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wein bridge oscillator

WEIN BRIDGE OSCILLATOR

An oscillator is a circuit that converts a dc input to an ac output. This project investigates sinusoidal, output oscillators. Sinusoidal oscillators consist of an amplifier with a positive feedback loop of a frequency selective network. The amplifier can be a transistor amplifier or an operational amplifier. The frequency of the oscillator is determined by the frequency selective network.

The criteria for an oscillator to produce sinusoidal oscillations are that

1.     The magnitude of the loop gain should be greater than or equal unity and

2.   The total phase difference between the output signal and the input provided through feedback should be integral multiple of 360⁰ or zero.

It is one of the most popular type of oscillators used in audio and sub-audio frequency ranges (20 – 20 kHz). This type of oscillator is simple in design, compact in size, and remarkably stable in its frequency output. Furthermore, its output is relatively free from distortion and its fre­quency can be varied easily. However, the maximum frequency output of a typical Wien bridge oscillator is only about 1 MHz. This is also, in fact, a phase-shift oscillator. It employs two transistors, each producing a phase shift of 180°, and thus producing a total phase-shift of 360° or 0°.

CIRCUIT DIAGRAM:-

The circuit diagram of Wien bridge oscillator is shown in the figure below.

It is essentially a two-stage amplifier with an R-C bridge circuit. R-C bridge circuit (Wien bridge) is a lead-lag network. The phase’-shift across the network lags with increasing frequency and leads with decreasing frequency. By adding Wien-bridge feedback network, the oscillator becomes sensitive to a signal of only one particular frequency. This particular frequency is that at which Wien bridge is balanced and for which the phase shift is 0°.If the Wien-bridge feedback network is not employed and output of transistor Q₂ is fed back to transistor Q₁ for providing regeneration re­quired for producing oscillations, the transistor Q₁ will amplify signals over a wide range of frequencies and thus direct coupling would result in poor frequency stability.

 Thus by em­ploying Wien-bridge feedback network frequency stability is increased.

In the bridge circuit R₁ in series with C_1, R₃, R₄ and R₂ in parallel with C₂ form the four arms.

This bridge circuit can be used as feedback network 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 figure. 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 Q₁ serves as an oscillator and amplifier whereas the transistor Q₂ as an inverter to cause a phase shift of 180°. The circuit uses positive and negative feedbacks. The positive feedback is through R₁ C₁ R₂, C₂ to tran­sistor Q₁ and negative feedback is through the voltage divider to the input of transistor Q₁. Resistors R₃ and R₄ are used to stabilize the amplitude of the output.

The two transistors Q₁ and Q₂ thus cause a total phase shift of 360° 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 case 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 amplifier voltage gain,

A = (R₃ + R₄) /  R₄ =  (R₃ / R₄) + 1 = 3

{Since R₃ = 2 R₄}

The above corresponds with the feedback network attenuation of 1/3. Thus, in this case, voltage gain A, must be equal to or greater than 3, to sustain oscillations.

To have a voltage gain of 3 is not difficult.

On the other hand,to have a gain as low as 3 may be difficult. For this reason also negative feedback is essential.

Wein Bridge Oscillator Using Op-Amp IC 741

Wein bridge oscillator is an audio frequency sine wave oscillator of high stability and simplicity. Before that let us see what is oscillator? An oscillator is a circuit that produces periodic electric signals such as  sine wave or square wave. The application of oscillator includes:

sine wave generator, local oscillator for synchronous receivers etc.

Here we are discussing wein bridge oscillator using 741 op amp IC. It is a low frequency oscillator. The op-amp used in this oscillator circuit is working as non-inverting amplifier mode. Here the feedback network need not provide any phase shift. The circuit can be viewed as a wein bridge with a series RC network in one arm and parallel RC network in the adjoining arm. Resistors Ri and Rf are connected in the remaining two arms.

Circuit Diagram:

Components Required:-

·         Resistors (1KΩ, 1.5KΩ x2)

·         Potentiometer(4.7KΩ)

·         Capacitor(0.1µF x2)

·         741 Op amp

Working of Wein bridge Oscillator:-

• The feedback signal in this oscillator circuit is connected to the non-inverting input terminal so that the op-amp works as a non-inverting amplifier.
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• The condition of zero phase shift around the circuit is achieved by balancing the bridge, zero phase shift is essential for sustained oscillations.
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• The frequency of oscillation is the resonant frequency of the balanced bridge and is given by the expression   fo = 1/2Πrc.
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• At resonant frequency ( ƒo), the inverting and non-inverting input voltages will be equal and “in-phase” so that the negative feedback signal will be cancelled out by the positive feedback causing the circuit to oscillate.
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• From the analysis of the circuit, it can be seen that the feedback factor β= 1/3 at the frequency of oscillation. Therefore for sustained oscillation, the amplifier must have a gain of 3 so that the loop gain becomes unity.

• For an inverting amplifier the gain is set by the feedback resistor network Rf and Ri and is given as the ratio -Rf/Ri.

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Design:-

The required frequency of oscillation fo=1kHz

we have,

Take C=0.01µF, then R=1.6kΩ (Use 1.5kΩ standard)

Gain of the amplifier section is given by,

Take Ri=1kΩ, then Rf=2.2kΩ (Use 4.7kΩ Potentio meter for fine corrections).