One of the most important analog building blocks is the current mirror. The basic topology of the current mirror is shown in Figure 4.1. The basic function of a current mirror is to generate an output current IOUT proportional to the input current IIN independent of the voltage VOUT over the output.
Figure 4.1: Simple N-channel MOS current mirror.
Intuitively, the input current IIN causes the voltage VIN over the diode coupled input transistor M1. The voltage is applied over the gate-source of the output transistor M2, which in turn generates the output current IOUT. As illustrated in Figure 4.1, the addition of extra outputs is simple: Just add an extra output transistor controlled by the same gate voltage, VIN.