current mirror

A current mirror is a small set of matched transistors that copies one current and reproduces it somewhere else in the circuit. Picture a master device that sets a reference current, and one or more twins standing right beside it that simply follow its lead: whatever the master carries, the copies carry too. This is one of the most-used building blocks in analog design — it is how you hand the same well-defined current to many parts of a chip at once, and how you bias amplifiers and other stages into their working state.

The simplest mirror is two transistors sharing a gate. The master is diode-connected (its gate tied to its own drain), so a reference current forced through it settles on exactly the gate voltage needed to carry that current. The second transistor sees the same gate voltage, so if the two devices are identical it carries the same current. Make the copy's W/L (width-over-length) ratio twice the master's and it carries twice the current; the mirror's copy ratio is just the ratio of the W/L sizes, so you can scale a single reference into many tidy multiples.

Accuracy lives and dies on matching: the two transistors must be truly alike — same temperature, same orientation, laid out close together — or the copy drifts off. The other limit is the output transistor's drain voltage. An ideal mirror would push the same current regardless of what voltage the load presents, but a plain mirror's copy creeps up as the output drain voltage rises (a finite output resistance). A cascode mirror stacks a second transistor on top to shield the mirroring device from those voltage swings, raising the output resistance and tightening the copy — the standard fix when you need a mirror that behaves like a stiff current source or a high-quality active load.