Re: a simple basic question
mujee,
The BJTs, including the Darlington, are current controlled devices. In other words, the collector current is proportional to the base current. The MOSFET is a voltage controlled device. The drain current is proportional to the gate voltage. In a "self biased circuit" where you have a resistor between emitter and ground, you can connect a voltage source to the base. The emitter voltage will be one diode drop below the base voltage. The emitter current wil be equal to the emitter to ground voltage divided by the emitter resistance. The base current will automatically adjust itself to provide the required emitter current.
Regards,
Kral
Added after 1 minutes:
I recommend "The Art of Electronics" by Horowitz and Hill as an introductory book on electronics.