Frequency compensation techniques using current buffers

Abstract

When attempting to stabilize a multi-stage amplifier, the introduction of a Miller capacitor may create a feed-forward path and Right-Half-Plane (RHP) zero, jeopardizing the stability of the control loop. In addition to a nulling resistor, a voltage buffer or a current buffer can be placed in series with the Miller capacitor to obviate the feed-forward path and introduce a Left-Half-Plane (LHP) zero. A LHP zero, placed in its proper location, can cancel the effect of a pole and/or an RHP zero, boosting the phase margin, and improving large-signal stability. This tutorial starts with the basics of frequency compensation. We explain some of the complex details in analyzing a wide range of compensation networks for multi-stage amplifiers. We introduce frequency compensation techniques using current buffers. In addition to the popular common-gate (or cascode) transistor topology used as a positive current buffer, we detail the recent technique of using current mirror as an inverting current buffer. Simple design equations and design examples of frequency compensation using current buffers are detailed. The tutorial concludes by introducing an effective method of using a series resistor for accurate placement of current buffer LHP zeros.