In video #324 (calibrating a Simpson 260), I used a 50uA Wilson current source/mirror. Several viewers asked for more detail. This video discusses the output impedance of BJT (bipolar junction transistor) current sources / mirrors. It presents the Early Effect and Early Voltage of BJTs, and then shows how the Wilson Current Mirror dramatically improves the output impedance of a BJT current mirror. A Curve Tracer is used to illustrate the transistor properties. Here are helpful links to the videos mentioned in this video, and a link to the notes.
#324: Calibrate the Simpson 260: https://youtu.be/TQ7w6_CgO0Q
#190: Current sources and mirrors: https://youtu.be/xR0RfmmRhDw