Transistor Base current

[Faizan Jawaid]

I want to know how to calculate Rb (base resistor) for the optimal transistor switching performance for the shown configuration. Formulas are appreciated.

Thanks.

 

[btbass]

There are so many variables that I would use rule of thumb methods.
You divide the collector current by the minimum current gain of the transistor to find the required base current.
Rule of thumb, multiply this value by ten to ensure the transistor is turned hard on.
To find the voltage accross the base resistor, subtract the maximum Base emiter votage (Vbe) from the minimum TTL driving voltage.
The resistor value is then V/I.

 

[FvM]

The required base current for a particular collector current and acceptable maximum saturation voltage can't be calculated from a formula but has to be checked with the transistor datasheet. You'll find, that 2N2222 is unsuitable to switch 500 mA at a moderate base current.

 

[Audioguru]

The base of the NPN transistor needs a fairly high positive current.
The TTL gate has a low positive current and a high negative-going current so it won't work well.