Analysis of Stability, Noise, and Design Guidelines for a Cascaded Floating-Inverter Amplifier

This brief analyzes the stability and noise in a cascaded floating-inverter amplifier (FIA). We show that the closed-loop stability of a switched-capacitor circuit using a cascaded FIA depends mainly on the device sizes and not on the reservoir capacitors of the FIA. The closed-loop settling is similar to that when the FIA is powered by a constant voltage source. This simplifies the problem of stabilizing a cascaded FIA to stabilizing a regular cascaded amplifier, which is well-known. We also do an accurate noise analysis using results from LPTV network theory. It is known that a cascaded FIA has an increased input-referred noise when used in a closed-loop switched-capacitor circuit. We show that, in a two-stage FIA, this noise penalty factor depends on the dc gain of the first stage, which is independent of the reservoir capacitor to a first-order, and the ratio of the reservoir capacitors in the second stage and the first stage. The settling speed, on the other hand, is shown to be mainly dependent only on the last stage reservoir capacitor. Simulation results are shown to validate all the claims. Using the above results, guidelines to design a cascaded FIA are provided.