BJT question

Hello all,

I have a question regarding the BJT configuration that I have posted. The values I had to find were the current Ie and the voltage V3, which is the same as Vc.

The answers are: Ie = Ve - (-10v) / 4.7K = 2mA

Vc = 10V - IcRc = 10V - 2MA*(3.3K) = 3.4V


these answers are correct. But what I am confused about is the following:

When I first look at this simple circuit, I see that the base is tied to ground. So to me, that mean that there is no current going into the base, therefore the transistor is in cut-off. If it is in cut-off, then the transistor behaves as an open switch and there-for no current through the collector or emitter.

i know I am wrong here, but this is my first assumption when I first look at it. Can someone shine some light for me here. Thanks

If the emitter was connected to 0V then what you are saying would be true as it isn't at 0V but -10V. The base is at 0V and the emitter is connected to -10V via a resistor meaning current is flowing from 0V to -10V.

Hero999 said:

If the emitter was connected to 0V then what you are saying would be true as it isn't at 0V but -10V. The base is at 0V and the emitter is connected to -10V via a resistor meaning current is flowing from 0V to -10V.

Click to expand...

So, it does not matter if the base is connected to 0V, as long as the Base-Emitter junction is forward biased. Which in this case it is because the N-type emitter is at a lower voltage potential than the P-type Base. Correct?

Of course, the above statement is assuming the Collector-Base junction is reverse biased.

jack23 said:

So, it does not matter if the base is connected to 0V, as long as the Base-Emitter junction is forward biased. Which in this case it is because the N-type emitter is at a lower voltage potential than the P-type Base. Correct?

Of course, the above statement is assuming the Collector-Base junction is reverse biased.

Click to expand...

Ground is just an arbitrary reference point. Look at the 3 circuits below. The transistor quiescent point is exactly the same in all three.

It makes me dizzy to see the ground bouncing around like that. Earthquake?

audioguru said:

It makes me dizzy to see the ground bouncing around like that. Earthquake?

Click to expand...

Yep. It all started like this...