Oscillators are circuits that produce specific, periodic waveforms such as square, triangular, saw tooth, and sinusoidal. They generally use some form of active device, lamp, or crystal, surrounded by passive devices such as resistors, capacitors, and inductors, to generate the output. There are two main classes of oscillator: relaxation and sinusoidal. Relaxation oscillators generate the triangular, saw tooth and other non sinusoidal waveforms and are not discussed in this note. Sinusoidal oscillators consist of amplifiers with external components used to generate oscillation, or crystals that internally generate the oscillation. The focus here is on sine wave oscillators, created using operational amplifiers op amps.
Op-amp sine-wave oscillators operate without an externally-applied input signal. Instead, some combination of positive and negative feedback is used to drive the op amp into an unstable state, causing the output to cycle back and forth between the supply rails at a continuous rate. The frequency and amplitude of oscillation are set by the arrangement of passive and active components around a central op amp.
Oscillators are created using various combinations of positive and negative feedback. The basic negative feedback amplifier block diagram with a positive feedback loop added is one of the type of oscillator. When positive and negative feedback are used, the gain of the negative feedback path is combined into a single gain term (representing closed-loop gain). You can look at the figure below. The figure is show Positive and Negative Feedback Loops.
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