8. Low-Voltage, Low-Power Operational Amplifiers

Integrated Circuit CAD has been driven by CMOS digital designers. Although electronic applications become supposedly more and more digital, there are applications that will require some sort of analog processing. In particular, A/D and D/A converters will always be required at the interface between the Digital Signal Processor (DSP) core and the off-chip peripherals [92]. Other examples where, interestingly, analog techniques are getting ahead, are those when power consumption is the main design concern, and linearity and Signal to Noise Ration (SNR) are in the range of less than 40dB, as in the case of disk-drive chips [93] and analog-assisted microprocessor chips [94].

Nonetheless, digital designed oriented CAD tools are becoming non usable for analog circuits that are forced to follow the power supply voltage reduction operated in the digital side. Indeed, the popular criterium for judging device models, such as the mean-square error for currents that are reasonable for digital modeling, fails the prediction of the analog behavior of circuits [95], as the very low supply voltages bring MOSFETs out of the traditional strong inversion regime.

In this chapter we will apply the methods described in Chapter 5 and Chapter 6 to analog circuits and demonstrate their usefulness in analog modeling. This is accomplished by designing a novel ultra-low-power operational amplifier, the fundamental analog building block, which is capable of 0.5V supply voltage operation.