## 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.

 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 = 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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