Three improved operational amplifiers with low voltage and rail-to-rail constant are proposed. Two of the amplifiers are modified from the amplifier with a level shifting circuit. One improved amplifier has fewer devices, higher speed, and reduced area and the other improved amplifier is added an additional adjustable gain. The third amplifier is a floating voltage controlled voltage source (FVCVS) amplifier, which has reduced area and improved frequency response. The first two level shifting operational amplifiers are designed in a 0.5£gm UMC CMOS process. They use about half number of devices. The supply voltage is 1.3V, and the current consumes just only 22.6¢H of the original circuits. The unity gain frequency increases 56.8%. The slew rate, CMRR and PSRR are higher. The 2nd amplifier still has a rail-to-rail constant gm; however, the gm can be adjusted. The third amplifier uses the 0.35£gm UMC CMOS process with 1.2V operating voltage. The gain-bandwidth product is 53.8¢H larger than the original circuits. No frequency compensation is used and the devices are fewer. The results are obtained in HSPICE simulation.

This work deals with issues of design and optimize of an operational amplifiers using CMOS transistor models. The main focus of work is to propose a circuit suitable for low voltage applications with low power. The proposed circuit consists of sub-circuits, each of which must operate in the desired voltage range. Detailed design of input and output stages will reach range of rail-to-rail type with a minimum quiescent current.