Current mirrors are essential in IC design. They provide stable current by "mirroring" current from a reference branch. They're constructed by shorting a collector and 2 bases of a pair of closely laid out BJTs or a drain and 2 gates of a pair of closely laid out MOS transistors.

       Below was one of my amateur attempts in simulating current mirrors back when I was still in university.



To understand how transistors provide equal constant currents, take Fig. 1.

Figure 1. An attempt at simulating a BJT current mirror

     Above are 2 2N2222 NPN transistors functioning as a current mirror. Q2 has its collector shorted to  the base. The 2 resistors are ohmic, so Ohm's law is valid over that branch. Current supplied is 5 mA and 6.667 mA (initially) to Q2 and Q1 respectively. The BJT is a current controlled device. If the collector of Q2 is controlling emitter output at both transistors (and since both have approximately the same beta, hfe, or operating characteristics) Ie flowing out of Q2's emitter approaches Ie flowing out of Q1's emitter. Voltmeter XMM1 detects a current of 4.615 mA at Q2's emitter. (The ideal maximum forward beta of 2N2222 is 153.575)

10=(2k)It+0.7

It=4.65 mA initially

Q1 and Q2 start conducting. (Vce=0.2 V)

Now that It=4.65 mA(KCL), It=(beta)Ib+2*Ib (remember 2 bases), Ib=0.029889121 mA.

Ic=4.59 mA

Ie=Ic+Ib= 4.62 mA



Now for Q1:

Since Q1 and Q2 are shorted, both power supplies are electrically connected, thus:

10=(2k+1.5k)*(It/2)

It=5.714 mA=Ic

Ib=0.029889121

Ie=5.745 mA


(Both values are a little off from simulation results. Anyhow, the current mirrors provide two equal current sources in spite of the 500 ohm difference between the 2 resistors.)

Below are are cases where current mirror loads have varying resistances:





Unfortunately, simulation results are spurious, since circuit implementation is awry.

Current mirrors can mirror current because the terminals controlling current through the emitter-collector or source-drain terminals (i.e. the base and gate) are common to each other. 

Also, there may be minute differences in the mirrored current due to the 2 bases/gates drawing current from the collector/drain.