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

The operational transconductance amplifier (OTA) is a core building block in most analog and mixed-signal electronic systems. Due to the increasingly demand of high-gain, large-bandwidth and wide-swing amplifiers, capable of driving large capacitive loads under low-voltage and low-power conditions, in recent years many researchers have proposed several compensation topologies for three-stage amplifiers. Indeed, a three-stage amplifier represents the only viable option the achieve DC gains in excess of 100dB in a low-voltage low-power environment, especially when deep sub-micron CMOS technologies are adopted.
Unfortunately, an effective design procedure based on closed-form equations involving the principal performance parameters is still missing. Besides, the compensation network design is usually carried out by neglecting the effect of zeroes and assuming a Butterworth unity-feedback frequency response. As a result, the designer cannot set the compensation network for the desired phase margin, as usually done in two-stage amplifiers.
Despite the transfer function complexity, which often made it difficult to derive stability conditions of reasonable practicality, this work provides a systematic design procedure exploiting the phase margin as main design parameter.
The present dissertation provides an overview and study on frequency compensation techniques for three-stage amplifiers and develops several novel techniques. The work is divided into two main parts.
In the first part the existing solutions are reviewed and discussed by developing a design-oriented methodology which exploits the phase margin as main design parameter. Besides discussing the mathematical basis of all the considered compensation methods, the dissertation supplies a strong theoretical analysis along with many transistor-level simulation examples to support the adopted compensation criteria. Furthermore, an analytical performance comparison was developed allowing a better understanding of the real benefits of a specific compensation strategy, irrespectively of the particular technology and topology used to implement the amplifier.
The second part of the dissertation proposes several original compensation topologies for three-stage amplifiers which are the direct result of a systematic research study. The differences as well as the advantages and drawbacks of each topology have been carefully investigated. Many of these solutions were exploited to fabricate several OTAs which were experimentally tested. A performance comparison with previously reported solutions showed a significant improvement of the proposed compensation topologies in terms of small-signal and large-signal performances.