base or fixed bias

i wonder if anyone could tell me the working of base bias in detailed.i am not able to understand it.
advanced thanks.

 

A transistor needs base current for it to turn on. But transistors even with the same part number have different amounts of current gain and different amounts of base-emitter voltage so a voltage divider is used to feed bias current to the base and an emitter resistor is used to reduce the variations among transistors.

 

how does base currrent remains constant and what makes it to be constant?
thanks.

 

If the supply voltage is regulated then the voltage divider feeding the base and the emitter resistor make the base-emitter current constant.