Low-voltage Low-power Operational Amplifier Design

Today's society portable electronic products has become a mainstream for consumer, in order to prolong battery life, IC products is being developed toward the low-voltage low-power direction. At the same time, in order to boost integration and reduce costs, transistor sizes continue to scale down, which make the supply voltage become more and more low as the threshold voltage of the transistors does not vary, as a result, which poses great challenges to the design of analog circuits in analog and digital mixed-signal system.The operational amplifier, which acts as a fundamental block in most analog system, is required to achieve hign gain and large bandwidth simultaneously in low voltage condition. To achieve hign gain, a conventional cascode structure is not suitable in low-voltage design as that results in small voltage swing, only by increasing the number of cascaded gain sta-ges to achieve the purpose of high gain. However, multistage amplifers suffer from the colsed loop stability problem due to the presence of multiple poles. Therefore, based on Miller compensation method, a active feedback frequency compensation technique is pres-ented in this paper, which not only ensures loop stability, but also creats a left half plane zero, increases phase margin, reduces the size of compensation capacitors and achieves the purpose of the broadband width, and boosts slew rate. In addition, feedforward gain stage is proposed in this paper, which effective controls Q value of the non-dominant pole, so at high frequency negative feedback compensation loop is ensured. At the same time class AB output stage is designed using feedforward transconductance and transconductance of the output stage, to improve the transmission efficiency. In order to improve signal to noise ratio in the low-voltage environment, the rail-to-rail input stage with constant transcon-ductance and output current is designed in this paper, which ensures that gain, bandwidth and slew rate is constant at the whole common mode input range, and reduces difficulty of the compensation. The compensation technique of the paper creats a left plane zero when compared with the internal Miller compensation, which preserves the stablity of the amplifier when output transconductance is in the same order of magnitude as transcon-ductance of the input stage as the former is much larger than the latter using NMC, therefore using the way can realize the objective of low power consumption.Based on csmc0.5um CMOS process, the detailed circuit simulation is carried out for the designed operational amplifier by using spectre simulation tool. As can be seen from the result, power consumption is 1.28 mW in 2.5V supply voltage, DC gain is 107dB, unity gain bandwidth is more than 4M, and the phase margin is 68°, input and output achieve full swing, and achieve the desired goals.