Wien-Bridge Oscillator - gain of RC network
For such a circuit (with positive and negative feedback) the oscillation condition is: Positive feedback factor kp (for the desired oscillation frequency) must be equal to the negative feedback factor kn.

The positive feedback factor for two equal R values and to equal C values is kp=1/3 for w=1/RC . Hence, for the circuit to oscillate we require also kn=1/3.

In reality (for a safe start of oscillation) it is common practice to use kn&lt;1/3 (R2>2R1).

Comment: The IDEAL oscillation condition kp=kn results from the fact that - for linear opamp operation - the signal voltages at the positive and negative input terminals must be equal (opamp ideal). In practice, however (due to real opamp properties), we choose kp slightly larger than kn.

Hence, the oscillation amplitude (overfullfillment of the Barkhausen condition) will rise until it is limited (clipped) because of the finite supply voltages - unless an additional amplituide control mechanism is included (antiparallel diodes, for example).