Wien Bridge Oscillator:
The Wien Bridge oscillator is a standard oscillator circuit for low to moderate frequencies, in the range 5Hz to about 1MHz. It is mainly used in audio frequency generators.
The Wien Bridge oscillator uses a feedback circuit called a lead lag network as shown in fig. 1.
At very low frequencies, the series capacitor looks open to the input signal and there is no output signal. At very high frequencies the shunt capacitor looks shorted, and there is no output. In between these extremes, the output voltage reaches a maximum value. The frequency at which the output is maximized is called the resonant frequency. At this frequency, the feedback fraction reaches a maximum value of 1/3.
At very low frequencies, the phase angle is positive, and the circuit acts like a lead network. On the other hand, at very high frequencies, the phase angle is negative, and the circuit acts like a lag network. In between, there is a resonant frequency fr at which the phase angle equals 0°.
The output of the lag lead network is
Fig. 1
The gain of the feedback circuit is given by
The phase angle between Vout and Vinis given by
These equations shows that maximum value of gain occurs at XC = R, and phase angle also becomes 0°. This represents the resonant frequency of load lag network. Fig. 2, shows the gain and phase vs frequency.
Fig. 2
