1. The Barkhausen Criterion: For oscillations to occur, the total phase shift around the feedback loop must be $360^\circ$ (or $0^\circ$). In this oscillator, a transistor amplifier is typically used in a common-emitter configuration, which inherently provides a $180^\circ$ phase shift. Therefore, the feedback network must provide the remaining $180^\circ$ phase shift.
2. Individual RC Section Phase Shift: A single RC network (one resistor and one capacitor) can theoretically provide a phase shift between $0^\circ$ and $90^\circ$. However, as the phase shift approaches $90^\circ$, the output voltage drops to zero. In practice, a single stage cannot reach $90^\circ$.
3. Why Three Stages?:
• One Stage: Max theoretical shift is $90^\circ$ (less in practice), which is insufficient.
• Two Stages: Max theoretical shift is $180^\circ$. To achieve exactly $180^\circ$ with just two stages, the attenuation would be infinite, meaning no signal would return to the amplifier.
• Three Stages: By using three RC sections, each section can provide a manageable $60^\circ$ phase shift ($60^\circ \times 3 = 180^\circ$). This configuration allows for the necessary phase shift while maintaining enough signal amplitude for the amplifier to sustain oscillations.
Thus, a minimum of three RC sections is required to create a stable $180^\circ$ phase shift in the feedback loop.