The circuit on the left shows a sin

The circuit on the left shows a single Resistor-Capacitor Network whose output voltage “leads” the input voltage by some angle less than 90o. An ideal single-pole RC circuit would produce a phase shift of exactly 90o, and because 180o of phase shift is required for oscillation, at least two single-poles must be used in an RC oscillator design.

However in reality it is difficult to obtain exactly 90o of phase shift so more stages are used. The amount of actual phase shift in the circuit depends upon the values of the resistor and the capacitor, and the chosen frequency of oscillations with the phase angle ( Φ ) being given as: