Nested Miller Compensation topologies: analysis, comparison, design and new solutions

In this chapter some of the most common frequency compensation techniques for three-stage operational transconductance amplifier, with the last stage inverting, will be considered. In particular, design equations introducing the phase margin as an explicit design variable will be carried out.
The performance achieved by each compensation technique is usually compared to the others only using experimental data, while analytical comparison is carried out solely for the counterpart of NMC. This type of evaluation may lead to imprecise results, as it strongly depends upon the technology used to implement each amplifier as well as on the supply voltage and current consumption. Moreover, from the designer’s point of view, a comparison based only on experimental results cannot indicate the real advantages of a particular compensation topology as it only allows a ranking based on the specific design conditions of different amplifiers. Hence, an analytical comparison among the most common solutions presented in literature is necessary. Indeed, only this type of evaluation can highlight the real benefits of a compensation network independently of the specific topology of the three gain stages, the transistor aspect ratio and the particular technology adopted to implement the amplifier. Moreover, an analytical comparison between different design solutions can help the designer to choose the compensation strategy which best satisfies a given set of design constraints. After deriving design equations that have the phase margin as the main design parameter, the different solutions are compared by evaluating a novel figure of merit that expresses a trade off between gainbandwidth product, load capacitance and total transconductance, for equal values of phase margin.
It is shown that there is no unique optimal solution as this depends on the load condition and the relative magnitude of the transconductance of each stage. From this point of view, the proposed comparison also provides useful design guidelines for the optimization of small-signal performance. Simulations confirming the effectiveness of the comparison are also given.