Op Amps for Everyone Design Guide (Rev. B)

1 Design Reference August 2002 Advanced Analog Products SLOD006B. IMPORTANT NOTICE. Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are sold subject to TI's terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI's standard warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty.

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4 TI is not responsible or liable for any such statements. Mailing Address: Texas Instruments Post Office Box 655303. Dallas, Texas 75265. Copyright 2002, Texas Instruments Incorporated Forward Everyone interested in analog electronics should find some value in this book, and an ef- fort has been made to make the material understandable to the relative novice while not too boring for the practicing engineer. Special effort has been taken to ensure that each chapter can stand alone for the reader with the proper background. Of course, this causes redundancy that some people might find boring, but it's worth the price to enable the satis- faction of a diversified audience. Start at Chapter 1 if you are a novice, and read through until completion of Chapter 9. After Chapter 9 is completed, the reader can jump to any chapter and be confident that they are prepared for the material.

5 More experienced people such as electronic technicians, digital engineers, and non-electronic engineers can start at Chapter 3 and read through Chapter 9. Senior electronic technicians, electronic engineers, and fledgling analog engi- neers can start anywhere they feel comfortable and read through Chapter 9. Experienced analog engineers should jump to the subject that interests them. Analog gurus should send their additions, corrections, and complaints to me, and if they see something that looks familiar, they should feel complimented that others appreciate their contributions. Chapter 1 is a history and story chapter. It is not required reading for anyone, but it defines the op amp's place in the world of analog electronics. Chapter 2 reviews some basic phys- ics and develops the fundamental circuit equations that are used throughout the book.

6 Similar equations have been developed in other books, but the presentation here empha- sizes material required for speedy op amp Design . The ideal op amp equations are devel- oped in Chapter 3, and this chapter enables the reader to rapidly compute op amp transfer equations including ac response. The emphasis on single power supply systems forces the designer to bias circuits when the inputs are referenced to ground, and Chapter 4. gives a detailed procedure that quickly yields a working solution every time. Op amps can't exist without feedback, and feedback has inherent stability problems, so feedback and stability are covered in Chapter 5. Chapters 6 and 7 develop the voltage feedback op amp equations , and they teach the concept of relative stability and com- pensation of potentially unstable op amps.

7 Chapter 8 develops the current feedback op amp equations and discusses current feedback stability. Chapter 9 compares current feedback and voltage feedback op amps. The meat of this book is Chapters 12, 13, and 14 where the reader is shown how Design the converter to transducer/actuator interface with the aid of op amps. The remaining chapters give support material for Chapters 12, 13, and 14. Chapter 18. was a late addition. Portable applications are expanding rapidly and they emphasize the need for low-voltage/low-power Design techniques. Chapter 18 defines some parameters in a new way so they lend themselves to low voltage Design , and it takes the reader through several low voltage designs. i Thanks to editor James Karki for his contribution. We never gave him enough time to do detailed editing, so if you find errors or typos, direct them to my attention.

8 Thanks to Ted Thomas, a marketing manager with courage enough to support a book, and big thanks for Alun Roberts who paid for this effort. Thomas Kugelstadt, applications manager, thanks for your support and help. Also many thanks to the contributing authors, James Karki, Richard Palmer, Thomas Ku- gelstadt, Perry Miller, Bruce Carter, and Richard Cesari who gave generously of their time. Regards, Ron Mancini Chief Editor ii Contents Contents 1 The Op Amp's Place In The World .. 1-1. 2 Review of Circuit Theory .. 2-1. Introduction .. 2-1. Laws of Physics .. 2-1. Voltage Divider Rule .. 2-3. Current Divider Rule .. 2-4. Thevenin's Theorem .. 2-5. Superposition .. 2-8. Calculation of a Saturated Transistor Circuit .. 2-9. Transistor Amplifier .. 2-10. 3 Development of the Ideal Op Amp equations .

9 3-1. Ideal Op Amp Assumptions .. 3-1. The Noninverting Op Amp .. 3-3. The Inverting Op Amp .. 3-4. The Adder .. 3-5. The Differential Amplifier .. 3-6. Complex Feedback Networks .. 3-7. Video Amplifiers .. 3-9. Capacitors .. 3-9. Summary .. 3-11. 4 Single Supply Op Amp Design Techniques .. 4-1. Single Supply versus Dual Supply .. 4-1. Circuit Analysis .. 4-3. Simultaneous equations .. 4-8. Case 1: VOUT = +mVIN+b .. 4-9. Case 2: VOUT = +mVIN b .. 4-13. Case 3: VOUT = mVIN + b .. 4-16. Case 4: VOUT = mVIN b .. 4-19. Summary .. 4-22. 5 Feedback and Stability Theory .. 5-1. Why Study Feedback Theory? .. 5-1. Block Diagram Math and Manipulations .. 5-1. Feedback Equation and Stability .. 5-6. iii Contents Bode Analysis of Feedback Circuits .. 5-7. Loop Gain Plots are the Key to Understanding Stability.

10 5-12. The Second Order Equation and Ringing/Overshoot Predictions .. 5-15. References .. 5-16. 6 Development of the Non Ideal Op Amp equations .. 6-1. Introduction .. 6-1. Review of the Canonical equations .. 6-2. Noninverting Op Amps .. 6-5. Inverting Op Amps .. 6-6. Differential Op Amps .. 6-8. 7 Voltage-Feedback Op Amp Compensation .. 7-1. Introduction .. 7-1. Internal Compensation .. 7-2. External Compensation, Stability, and Performance .. 7-8. Dominant-Pole Compensation .. 7-9. Gain Compensation .. 7-12. Lead Compensation .. 7-13. Compensated Attenuator Applied to Op Amp .. 7-16. Lead-Lag Compensation .. 7-18. Comparison of Compensation Schemes .. 7-20. Conclusions .. 7-21. 8 Current-Feedback Op Amp Analysis .. 8-1. Introduction .. 8-1. CFA Model .. 8-1. Development of the Stability Equation.