Extremely low voltage operational amplifier design with rail-to-rail input common mode range

Abstract

Increasing trends towards battery operated systems demand circuits to be designed at low voltages. Low voltage operation severely limits the operational amplifier as a voltage buffer as the input common mode range available is very limited. This work deals with designing a very low voltage amplifier that can be used as a unity gain buffer. The architecture is based on using an operational amplifier in conjunction with an adapter circuit. The compliance voltage of the tail current source is maintained constant by comparing with a reference voltage using negative feedback action. The amplifier has been designed in 0.18µm technology at a supply voltage of 0.8 Volts. The amplifier gives a constant performance for varying common mode voltage as is demanded for a rail-to-rail amplifier. Designed amplifier gives a gain of 77.4dB with an input stage transconductance ‘Gm’ variation of just 1.29 % over the entire input common mode range.