Q1 and Q2 form a half-supply voltage generator (refer to the circuit CMOS Half-Supply-Voltage Generator). In this circuit Q1 and Q2 are complementary so the voltage at the gate/drain of Q2 is ½ of the supply voltage. The voltage at the gate/drain of Q2 is also applied to the gate of Q3 so these two MOSFETs form a current mirror.
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