Overview

The purpose of this chapter is to consider the consequences involved when an operational amplifier ^[1] has a finite, frequency-dependent gain function. Introductory circuit courses do not discuss this effect of the opamp model. As long as the designer is not too concerned with frequency response behavior, a simple model is adequate to accomplish many useful instrumentation (low-frequency or dc) applications. However, when the goal is to design frequency-dependent network functions for filters, the opamp nonideal frequency response becomes an important issue to be aware of and to understand.

Many parameters are actually used to describe and specify the performance of a practical opamp (e.g., common mode rejection, input offset currents). The discussion presented here will not go into the different parameters, even though they are all important considerations for the proper utilization of such devices. The student is referred to other books (or courses) for information on the parameters not treated here. References [1 8] are listed at the end of the chapter. Reference [6] defines parameters used in the industry to specify opamp performance, and test circuits are provided in an appendix. James K. Roberge discusses the meaning and describes measurement techniques for important parameters in Chapter 11 of [4]. Review of these references will provide perspective on how practical opamps arc modeled by designers and manufacturers.

This chapter looks at the effects of finite frequency-dependent gain for an otherwise ideal circuit model. Some examples are analyzed to show why the high-gain wide-bandwidth criteria evolved. These examples...