A fully differential 1.5 V low-power CMOS operational amplifier with a rail-to-rail current-regulated constant-gm input stage

This paper presents a compact fully differential amplifier with a rail-to-rail input stage and a class AB output stage. A new biasing scheme for a complementary rail-to-rail input stage with constant g_m is introduced. The proposed biasing scheme uses a current regulating loop to keep the sum of the biasing currents of the complementary input pairs constant. This results in a g_m-variation below 4% for input pairs operating in weak inversion. In a standard 0.7 μm CMOS technology the circuit can handle power supply voltages ranging from 1.5 V up to 3.3 V. For a power consumption of 300 μW and a load capacitance of 15 pF a gainbandwidth (GBW) of 4.3 MHz is achieved. This results in a GBWCL-to-supply-power ratio of 210 MHz·pF/mW. The total die area of the amplifier is 0.25 mm²