Another type of popular audio frequency oscillator is the Wien bridge oscillator circuit. This is mostly used because of its important features. This circuit is free from the circuit fluctuations and the ambient temperature.

The main advantage of this oscillator is that the frequency can be varied in the range of 10Hz to about 1MHz whereas in RC oscillators, the frequency is not varied.

Construction

The circuit construction of Wien bridge oscillator can be explained as below. It is a two-stage amplifier with RC bridge circuit. The bridge circuit has the arms R₁C₁, R₃, R₂C₂ and the tungsten lamp L_p. Resistance R₃ and the lamp L_p are used to stabilize the amplitude of the output.

The following circuit diagram shows the arrangement of a Wien bridge oscillator.

The transistor T₁ serves as an oscillator and an amplifier while the other transistor T₂ serves as an inverter. The inverter operation provides a phase shift of 180^o. This circuit provides positive feedback through R₁C₁, C₂R₂ to the transistor T₁ and negative feedback through the voltage divider to the input of transistor T₂.

The frequency of oscillations is determined by the series element R₁C₁ and parallel element R₂C₂ of the bridge.

$$f = frac{1}{2 pi sqrt{R_1C_1R_2C_2}}$$

If R₁ = R₂ and C₁ = C₂ = C

Then,

$$f = frac{1}{2pi RC}$$

Now, we can simplify the above circuit as follows −

The oscillator consists of two stages of RC coupled amplifier and a feedback network. The voltage across the parallel combination of R and C is fed to the input of amplifier 1. The net phase shift through the two amplifiers is zero.

The usual idea of connecting the output of amplifier 2 to amplifier 1 to provide signal regeneration for oscillator is not applicable here as the amplifier 1 will amplify signals over a wide range of frequencies and hence direct coupling would result in poor frequency stability. By adding Wien bridge feedback network, the oscillator becomes sensitive to a particular frequency and hence frequency stability is achieved.

Operation

When the circuit is switched ON, the bridge circuit produces oscillations of the frequency stated above. The two transistors produce a total phase shift of 360^o so that proper positive feedback is ensured. The negative feedback in the circuit ensures constant output. This is achieved by temperature sensitive tungsten lamp L_p. Its resistance increases with current.

If the amplitude of the output increases, more current is produced and more negative feedback is achieved. Due to this, the output would return to the original value. Whereas, if the output tends to decrease, reverse action would take place.

Advantages

The advantages of Wien bridge oscillator are as follows −

• The circuit provides good frequency stability.

• It provides constant output.

• The operation of circuit is quite easy.

• The overall gain is high because of two transistors.

• The frequency of oscillations can be changed easily.

• The amplitude stability of the output voltage can be maintained more accurately, by replacing R₂ with a thermistor.

Disadvantages

The disadvantages of Wien bridge oscillator are as follows −

• The circuit cannot generate very high frequencies.

• Two transistors and number of components are required for the circuit construction.