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
is needed.

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.