Stability of Power Supply Bootstrapped Unity-Gain Buffers

Power-supply bootstrapping is a technique based on applying a DC-shifted copy of the input signal to the supply terminals of an operational amplifier (OA). It is usually applied configuring the OA as a unity-gain buffer, and achieves an increased input impedance or an extended voltage range for a device. However, bootstrapped circuits can become unstable for reasons that are not evident when considering the most simple OA models. In this brief, we show the mechanism producing instabilities in these circuits and the necessary equations for stability analysis are determined. Stability can be studied by considering the power-supply and common-mode rejection ratio curves thanks to an intrinsic relationship between them. A realistic OA model from the literature was chosen as validation for the presented method and as a study-case to show the rationale for the stability analysis and possible compensation strategies. The proposed method was successfully applied to compensate bootstrapped buffers in experimental tests.