They change a bit, but primarily at startup. After that they're controlled by the stability of the biasing. I'd recommend "Designing Analog Chips" by Hans Came

They change a bit, but primarily at startup. After that they're controlled by the stability of the biasing. I'd recommend "Designing Analog Chips" by Hans Camenzind[0], the designer of the 555 timer. Particularly chapter 3, "Current Mirrors". [0] http://www.designinganalogchips.com/ http://www.designinganalogchips.com/ has a free pdf