Current minor circuit: The ckt in which the o/p current can be forced to become equal to i/p current called as current mirror ckt.

In the other words a current mirror ckt the o/p current is mirror image of i/p current.

Operation of current mirror ckt: the current mirror is a special type of constant bias and hence can be med to provide a constant current bias for two transistors $Q_1$ and $Q_2$ which differ ampr ckt.

$VBEq_3 = VBEq$

$Ic_3 = Ic_4$

And $IB_3 = IB_4$

Applying kcl @ node A.

$I_2 = Ic_4 + I$

$I_2 = Ic_4 + IB_3 + IB_4$

$IB_3 = IB_4$

$\therefore$ $I_2 = Ic_3 + 2 IB_3$

$I_2 = IC_3 + 2 \frac{Ic_3}{\beta dc}$

$I_2 = Ic_3 ( 1+ \frac{2}{\beta dc})$

As $\beta$ dc is very large $\frac{2}{\beta dc}$ term cause neglected.

$\therefore$ $I_2 \approx Ic_3$

Current mirror ckt is easy to fabricate. Therefore it is widely used as constant current bias in integrated amplifiers.