Table of Contents
A current source provides a certain amount of current to a load instead of a certain voltage. Of course, the current source needs to be able to provide enough voltage to push that current through a load.
BJT Current Source:
A Bipolar Junction Transistor Current Source has an Emitter resistor.
Setting a voltage at the Base will transfer that voltage to the Emitter resistor but with a 0.6V offset. There will be 0.6V less at the NPN BJT Current Source Emitter than the Base voltage.
The NPN transistor will pass current through the Collector to Emitter as needed to maintain that voltage. The Base does not provide the current.
2.6V at the Base, will put 2V across the Emitter resistor. If the Emitter resistor is 220Ω (as shown in diagram) then there will be 2V/220Ω = 0.00909A (approx. 9.1mA). With a 5V supply, a forward biased red LED at the Collector can be expected to have about 9.1mA pass through it. A blue LED will likely drop too much voltage, so a 6V supply might be needed.