What is the difference between amplifier and oscillator?

Answer

The (very) basic answer is that an oscillator is an amplifier with a positive feedback path engineered into. This sets up a situation where the feedback returns a bit of the output signal to the input of the stage to "keep it going" and to permit it to sustain the "continuous" output of a (frequency controlled) signal.

All amplifiers can be made to oscillate, and all oscillators amplify. The difference is in how we set them up, which we will do in accordance with what we want out of them.

Answer

To understand the differences it is helpful to look at the similarities. They both need some form of amplifier to work.

The "goodness" of an amplifier is specified by the amount of gain (among other things) it possesses ie by how much it amplifies which is measured by seeing how much bigger the output is compared with the input.

All circuits contain feedback paths where part of the output signal finds its way back to the input - some intentional (as in an oscillator) and some unwanted (as in an amplifier).

If we start with an ordinary amplifier circuit and gradually increase the gain of the amplifying bit we will eventually get to the point where the specific combination of that amount of gain and the characteristics of the feedback path result in enough energy travelling through the feedback path to cause the amplifier to become unstable. In other words it oscillates! The frequency of oscillation is largely determined by the characteristics of the feedback path and when the feedback signal is big enough and is in phase with the original input signal it oscillates.

This answer is grossly over-simplified. If you really want to know whats going on, get stuck into the maths!

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