Low-Drop Current Source

All simple constant-current sources generally operate on the same principle: a current is allowed to flow through a resistance and some sort of regulator is used to try to hold the voltage across this resistance constant. If this is done using a transistor, there must be a voltage drop of approximately 0.6 V over the resistor in order to forward bias the base. However, in some cases this yields an excessive loss, so an opamp with a reference source is used instead.