Compact 1.8 V low-power CMOS operational amplifier cells for VLSI

The continuing down-scaling of CMOS processes has two consequences for the design of mixed-mode VLSI circuits. First, the supply voltage will go down from the present 5 V to 3 V and subsequently to 2 V, because the smaller dimensions result in a reduction of the breakdown voltage. Second, the circuit density possible increases. To meet the VLSI demands, compact topologies are needed that can operate on supply voltages between 1 and 2 V, depending on threshold voltages of the transistors. Existing CMOS opamps are compact but cannot operate at supply voltages below 2.5 V or otherwise have a complicated structure. The authors present a compact opamp which operates on supply voltage down to one gate-source voltage and two saturation voltages which amounts to 1.8 V. The opamp consists of two stages, an input stage and, a rail-to-rail output stage. An intermediate part contains current sources and a current mirror that sums the differential signals from the input stage and, cascodes that provide gain.