Electronic oscillators are commonly found in everyday circuits, ranging from antique radios to the transmitter in your TV remote. The basic job of an electronic oscillator is to generate an oscillating output often at a constant frequency. Various outputs can be sine, square, saw tooth, triangle, or complex waveforms. This article explains basic sinusoidal LC oscillators. Electronic Communication systems could not operate without sources of sinusoidal electrical waves. Many types of oscillator circuits are used to produce these sinusoids.

Feedback Oscillators, RC and tuned LC types are applicable to frequencies from the audio range up to the VHF range, and may use any convenient three terminal amplifying devices.

POSITIVE FEEDBACK OSCILLATOR

Another type of feedback, namely positive feedback, also finds application in op-amp circuits. Unlike negative feedback, where the output voltage is "fed back" to the inverting (-) input, with positive feedback the output voltage is somehow routed back to the non-inverting (+) input. In its simplest form, we could connect a straight piece of wire from output to non-inverting input and see what happens:

The inverting input remains disconnected from the feedback loop, and is free to receive an external voltage. Let's see what happens if we ground the inverting input:

With the inverting input grounded (maintained at zero volts), the output voltage will be dictated by the magnitude and polarity of the voltage at the non-inverting input. If that voltage happens to be positive, the op-amp will drive its output positive as well, feeding that positive voltage back to the non-inverting input, which will result in full positive output saturation. On the other hand, if the voltage on the non-inverting input happens to start out negative, the op-amp's output will drive in the negative direction, feeding back to the non-inverting input and resulting in full negative saturation. 

What we have here is a circuit whose output is bistable: stable in one of two states (saturated positive or saturated negative). Once it has reached one of those saturated states, it will tend to remain in that state, unchanging. What is necessary to get it to switch states is a voltage placed upon the inverting (-) input of the same polarity, but of a slightly greater magnitude. For example, if our circuit is saturated at an output voltage of +12 volts, it will take an input voltage at the inverting input of at least +12 volts to get the output to change. When it changes, it will saturate fully negative. 

Another application of positive feedback in op-amp circuits is in the construction of oscillator circuits. An oscillator is a device that produces an alternating (AC), or at least pulsing, output voltage. Technically, it is known as a stable device: having no stable output state (no equilibrium whatsoever). Oscillators are very useful devices, and they are easily made with just an op-amp and a few external components. 

When the output is saturated positive, the Vref will be positive, and the capacitor will charge up in a positive direction. When Vramp exceeds Vref by the tiniest margin, the output will saturate negative, and the capacitor will charge in the opposite direction (polarity). Oscillation occurs because the positive feedback is instantaneous and the negative feedback is delayed (by means of an RC time constant). The frequency of this oscillator may be adjusted by varying the size of any component.

So, that's it for the oscillator. Next tutorial is about RC Phase shift oscillator.