Feedback resistance condition (Rf > 29R) in Phase-shift oscillators
For each oscillator circuit the designed loop gain has to be slighly larger than the theoretical value of "1" (oscillation condition). This is because (a) in most cases, the calculation of the circuits assumes no parts tolerances as well as IDEAL opamp properties (input/output impedances, infinite gain) and (b) a safe start of oscillations must be ensured with a pole pair slightly into the right half of the s-plane (loop gain >1). As a consequence, oscillation will safely start and amplitudes will rise continuously until an amplitude limitation will take place (supply voltage limitation or other non-linear parts like diodes) - thereby reducing the loop gain to a mean value of unity.