Principle of the Wien bridge oscillation circuit
The basic composition
The figure on the left is the basic form of the Wien bridge oscillator which used the operational amplifier.The part which is composed of C1, C2, R1, R2 becomes the positive feedback circuit of the operational amplifier. With it, the operational amplifier works in the oscillation. Also, the circuit sets the oscillation frequency in the damping time constant of the CR.On the other hand, the circuit of R3, R4 controls the mu factor of the operational amplifier.This oscillator begins the oscillation when it makes the mu factor equal to or more than 3. At less than 3, it doesn't oscillate. The beautiful signal doesn't come out by output's being saturated when the mu factor is more than 3.The mu factor must be controlled to 3.Because it is, only this basic circuit can not use for the practical use.

Where is the bridge?The part of the circuit diagram becomes the figure on the left when it changes the position.The balance condition of this bridge becomes the oscillation condition.The oscillation frequency can be calculated by the following formula.In case of C=C1=C2, R=R1=R2, the formula becomes the following.The condition of the operational amplifier which is necessary to do the stable oscillation is the mu factor=3.It becomes the following when finding the relation between R3 and R4 from the mu factor.Gain 3=(1+R4/R3) That is, the condition of R4=2xR3 is necessary.