In the second figure of the book section that I'm including a link to, the author shows a diode connected from the base termination to the emitter termination of a transistor configured to be a current source. Given that the base to emitter junction is a diode of sorts, what purpose does it serve to connect a diode across that junction? It seems to me that operation of the transistor would be the same with or without the diode, except that the current through the bias resistor would be divided between the diode and the base-emitter junction of the transistor.
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/current-mirrors/
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/current-mirrors/