Set bias to your transistor
We have to set bias to a transistor to let it work.
That doesn't work ,when we set 12V
between collector and emitter.But it works,
if we set bias to base terminal. In case of bipolar transistor positive bias over 0.6V
Destination : Current between collector and emitter is 4mA
Firstly we get 2V by dividing resistor .
If we set ratio to be R1/R2=5/1,we can
get 2V by the law of Ohm.
If we set 2V to base terminal,channel
between collector and emitter is on,eventually
current flows to make 0.6V between base and emitter.Because Vbe is constant,voltage
appearing at emitter is 1.4V.
Secondly we calculate R3.
Therefore R3 is 350Ω.
This is the most simple current regulator at 4mA.