Basic circuit diagram of mirror constant current source

The figure shows the basic circuit of a mirror constant current source , in which VT1 and VT2 are matched pairs of tubes. It can be seen from the figure that Ir=Ic2+IB1+IB2

Since VT1 and VT2 are symmetrical, their collector current and base current are equal respectively, so there is

When Ir is determined, the output current Io of the constant current source is also determined. When β is large enough, Io≈Ir, that is, the output current is approximately equal to the reference current, so this circuit is often called a current mirror circuit.

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