This preview shows pages 1–2. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: TL/H/6822 AnApplicationsGuideforOpAmpsA N-20 National Semiconductor Application Note 20 February 1969 An Applications Guide for Op Amps INTRODUCTION The general utility of the operational amplifier is derived from the fact that it is intended for use in a feedback loop whose feedback properties determine the feed-forward characteristics of the amplifier and loop combination. To suit it for this usage, the ideal operational amplifier would have infinite input impedance, zero output impedance, infinite gain and an open-loop 3 dB point at infinite frequency rolling off at 6 dB per octave. Unfortunately, the unit costin quan- titywould also be infinite. Intensive development of the operational amplifier, particu- larly in integrated form, has yielded circuits which are quite good engineering approximations of the ideal for finite cost. Quantity prices for the best contemporary integrated amplifi- ers are low compared with transistor prices of five years ago. The low cost and high quality of these amplifiers allows the implementation of equipment and systems functions im- practical with discrete components. An example is the low frequency function generator which may use 15 to 20 opera- tional amplifiers in generation, wave shaping, triggering and phase-locking. The availability of the low-cost integrated amplifier makes it mandatory that systems and equipments engineers be fa- miliar with operational amplifier applications. This paper will present amplifier usages ranging from the simple unity-gain buffer to relatively complex generator and wave shaping cir- cuits. The general theory of operational amplifiers is not within the scope of this paper and many excellent refer- ences are available in the literature. 1,2,3,4 The approach will be shaded toward the practical, amplifier parameters will be discussed as they affect circuit performance, and applica- tion restrictions will be outlined. The applications discussed will be arranged in order of in- creasing complexity in five categories: simple amplifiers, op- erational circuits, transducer amplifiers, wave shapers and generators, and power supplies. The integrated amplifiers shown in the figures are for the most part internally compen- sated so frequency stabilization components are not shown; however, other amplifiers may be used to achieve greater operating speed in many circuits as will be shown in the text. Amplifier parameter definitions are contained in Appendix I. THE INVERTING AMPLIFIER The basic operational amplifier circuit is shown in Figure 1 . This circuit gives closed-loop gain of R2/R1 when this ratio is small compared with the amplifier open-loop gain and, as the name implies, is an inverting circuit. The input imped- ance is equal to R1. The closed-loop bandwidth is equal to the unity-gain frequency divided by one plus the closed-loop gain....
View Full Document