Block diagram of oscillator consists of

Explanation

An oscillator is an electronic circuit designed to convert dc power into ac power by feeding some of the output of an amplifier back as its input in phase. There must be an electrical connection or some kind of link (e.g, inductive coupling) between output and input of the amplifier so that there is sufficient feedback. When the feedback voltage is in phase with the input, it is called positive feedback. Then, application of direct-voltage supply to the circuit is sufficient to start and maintain electrical oscillations at some (usually) high frequency to which the system is resonant.
A simple oscillator consists essentially of :
(1) a frequency-determining network, such as a resonant tank circuit, which also works as feedback network
(2) a transistor amplifier which is the active element.
With positive feedback, the output current of the amplifier will be right phase to increase the alternating current in the resonant circuit. The oscillations then build up in amplitude until the power osses in the circuit ure equal to the power that the amplifer can develop. The natural frequency of the oscillator is close to the resonant frequency of the resonant circuit

Suppose he voltage gain without feedback of the amplifier is`A=V_(o)/V_(i)` The feedback factor `beta` back to the input, `V_(i)=V_(f)+betaV_(o).`
`therefore A=(V_o)/(V_1)=1/beta or Abeta=1`
For `Abeta=1`, Barkhausen's criterion states that the phase shift of the feedback voltage will be zero or integral multiple of `2pi` rad, i.e., there will be positive feedback
The voltage gain with feedback, `A_f`, for the system is `A_(f)=A/(1-Abeta)`
Thus, for the frequency for which `Abeta=1, A_(f)`, will be infinite, i.e., the circuit will operate without any external signal voltage, which means to circuit will oscillate at that frequency.