an electronic oscillator is a device which can generate some signals without input signals. It is an unstable system realized with positive feedback, as shown in the following scheme:

[![Part of the output signal is fed back to the amplifier input in such a way that the feed back signal is regenerated, re-amplified and fed back again to maintain a constant output signal.](https://i.stack.imgur.com/qfqjw.png)](https://i.stack.imgur.com/qfqjw.png)

Now I have two questions:

* How is the closed loop transfer function of this device defined? The input signal does not exist.
* How can we implement negative or positive feedback, if the input signal does not exist?
I know that in negative feedback the feedback signal is added to the input signal, in positive feedback is subtracted. So in the first situation we use and adder circuit, in the second one a subtractor circuit. But what do we do in this case?