Eliminating Oscillation in Power Analog Designs

When integrating analog drivers, designers are often faced with unwanted oscillation caused by instability. Download Eliminating Oscillation in Power Analog Designs to learn about instability in capacitive loads, closed loop gain (1/ß) and open loop gain (AOL) and their relationships to stability, a capacitor’s impact the open loop gain (AOL) curve, and three ways to adjust the closed loop gain and the open loop gain responses to regain phase margin and stability.

Introduction

It is unlikely that a power op amp would always be employed to drive a purely resistive load. For if that were the case, stability would never become a problem. On the other hand, a load which is largely capacitive does present stability issues as will become clear in this white paper. In a pure capacitor, the voltage lags the charging current by 90 degrees, so there is a large phase delay in the loop response of a feedback circuit. Such a phase delay can be a significant contributor to instability. Let’s get started by considering any multistage amplifier. It will begin to exhibit a roll off in open loop gain (AOL), as well as an increase in phase delay at higher frequencies because of the low-pass filters that are formed by nodes with finite source impedances driving capacitive loads within the amplifier stage1. So a load that is largely capacitive, together with the capacitance within a multistage amplifier, can bring about instability since the phase lag may be approaching, and perhaps exceed 180 degrees. It is this phase delay that is a key contributor to instability. When instability occurs it can inadvertently transform a power amplifier into an oscillator, and the device is likely to become quite hot and fail in as little as one second. In the discussion that follows, there are some simple techniques for managing phase and gain relationships to maintain a stable circuit.