Feedback Circuit Techniques

All electronic systems employ feedback. All analog circuits, including op amps, oscillators, filters, and power supplies (just to name a few), critically depend on feedback. Some of these feedback loops are explicit, some are implicit, some are intentional, and some are unintentional. In all cases, the analysis and design of circuits using feedback requires the knowledge of analytical techniques, creative design, and an appreciation for possible trade-offs. Understanding feedback theory, and its use in practical applications, is the key to successful system design.

1. Introduction to Feedback Circuit Techniques
   • Some op-amp history
   • Simple op-amp circuits
   • Feedback systems
   • Block diagrams
2. Desensitivity and Return Ratio
   • Benefits of feedback
   • Return ratio
   • Blackman
   • Middlebrook
3. Linear System Behavior
   • Transfer functions
   • The Laplace transform
   • Time and frequency response
   • Elmore delay
4. Feedback Analysis Tools
   • Closed-loop poles and zeros
   • Phase margin
   • Nyquist criterion
   • Nichols plots
   • Root-locus plots
   • Crossover frequency and phase margin
5. Op-Amp Transfer Function
   • General-purpose transfer function
   • Integrators and differentiators
   • Decompensated op amps (OP27 vs. OP37)
   • Good generality, bad optimality
6. Compensation and Design
   • Phase and gain margin
   • Gain setting
   • Dominant pole
   • Lag and lead
7. Internal Op-Amp Compensation
   • Op-amp transfer function
   • Pole splitting
   • Minor-loop feedback
   • Reducing steady-state errors
8. Driving Capacitive Loads
   • Gain reduction and overcompensation
   • Out-of-the-loop and in-the-loop resistance
   • Output snubber circuit
   • Minor-loop compensation
9. Current Feedback Amplifiers
   • Voltage op amps
   • Current-feedback amps
   • Implementation
10. Oscillators
    • Amplitude control
    • Shape
    • Limit
    • Feedback

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Lecturer

Kent H. Lundberg is an educator, consultant, and historian. He is president of Keeling Flight Hardware, Ltd., which provides design, research, and educational consulting services in the fields of aerospace, electronics, and control systems for companies, universities, and government organizations.

Since 2008, Dr. Lundberg has been a Visiting Professor at Olin College of Engineering, where he teaches courses in controls, circuit design, and instrumentation. From 2002 to 2005 and in 2011, he was a Lecturer with the Department of Electrical Engineering and Computer Science at the Massachusetts Institute of Technology. His research and teaching interests include the application of classical control theory to problems in analog circuit design, and the development of educational toys (lecture demos, take-home laboratory kits, and tutorial computer applications) for feedback systems and control engineering.

Dr. Lundberg was the Associate Editor for History of IEEE Control Systems Magazine from 2004 to 2011. He attended M.I.T. earning a Bachelor's degree in physics in 1992, and a Ph.D. in electrical engineering in 2002. He owns 43 Tektronix oscilloscopes, and he obsessive-compulsively collects analog synthesizers, technology artifacts, and classic textbooks on radar, nuclear energy, analog computing, and control.