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# finding Vbe in NPN Biasing

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#### yefj

Hello,I am trying to bias BC547B NPN transistor.
We have Vcc=10V in the middle we have a voltage drop between 47k and the resistance of BE junction.
How do i know the resistance of the BE juction ?
I cant see how to get the simations result that Vb=0.76

The voltage dropped across B-E isn't a resistance, that's why it isn't easy to calculate. There is a PN junction between base and emitter so the 0.76V comes from the forward voltage dropped across it. The voltage will remain fairly constant across a wide range of base currents.

Brian.

Hi,

What do you want to achieve with this circuit?
Switching application? Some linear regulation?

Over the thumb:
From datasheet you know V_BE is 0.55V ... 0.77V.
Start to calculate with 0.65V.
Then you can calculate I_B.
With the DC gain from the datasheet you can calculate the expected collector current.
Then you may start again by getting V_BE from datasheet, now with the use of collector current.

The problem here is that - as Brian already mentioned - the calculation is difficult.
V_BE depends on base current, collector current, temperature, production tolerances ....

So the value of the simulation ... in reality may vary a lot from transistor to transistor.

Klaus

Added: Don't use datasheets from third party. Better use datasheets directly from the manufacturer. They are most up to date, most reliable...

Vbe :

Biasing :

Regards, Dana.

The problem here is that - as Brian already mentioned - the calculation is difficult.
V_BE depends on base current, collector current, temperature, production tolerances ....

So the value of the simulation ... in reality may vary a lot from transistor to transistor.
And this is exactly the reason that a circuit as shown should NOT be used for amplifying purposes.
Instead, it is recommended to use DC negative feedback (emitter resistor) to make the whole circuit less sensitive to parameter tolerances of the active device (BJT).

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