Frequently Asked Questions about Op-Amps

Q. What is the golden rule of op-amps?
A. The two inputs always at the same voltage.

Q. What good is the golden rule?
A. It is a good starting place to find the gain of a amplifier configuration. Examples.

Q. Is the golden rule always true?
A. NO! Its only true with special conditions that we must work hard to maintain. With these conditions the two inputs are within several tens of microvolts of each other. The inputs are not equal under these conditions.

Q. How can the op-amp make its inputs equal, if it can only change its output?
A. There must be an external circuit connection from the output to the input. This is called feedback. Generally if the connection is to the negative input, the result is negative feedback and positive feedback if connected to the positive input.

Q. Does feedback change the way the amplifier or system works?
A. Yes. Feedback changes almost every aspect of amplifier or system. The changes can be very large and are usually improvements.

Q. What are some other examples of feedback?
A. Feedback occurs in many parts of daily life. Examples.

Q. Why do op-amps use negative feedback?
A. Negative feedback makes a system more stable. Examples.

Q. What is positive feedback?
A. Positive feedback drives a system toward extremes. It is useful in some circuits. Examples.

Q. Since a negative feedback system is stable, is there a connection to 'poles in the left half plane'?
A. Yes, these are two way of looking at the same thing. The poles of a positive feedback system are in the right half plane.

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