13.1: INTRODUCTION

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James K. Roberge
Massachusetts Institute of Technology via MIT OpenCourseWare

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Proper compensation is essential for achieving optimum performance from virtually any sophisticated feedback system. Objectives extend far beyond simply guaranteeing acceptable stability. If stability is our only concern, the relatively unimaginative approaches of lowering loop-trans mission magnitude or creating a sufficiently low-frequency dominant pole usually suffice for systems that do not have right-half-plane poles in their loop transmissions. More creative compensation is required when high de- sensitivity over an extended bandwidth, wideband frequency response, ideal closed-loop transfer functions with high-pass characteristics, or operation with uncertain loop parameters is essential. The type of compensation used can also influence quantities such as noise, drift, and the class of signals for which the system remains linear.

A detailed general discussion has already been presented in Chapter 5. In this chapter we become more specific and look at the techniques that are most appropriate in the usual operational-amplifier connections. It is assumed that the precautions suggested in Section 11.3.2 have been observed so that parasitic effects resulting from causes such as inadequate power-supply decoupling or feedback-network loading at the input of the amplifier do not degrade performance.

It is cautioned at the outset that there is no guarantee that particular specifications can be met, even with the best possible compensation. For example, earlier developments have shown how characteristics such as the phase shift from a pure time delay or a large number of high-frequency poles set a very real limit to the maximum crossover frequency of an amplifier-feedback network combination. Somewhat more disturbing is the reality that there is usually no way of telling when the best compensation for a particular application has been realized, so there is no clear indication when the trial-and-error process normally used to determine compensation should be terminated.

The attempt in this chapter is to introduce the types of compensation that are most likely to succeed in a variety of applications, as well as to indicate some of the hazards associated with various compensating techniques. The suggested techniques for minor-loop compensation are illustrated with experimental results.