Q. What are the necessary conditions to maintain sustained oscillation?

The use of positive feedback that results in a feedback amplifier having closed-loop gain |Af| greater than 1 and satisfies the phase conditions will result in operation as an oscillator circuit. An oscillator circuit then provides a varying output signal. If the output signal varies sinusoidally the circuit is referred to as a sinusoidal oscillator. If the output voltage rises quickly to one voltage level and later drops quickly to another voltage level, this circuit is generally referred to as a pulse or square wave oscillator.

Feedback circuit used as an oscillator

 To understand how a feedback oscillator circuit performs as an oscillator consider the feedback circuit of the figure. When the switch at the amplifier input is open, no oscillation occurs. Consider that we have a fictitious voltage at the amplifier input(Vi). This result is an output voltage V0=AVi. After the amplifier stage and in a voltage Vf ?(AVi) after the feedback stage. Thus we have a feedback voltage Vf ?AVi where BA is referred to as the loop gain. If the circuits of the base amplifier and feedback network provide ?A of a correct magnitude and phase, Vf can be made equal to Vi. Then, when the switch is closed and fictitious voltage Vi is removed, the circuit will continue operating since the feedback voltage is sufficient to drive the amplifier and feedback circuits resulting in a proper input voltage to sustain the loop operation. The output waveform will still exist after the switch is closed if the condition ‘?A=1' is met. This is known as the Barkhausen criterion for oscillation.