resistance of a constant current source

[sabahan]

I try to figure out how to I calculate the resistance of a constant current source,I am using a LED as a voltage reference to control the voltage,Iam tring to calculate the gain of a common emitter amplifier,I know that the gain is RL/(RE+re),but how I know the value of RL if i am using a constant current source ,thank you

 

[FvM]

Both common emitter amplifier and current source have a finite output resistance ro, review your text books. Only a simplified transistor model has infinite output resistance, in standard models, the Early voltage defines the output resistance.