CU-Boulder PHYS 3330 - Positive Feedback and Oscillators (11 pages)

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Positive Feedback and Oscillators



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Positive Feedback and Oscillators

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Pages:
11
School:
University of Colorado at Boulder
Course:
Phys 3330 - Electronics for the Physical Sciences

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Physics 3330 Experiment 5 Fall 2006 Positive Feedback and Oscillators Purpose In this experiment we will study how spontaneous oscillations may be caused by positive feedback You will construct an active LC filter add positive feedback to make it oscillate and then remove the negative feedback to make a Schmitt trigger Introduction By far the most common problem with op amp circuits and amplifiers in general is unwanted spontaneous oscillations caused by positive feedback Just as negative feedback reduces the gain of an amplifier positive feedback can increase the gain even to the point where the amplifier may produce an output with no input Unwanted positive feedback is usually due to stray capacitive or inductive couplings couplings through power supply lines or poor feedback loop design An understanding of the causes of spontaneous oscillation is essential for debugging circuits On the other hand positive feedback has its uses Essentially all signal sources contain oscillators that use positive feedback Examples include the quartz crystal oscillators used in computers wrist watches and electronic keyboards traditional LC oscillator circuits like the Colpitts oscillator and the Wien bridge and lasers Positive feedback is also useful in trigger and logic circuits that must determine when a signal has crossed a threshold even in the presence of noise In this experiment we will try to understand quantitatively how positive feedback can cause oscillations in an active LC filter and how much feedback is necessary before spontaneous oscillations occur We will also construct a Schmitt trigger to see how positive feedback can be used to detect thresholds Readings The readings in H H and Bugg are optional for this experiment 1 Horowitz and Hill Section 5 12 to 5 19 If you are designing a circuit and want to include an oscillator look here for advice Amplifier stability is discussed in Sections 4 33 4 34 2 Bugg discusses the theory of spontaneous oscillations in Chapter 19



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