Low frequency Wein-bridge Oscillator and it’s Working

 ·         Wein Bridge Oscillator

The Wein bridge oscillator is the standard and simple oscillator circuit for low to moderate frequencies (10 Hz to 1 MHz). It is most frequently used type of audio oscillator. It is found that the oscillator is free from circuit fluctuations and ambient temperature.

The oscillator circuit consists of an amplifier and an RC bridge is used in the feedback network of an oscillator.

Following fig. shows the circuit of Wein Bridge Oscillator.


•      Wein bridge oscillator is essentially a two-stage amplifier with RC bridge circuit.

•      The transistor Q1 serves as an oscillator and amplifier, while the other transistor Q2 serves as an inverter i.e. to produce a phase shift by 1800.

•      This circuit uses positive and negative feedbacks.

•      The positive feedback is through R3C3, R4C4 to the transistor Q1. The negative feedback is through the voltage divider to the input of transistor Q2.

•      The frequency of oscillation is determined by the series element R3C3 and parallel element R4C4  of the bridge and is given by-

f = 1/ 2π√(R3C3 R4C4)

If R3 = R4 and C3 = C4

f = 1/ 2πRC

•      Working :

•      When the circuit is switched ON, the bridge circuit produces oscillations of frequency determined by equation f = 1/ 2πRC. The two transistors produce a phase shift of

3600  so that proper positive feedback is ensured. The negative feedback in the circuit ensures constant output. This is achieved by the replacement of R1 with the temperature sensitive tungsten lamp. Its resistance increases with current.

•      When the amplitude of the output increases, more current will provide more negative feedback. The result is that, the output would return to original value. When the output amplitude decreases, less current provides less negative feedback, the result is that output will return to original value. Thus, temperature sensitive lamp provides amplitude stability.

•      The frequency of oscillations of Wein bridge oscillator can be varied by changing C3 and C4 or R3 and R4 simultaneously. Then if we use dual pot whose value is varied by changing the shaft position in place of R3 and R4, we can obtain variable frequency Wein bridge oscillator. The Wein bridge oscillator is practically built using operational amplifiers.

 

•      Advantages

Ø  Output stability is more.

Ø  The overall gain is high due to use of two transistors.

Ø  The frequency of oscillation can be changed by using a potentiometer.

 

•      Disadvantages

Ø  The circuit cannot generate very high frequencies.

Ø  At high frequency, the shunt capacitor gets shorted and there is no output. Even at very low frequency, the series capacitor appears open to the input signal and there is again no output signal.

Ø  The circuit requires two transistors and a large number of components.

 






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