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18/08/2021

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 RF is connected in the remaining arms. The total phase shift around the circuit must be 0 degree.


wien-bridge-oscillator-using-op-amp.png




wien-bridge-oscillator-using-op-amp


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. 

frequency-response-of-wien-bridge-oscillator


  • At resonance condition, the bridge becomes balance and at this time total phase shift become zero. 
  • If R = R1 = RF 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 AV = 1 / B

                                    = 3


                                   OR


                         1 + RF / R1 = 3

                            RF = 2R1

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