**Wien Bridge Oscillator using Op Amp:**

This is also RC oscillator which uses RC type of feedback network. The main difference between phase shift and Wien bridge oscillator is that the R-C phase shift oscillator introduces 180° phase shift during the amplifier stage, while Wien bridge oscillator uses a **non-inverting amplifier. **So in Wien bridge type there is no phase shift necessary through the feedback network. Let us see the basic version of the Wien bridge oscillator and its analysis.

A basic Wien bridge and an amplifier stage is shown in the Fig. 2.94.

The output of the amplifier is applied between the terminals 1 and 3, which is the input to the feedback network. While the amplifier input is supplied from the diagonal terminals 2 and 4, which is the output from the feedback network. Thus amplifier supplies its own input through the Wien bridge as a feedback network.

The two arms of the bridge, namely R_{1},C_{1} in series and R_{2},C_{2} in parallel are called **frequency sensitive arms**. This is because the components of these two arms decide the frequency of the oscillator. Such a feedback network is called **lead-lag network**. This is because at very low frequencies it acts like a lead while at very high frequencies it acts like lag network

**Wien Bridge Oscillator using Op amp:**

The Fig. 2.95 shows the Wien bridge oscillator using an op-amp.

The resistance R and capacitor C are the components of frequency sensitive arms of the bridge. The resistance R_{f} and R_{1} form the part of the feedback path. The gain of noninverting op-amp can be adjusted using the resistance R_{f}** **and R_{1}. The gain of op-amp is,

To satisfy Barkhausen criterion that Aβ ≥ 1 it is necessary that the gain of the noninverting op-amp amplifier must be minimum 3.

Thus ratio of R_{f}** **and R_{1} must be greater than or equal to 2.

The frequency of oscillations is given by,

The feedback is given to the noninverting terminal of op-amp which ensures zero phase shift. It is used popularly in laboratory signal generators.

If in a Wien bridge feedback network, two resistances are not equal i.e. they are R_{1} and R_{2} while two capacitors are not equal i.e. they are C_{1} and C_{2} then the frequency of oscillations is given by,

With R_{1} = R_{2} = R and C_{1} = C_{2} = C we get it as 1/2πRC as stated earlier.

**Advantages of ****Wien Bridge Oscillator****:**

The various advantages of Wien bridge oscillator are,

- By varying the two capacitor values simultaneously, by mounting them on the common shaft, different frequency ranges can be obtained.
- The perfect sine wave output is possible.
- It is useful audio frequency range i.e. 20 Hz to 100 kHz.

**Disadvantages of ****Wien Bridge Oscillator****:**

If instead of op-amp, transistorised amplifier is to be- used then more stages are required to obtain 0° phase shift between input and output. This increases the number of components and cost. The frequency stability is poor.