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Oscillators are signal sources. Many times, it is necessary to generate waveforms with a known wave shape, frequency, and amplitude. A laboratory signal generator perhaps first comes to mind, but there are many other applications. Signal sources are needed to create and receive radio and television signals, to time events, and to create electronic music, among other uses. Oscillators may produce very low frequencies (a fraction of a cycle per second) to very high frequencies (microwaves, > 1 GHz). Oscillators employing op amps are generally used in the area below 1 MHz. Specialized linear circuits may be used at much higher frequencies. The output wave shape may be sinusoidal, triangular, pulse, or some other shape. Oscillators can generally be broken into two broad categories: fixed frequency or variable. For many fixed frequency oscillators, absolute accuracy and freedom from drift are of prime importance. For variable oscillators, ease of tuning and repeatability are usually important. Also, a variable frequency oscillator might not be directly controlled by human hands, rather, the oscillator may be tuned by another circuit. A VCO, or voltage-controlled oscillator, is one example of this. Depending on the application, other factors such as total harmonic distortion or rise time may be important.
No matter what the application or how the oscillator design is realized, the oscillator circuit will normally employ positive feedback. Unlike negative feedback, positive feedback is regenerative - it reinforces change. Generally speaking, without some form of positive feedback, oscillators could not be built. In this chapter we are going to look at positive feedback and the requirements for oscillation. A variety of small oscillators based on op amps will be examined. Finally, more powerful integrated circuits will be discussed.