The input voltage for starting oscillations in an oscillator is caused by:

Introduction:
An oscillator is an electronic circuit that produces a continuous and repetitive waveform without any external input. It generates its own periodic signal, typically a sinusoidal waveform. The input voltage required to start the oscillations is known as the "trigger" or "initiating" voltage.

Explanation:
The input voltage for starting oscillations in an oscillator is caused by the noise voltage in resistors. The noise voltage is an inherent random voltage that exists in all resistors due to the thermal agitation of charge carriers (electrons) within the resistor.

Noise voltage in resistors:
When an electric current flows through a resistor, the random motion of charge carriers causes fluctuations in the voltage across the resistor. These fluctuations result in a noise voltage superimposed on the desired signal. The noise voltage is typically small and appears as random variations around the DC voltage level.

Role of noise voltage in starting oscillations:
In an oscillator circuit, the noise voltage in resistors can act as an initial disturbance or perturbation to initiate the oscillations. The noise voltage provides the necessary energy or kick-start to break the initial symmetry or balance of the circuit and start the feedback process.

Positive and negative feedback:
Positive feedback and negative feedback are two common types of feedback used in oscillators. Positive feedback amplifies and reinforces the input signal, while negative feedback reduces the output signal. In an oscillator, positive feedback is required to sustain and maintain the oscillations.

Why noise voltage and not feedback?
While feedback is essential for sustaining oscillations, it is not responsible for initiating the oscillations. Feedback alone cannot create an oscillatory signal from scratch. It requires an initial disturbance or input voltage to start the process. In an oscillator, this initial input voltage is provided by the noise voltage in resistors.

Conclusion:
The input voltage for starting oscillations in an oscillator is caused by the noise voltage in resistors. The random fluctuations in voltage due to the thermal agitation of charge carriers within the resistors act as an initial disturbance, breaking the symmetry of the circuit and initiating the oscillatory process. Positive feedback is then used to sustain and maintain the oscillations.