Wien’s Bridge Audio Frequency Oscillator

Wien’s Bridge Audio Frequency Oscillator Points : wien’s bridge audio frequency oscillator It is one of the most popular type of oscillators used in audio and sub-audio frequency ranges (20^HZ - 20 KH). This type of oscillator is simple in design, compact in size, and remarkable stable in its frequency output. Furthermore, its output is relatively free from distortion and its frequency can he varied easily. However, the maximum frequency output of a typical wien bridge oscillator is only about 1 MHz.

The circuit arrangement of a wien bridge oscillator is shown in fig. It is essentially a two — stage amplifier with R.C bridge circuit. R.C bridge circuit (Wien bridge) works as the feedback network between the input and output.

In the bridge circuit R₁, c₁, R₃, R₄ and R₂ are in parallel with C₂ form the four arms. From the analysis of the bridge circuit it is obvious that the bridge will be balanced only when:

Thus we see that in a bridge circuit the output will be in phase with the input only when the bridge is balanced i.e, at resonant frequency given by expression (iii).

So this bridge circuit can be used as feedback for an oscillator, provided that the phase shift through the amplifier is zero. This requisite condition is achieved by using a two stage amplifier, as illustrated in the fig. In this arrangement the output of the second stage is supplied back to the feedback network and the voltage across the parallel combination C₂ R₂ is fed to the input of the first stage. Transistor T₁, serves as an oscillator and amplifier whereas the transistor T₂ as an inverter to cause a phase shift of 1800 degree. The circuit use positive and negative feedbacks. The positive feedback is through the voltage divider to R₁, C₁, R₂, C₂ to transistor T, and negative feedback is through the voltage divider to the input of transistor T₂. Resistors R₃ and R₄ are used to stabilize the amplitude of the output.

The two transistors T₁, & T₂ thus cause a total phase shift of 360° degree and ensure proper positive feedback. The negative feedback is provided in the circuit to ensure constant output over a range of frequencies. This is achieved by taking resistor R4 in the form of a temperature sensitive lamp, whose resistance increases with the increase in current. In the amplitude of the output tends to increase, more current would provide more negative feedback. Thus the output would regain its original value.) A reverse action would take place in case the output tends to fall.

The continuous frequency variation in this oscillator can be had by varying the two capacitors C₁ and C₂ simultaneously. These capacitors are variable air-gang capacitors. We can change the frequency range of the oscillator by switching into the circuit different values of resistors R₁ & R₂.