Category:Simple BJT current mirrors
The current mirror is perhaps one of those topologies inherently well suited to IC design.
In the not so far away past the use of passives in discrete or hybrid circuits allowed for the biasing of analog circuits with high-precision. Unfortunately, in ICs fabricated primarily in standard digital processes, passives are known for having sub-par performance and large temperature coefficients: hence making them poor choices for current references.
The genius of Bob Widlar was to recognize that ICs offered the ability to finely control the matching of devices. Henceforth, traditional topologies requiring the absolute precision of component values would turn out to be difficult to integrate while maintaining performance. On the other hand, new or revised topologies exploiting matching and/or relative ratios among components would pave the way forward in analog IC design. The simple current mirror (shown below) exemplifies this concept and would become a staple of analog IC design for generations to come.
The diode-connected reference transistor transduces the reference current to a diode VBE voltage (or alternatively said, it calculates the VBE required to yield the applied collector current) which is then used subsequently to mirror the reference current to other matched devices in other biasing branches.