biasing A Transister

[geo4allah]

Asalam-o-Aliakum
I am Mujahid Mir and want to bias a transistor in every accept completely by using theoretically method (using formulas for base, collector, and emitter bias resistors) for the following circuit.
I wan to derive a Electro Magnetic relay of 12V with a c1383 transistor. With following specifications.

Silicon npn transistor
Collector to base voltage

[audioguru]

Hi Allah,
You have listed the limits for the transistor beyond which damage will occur. But you did not list its range of current gain, which determines how it operates.

The transistor's base resistor must pass enough base current so that the transistor's current gain makes it conduct enough to pass the current of the relay's coil. You don't list the current of the relay's coil.

It's easy.
1)  Assume that the relay coil is 120 ohms. Then with 12V it conducts 100mA.
2) The datasheet for the 2SC1383 transistor shows a minimum current gain (hFE) of 85 near a collector current of 100mA. Therefore the base current must be at least 1.18mA. If you want the transistor to conduct better, use 4.72mA.
3) If the base resistor connects to +12V to turn on the transistor, it will have about 11.3V across it. Ohm's Law calculates the base resistor to be 2400 ohms. 

[Kevin Weddle]

I think I get what you are saying. The voltage at the base is found by taking the collector current, dividing by beta, getting the base current, and calculating. Only multiply the emitter resistor by beta when you are examining the change in base current with a signal. The reason is you cannot get a DC value of current without using the .7 as reference. The actual value of PN voltage can only be found by using a formula that tells you what the PN voltage is with a certain amount of current.