Please explain me transistor as an oscillator

Please explain me transistor as an oscillator.

In an amplifier, an externally applied ac input voltage is amplified at the output. An oscillator is a device, which produces an output of a desired frequency without any external input voltage.
This can be achieved if a part of the output of a transistor is fed back into the input circuit. If the feedback voltage is in the correct phase, oscillations of a transistor oscillator. The oscillator circuit has two principal sections.

1. Amplifying Section
This section is just a common-emitter amplifier having a high voltage gain.

2. Feedback Section
It comprises a network of LC oscillatory circuits to provide the necessary positive feedback in the correct phase.
A sinusoidal input is given which appears as an amplified signal in the output. This means that an external input is necessary to sustain ac signal in the output for an amplifier.
In an oscillator, we get ac output without any external input signal. In other words, the output in an oscillator is self-sustained.