Introduction

Selecting the correct op amp for an application requires investigation of many different parameters. Voltage offset, bias currents and similar parameters are easy to evaluate because they are dc parameters that do not vary with frequency. Accuracy, on the other hand, is hard to specify -- and comply with -- because it is a function of frequency; hence, accuracy specifications involve the knowledge of frequency-dependent feedback circuits that are bandwidth-dependent.

The bandwidth (BW) problem is complicated by the op amp's feedback because it hides decreasing BW until accuracy problems become apparent. If op amps had a constant open-loop gain the accuracy of an op amp circuit would remain constant. The open-loop gain of any op amp decreases with increasing frequency. Except for a phenomenon called "peaking" all op amps lose accuracy at high frequencies. The designer's problem is in selecting an op amp that has an acceptable accuracy loss at the frequencies of interest. Proper analysis of this problem requires an understanding of feedback, loop gain, and frequency dependence.

Preserving the signal integrity or accuracy during amplification is an essential part of the design but in order to preserve the signal one must first define it. That sounds like a simple task, but it is complicated and must be performed in several different ways. Various methods used for defining the frequency content of the signal are examined in detail in this article because no single method works for every case.

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