You can find a very easy introduction to Op Amps in Lessons in Electric Circuits (allaboutcircuits) by Kuphaldt . Have a look at the chapter Operational Amplifiers. If you think you have understood it, test your knowledge in the Socratic Electronics project, look at the Analog Integrated Circuits chapter. You should also try to build the Op Amp circuits in the Experiments volume. One thing you will not find in Lessons In Electric Circuits is a stability analysis of Op Amp circuits. It would probably be outside the scope of the book, but it is an important concept. Anyway, you can find a very good treatment of this topic in the following book by Mancini.

A very popular book is Op Amps For Everyone by Mancini that you can download for free from Texas Instruments. It is more demanding and longer than the Op Amp chapter in Lessons In Electric Circuits but it will also take you much further. If you like it, you might also want to take a look at the 3rd edition of Op Amps for Everyone - you have to pay for it, but it contains some additional chapters.

My favorite treatment of Op Amps is contained in The Art of Electronics by Horowitz and Hill (Note: the following refers to the 2nd edition. For the 3rd edition see the next paragraph). Chapter 4 (Feedback and Operational Amplifiers), is really great if you want to learn how to design op amp circuits (not so great if you need to prepare for an exam and you need to focus on the theory and mathematics). Make sure you understand section 4.03 The golden rules. They are the key for understanding the (ideal) Op Amp. If you worked through chapter 4 and think you understood it, make sure to have a look at the Bad Circuits (p. 258) and try to find the errors in them. If you are in a hurry, working through chapter 4 may be enough. But it would be better to also study chapter 7 (Precision Circuits and Low Noise Techniques). It will teach you how to select the right Op Amp if you want to build circuits of high precision. If you are interested in Low Power Design you should also have a look at sections 14.12 and 14.13. You can also find some interesting applications of Op Amps by browsing through chapters 5, 6, 9 and 15. A warning: Please keep in mind that the book was written in 1989. So if you search through the large tables that list the specifications of all kinds of Op Amps, and find an Op Amp that you like, it might not be available anymore. But the foundations and design techniques that the book teach you are as valuable now as they were 20 years ago. As a final remark, in section 4.02 Horowitz and Hill recommend the LF411 as a good all-around performer (and they will probably do so again in the coming 3rd edition, at least Hill said so in a post in 2007 Update: Yes indeed, please see p.225 and p.243 of the 3rd edition). I think this is a much better choice than the antique 741 that so many other authors adopt as their standard Op Amp, and I would advise you to follow their recommendation.

In the 3rd edition of The Art of Electronics the golden rules (now section 4.1.3, p.225) are still the foundation for everything Op Amp related (with the ideal Op Amp in mind). The next pages show you how the golden rules help you to build basic Op Amp circuits. For a deeper understanding, see section 4.4 A detailed look at Op Amp behaviour (talking about the parameters of real Op Amps) and 4.5 A detailed look at selected Op Amp circuits. Also take a very close look at 4.9 Feedback amplifier frequency compensation. If you want to build precision circuits, take a look at 5.7 Amplifier Input errors and 5.8 Amplifier output errors. And then at 5.10 Choosing a precision op-amp (including the glorious table with 67 such op amps, p.320 and 321). Only if you are truly fearless should you take a look at 8.9 Noise in operational amplifier circuits. And of course there are many, many more places in the book where Op Amps are used in various circuits, with very detailed descriptions.

You can find a short review of the DC parameters of an Op Amp in AN722 Operational Amplifier Topologies and DC Specifications, and of the AC parameters in AN723 Operational Amplifier AC Specifications and Applications (both by Bonnie Baker).

If you already have a solid knowledge of the basics of Op Amps and you want to expand your knowledge, you should have a look at the free Op Amp Applications by Walt Jung. You can also can get it as a book: Op Amp Applications Handbook.

A very nice textbook is Design with Operational Amplifiers and Analog Integrated Circuits by Franco if you already know a bit about electronics and Op Amps and are not afraid of a little math (for a refresher, you might find some of the material in Math for Physics and Engineering interesting).

After you have understood how Operational Amplifiers work, you may be interested in some Op Amp Applications. And if you want to design circuits of high quality, you should also pay close attention to the passive components Resistor and Capacitor: Even if you carefully select your op amp for minimum noise, you can still ruin the overall performance of the circuit with a noisy resistor...

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