Q. Explain the operation of oscillators.

Oscillator is a circuit that changes dc energy from the power supply into ac energy. An amplifier provided with a positive feedback becomes an oscillator and it produces an output signal even though there is no external input signal. The function of the oscillator is just the reverse of a rectifier. The block diagram of an oscillator is given below:

Consider the block diagram of figure. A voltage source Vin drives the input terminals of an amplifier. The amplifier output voltage is AVin and it drives the feedback network producing a feedback voltage Vf = AbVin. If the circuit of the amplifier and the feedback network provide correct phase shift, then this feedback voltage Vf will be in phase with signal Vin that drives the input terminals of the amplifier. Now, if switch is closed and simultaneously the voltage source Vin is removed, the feedback voltage Vf will drive the input terminals of the amplifier. If Ab is less than 1, AbVin will be less than Vin, the signal feedback is not sufficient to drive the amplifier and the feedback circuits and the output signal will die out. If Ab is greater than 1, the output signal builds up resulting in oscillations with growing magnitude. If Ab is equal to 1, AbVin = Vin and the output voltage is a sine wave whose amplitude remains constant.

When the oscillator circuit is switched on, the only signal source at input is the noise voltage generated by the resistors. This noise voltage gets amplified and then appears at the output. This amplified noise now drives the feedback circuit, which can be a resonant, or a phase shift network. The feedback voltage is maximum at a particular frequency and also the phase shift for positive feedback is correct at this frequency. So we get oscillations at one frequency only though noise voltage contains all frequencies.