| Operational amplifier is the kernel module of integrated circuits, which is applied to digital analog converter(DAC),analog digital converter(ADC) and Power Management IC. The performance of operational amplifier directly affects the performance of the whole circuit. Therefore, the design of operational amplifier becomes the hot spot in the integrated circuit field.With the development of integrated circuit technology, circuit integration is getting higher and higher, the size of transistor becoming smaller and smaller, and VLSI technology enters ultra-deep sub-micron regime. Effect of short channel becomes prominent, which makes the open-loop gain of opamp reduced. The traditional two-stage amplifier cannot meet the requirements. In order to increase the open-loop gain of opamp, the multistage amplifier is necessary. However, the extra zero and pole are introduced by cascade of operational, which affects the stability of the amplifier. Therefore, it need to adopt frequency compensation technology to guarantee the closed-loop stability of the amplifiers.This paper expounds the frequency compensation theory of amplifier and introduces several kinds of structure of multistage amplifier frequency compensate, according to the traditional two-stage amplifier. A three stage operational amplifier is designed in this paper, which has adequate gain in deep sub-micro level. This amplifier is designed and simulated based in SMIC0.18μm CMOS technology which provides enough dc gain, wide input and output swings with suitable frequency compensation structure. With supply voltage3.3V and capacitive load5pF, the amplifier shows155dB open-loop dc gain,32MHz unity gain-bandwidth and56.09°phase margin. The simulation results indicate its ideal frequency response as well as transient response. It needs smaller compensate capacitor with the benefit of decreased die area, which is easily implemented in current CMOS technology. Gain and phase margin both meet the design requirements... |
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