:57
7
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https://upload.wikimedia.org/wikipedia/en/thumb/4/46/BJT_Switch.jpg/800px-BJT_Switch.jpg

In this circuit, the base current would be approximately (Vs - 0.7) / R2. The BE junction (with the arrow) behaves like a diode, so as long as Vs is greater than 0.7 V, then current will flow through the diode and there will be a drop of about 0.7 V (depending on the part) and then the current is just limited by the resistor according to Ohm's law.

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    \$\begingroup\$ I find when building circuits using .7 works a lot better. \$\endgroup\$ – Kortuk Sep 29 '10 at 15:48
  • \$\begingroup\$ This is one thing that I can really teach well in person, the internet is hard. \$\endgroup\$ – Kortuk Sep 29 '10 at 15:48
  • \$\begingroup\$ 0.65V is more commonly found on the 'net and is a compromise. \$\endgroup\$ – Thomas O Sep 29 '10 at 20:19
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    \$\begingroup\$ The exact value varies depending on the part, the amount of current, and the temperature. \$\endgroup\$ – endolith Sep 29 '10 at 21:33

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