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Explain wein bridge oscillator in brief and give expression for frequency of oscillation.

Mumbai University > Electronics Engineering > Sem 4 > Discrete Electronic Circuits

Marks: 10M

Year: May 2015,Dec 2016

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1. Wein bridge oscillator consist of two stages of RC coupled amplifier and feedback network.
2. These two stage amplifier provides $360^0$ phase shift and positive feedback comprises of lead and lag network which introduces phase shift of $0^0$.
3. Output of amplifier is connected to input of feedback network and voltage across $R_2 , C_2$ acts as input to amplifier.

Figure 1: Circuit diagram of wein bridge oscilator.

Operation:

The noise present generated due to movement of electrons in resistor or power supply noise gets amplified which cover wide range of frequencies up to GHz. At one frequency bridge gets balanced,

$\frac{R_3}{R_4} = \frac{Zsc}{Zparallel}$ i.e output of the bridge will be in phase with input. Hence at that frequency total phase shift around close loop becomes $0^0$ and circuits starts oscillating.

Frequency of oscillation given by,

$F = \frac{1}{2\pi\sqrt{R_1R_2C_1C_2}}$

when $R_1 \neq R_2$ and $C_1 \neq C_2$ then,

$F = \frac{1}{2\pi \times RC}$

When condition is satisfied, we must have $\beta = \frac{1}{3}$. This means that amplifier must have gain at least 3 ($A\beta = 1$).

Features of Wein bridge oscillator:

• Frequency range 10Hz to around 1MHz.
• Used in commercial audio generator.
• It consist of +ve feedback for sustained oscillation and -ve feedback to improve the stability of oscillator.
• Gain should be atleast 3 and feedback of 1/3.
• Circuit osillate at frequency given by $F = \frac{1}{2\pi\sqrt{R_1R_2C_1C_2}}$