Operational Amplifier Stability and Common-Mode Noise Rejection

George Rutkowski, P.E.

Course Outline

This course includes essential information of operational amplifier (Op Amp) parameters and their stabilities. It emphasizes Op Amp specifications that are published by the manufacturers. Here’s a brief review of material covered in the previous course E272 Operational Amplifier Fundamentals and Design. This review includes Op Amps as used in the design of basic inverting and noninverting amplifiers. And extending beyond this, this course emphasizes performance of practical Op Amp’s which can behave in less than the ideal text book versions. These behaviors, inherent instabilities, can be predicted by referring to specified parameters and external conditions such as temperature and power supply variations. You will learn how the effects of these instabilities can be minimized by proper selection of circuit components.

Also, you’ll learn how Op Amps are used as differential amplifiers and how this application serves to minimize, if not practically eliminate, induced noise. Time-varying electric and magnetic fields are in every industrial, medical and commercial environment. They induce intolerable noise with amplitudes much larger than the intended signals that are to be amplified. You will learn how differential amplifiers can make this induced noise common mode which differential amplifiers can reject. Ample example problems with solutions are provided that reinforce the concepts covered.

This course covers the following topics:

• Input Offset Voltage Vio
• Input Bias Current IB
• Input Offset Current Iio
• Combined Effects of Vio and Iio
• Differential-Mode Op Amp Circuit
• Common-Mode Rejection Ratio, CMRR
• Maximum Common-Mode Input Voltages
• Op Amp Instrumentation Circuits

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the end of this course, the student will:

• Know the gain equations for Inverting, Noninverting Differential mode amplifiers;
• Understand the significance of manufacturers’ specifications (Specs) such as the Input Offset Voltage Vio, Input Bias Current IB and Input Offset Current Iio;
• Be able to predict amount and to minimize the effects of an Op Amp’s specified Vio, IB and Iio;
• Know the difference between Open-Loop voltage gain AVOL and Closed-Loop voltage gain Av;
• Become familiar with Null Adjustment, why often necessary, how implemented;
• Know that induced noise voltages are usually present where electrical lighting and switching systems are working;
• Become aware that, in many environments, voltage amplitudes of induced noises often exceed the values of signal voltages that are intended to be amplified;
• See how Op Amps are wired work as a differential Amplifier;
• Learn the meaning of the Spec term Common-Mode Rejection Ratio (CMRR);
• And learn the importance of resistor values in Differential Amplifiers;
• Be able to predict the amount of noise at the output of a Differential Amplifier given its component values and specified CMRR;
• Given an Op Amp’s specified CMRR, be able to determine its equivalent CMR(dB) and vice-versa;
• Be able to determine a Differential Amplifier’s Common-Mode Gain from its Component Values and specified CMRR; and
• Compare (calculate) effects of induced noise with Inverting, Noninverting and Differential Amplifiers to re-emphasis to which circuit configuration the Spec CMRR applies.

Intended Audience

Engineers, technologists and anyone familiar with Ohm’s and Kirchoff’s laws (basic electrical circuits) that are interested in becoming competent with this very flexible electronic component.

Benefit for Attendee

A powerful feature of Operational Amplifiers includes their differential inputs. You will learn component selection and circuit configurations that enable the Op Amp to function as a differential amplifier. Working in a differential-mode configuration, an Op Amp can select and amplify intended weak signals in an electrically noisy environment. The amplitude of the induced noise is often very much larger than the signal we want to  amplify. Most industrial, medical and commercial environments are electrically very noise; that is, time-varying induced noise from lighting, switching and other radiating equipment can interfere with intended operation electronic equipment. After completing this lesson, you will be in a strong position to solve noise related problems associated with electronic equipment.. You’ll also learn how variation in temperature and dc supply voltages can affect operation, including stability and accuracy, of Op Amp applications and how to minimize the effects of these variations.

Course Introduction

The practical Op Amp, unlike the hypothetical ideal version, has some dc output voltage we are calling output offset voltage, even though both of its inputs are grounded. Such an output offset is an error voltage and is generally undesirable. The causes and cures of output offset voltages are the subjects of this chapter. Here we will become familiar with parameters that enable us to predict the maximum output offset voltage that a given Op Amp circuit can have. On the foundation laid in this chapter, we will build an understanding of why, and an ability to predict how much, a given Op Amp's output voltage tends to drift with power supply and temperature changes, which are important subjects discussed later.

Course Content

The course content are in the following PDF files:

Chapter 5: COMMON-MODE VOLTAGES AND DIFFERENTIAL AMPLIFIERS

Appendix A

Appendix B

Appendix C

Please click on the above underlined hypertext to view, download or print the document for your study. Because of the file size, we recommend that you first save the file to your computer by right-clicking the mouse and choosing "Save Target As ...", and then open the file in Adobe Acrobat Reader from your computer.

Course Summary

An Op Amp’s output voltage Vo can vary with variations in dc supply voltages, including their ac ripple, and temperature. These variations can be minimized by including stabilizing resistors. Op Amps wired in differential-mode make induced noise voltage common-mode and thereby can be rejected. That is, intended signals VS can be amplified while the noise is attenuated.