21-04-2012, 01:00 PM
PULSE AND DIGITAL CIRCUITS
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THEORY:
The Wien bridge oscillator, the same circuit can be drawn as shown below. Observe
the circuit diagram carefully. The lead lag circuit is used as feedback network about which we have
already discussed. Here, no inverting amplifier stage is used to achieve oscillations.
The Wien bridge oscillator circuit consists of two feedbacks, positive as well as negative. A
positive feedback is between output of two stage rc –coupled amplifier
You know that for oscillator, positive feedback is essential. Here, positive feedback is used to
produce zero degree phase shift between amplifier and feedback network.
Perhaps you would like to know why negative feedback is required here. To ensure sustained
oscillations, the loop gain must be slightly greater than one when circuit is turned on for the
first time. For Wien bridge oscillator, the gain of the amplifier must be greater than three
(A>3), which will ensure that sustained oscillations build up in the circuit. Hence, to set this
gain negative feedback is essential.
In practice, to obtain the sustained oscillations at the desired frequency of oscillations, the
product of the voltage gain A and the feedback gain β must be one or greater than one. In this
case, the amplifier gain A must be 3.
Hence, to satisfy the product condition, feedback gain β must be 1/3. Here, the gain of the
amplifier is decided by resistor 'R3' and 'R4'.
Frequency components selection
We get the maximum output Vo only if resistive value is equal to reactance value. Hence, the
frequency of oscillation is decided by resistor R and capacitor C. Actually, the oscillation
frequency