Wien Bridge Oscillator

 

• The function of the Wien bridge oscillator is to generator sinusoidal frequency. 
• It is audio frequency oscillator.

Circuit Connection Diagram

• The Wien bridge circuit is connected between amplifier input and output terminal. 
• This bridge has series RC circuit in one arm and parallel RC circuit in the other arm. 
• The resistor R1 and R_F is connected in the remaining arms. The total phase shift around the circuit must be 0 degree.

Feedback

 

• The output of the operational amplifier is fed back to the both the inputs of the amplifier. 
• One part of the feedback signal is connected to the inverting input ( negative ) terminal through feedback network R1 and RF. 
• The function of this feedback network is to adjust amplifier gain into narrow limits.
• The series RC and parallel RC circuit forms second feedback circuit and it is fed back to the non – inverting input ( positive ) terminal from the output of the operational amplifier.  
• The function of this feedback circuit is to rise oscillation.

RC Phase Shift Circuit Feedback

 

• The series RC circuit is connected to parallel RC circuit. 
• It is actually high pass filter ( Series RC circuit ) is connected to low pass filter ( Parallel RC circuit ). 
• This band pass filter allows only selective frequency to pass.

Low frequency

• The reactance of the series capacitor is vey high so it acts like open circuit which blocks input signal therefore no output at low frequency.

High frequency

• The reactance of the parallel capacitor is very low, therefore Parallel RC circuit acts like short circuit across output therefore no output at high frequency.

Resonance frequency

• At resonance frequency, the circuit reactance is equal to resistance, the phase difference between input and output is equal to zero. 
• The magnitude of the output voltage is 1/3 that of input voltage. 
• The voltage gain of the amplifier must be equal to or greater than 3 for oscillation to start. 
• The voltage gain ≥ 3 is set by feedback resistor R1 and RF. 

• At resonance condition, the bridge becomes balance and at this time total phase shift become zero. 
• If R = R₁ = R_F and C = C₁ = C₂ , the frequency of oscillation is equal to resonance frequency is given by

       f = 1 / 2πRC

         = 0.159 / RC

• The gain of the sustained oscillation is given by

        Voltage Gain A_V = 1 / B

                                    = 3

                                   OR

                         1 + R_F / R₁ = 3

                            R_F = 2R₁

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