GATE EC 2016 Set 1 — Question 25
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Analog Circuits → Feedback & Oscillators → Wien Bridge Oscillator
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Question
Consider the oscillator circuit shown in the figure. The function of the network (shown in dotted lines) consisting of the 100 k resistor in series with the two diodes connected back-to-back is to:
Correct answer
(A) introduce amplitude stabilization by preventing the op amp from saturating and thus producing sinusoidal oscillations of fixed amplitude
Solution
The circuit shown is a Wien bridge oscillator. For stable sinusoidal oscillations, the loop gain must be exactly 1.
1.The network in the dotted lines consists of two back-to-back diodes in series with a resistor, all in parallel with the feedback resistor.
2.As the output voltage amplitude increases, the voltage across the feedback network increases. Once it exceeds the turn-on voltage of the diodes, they begin to conduct.
3.This effectively places the resistor in parallel with the resistor, reducing the overall feedback resistance and thus reducing the closed-loop gain of the op-amp.
4.This automatic gain control (AGC) prevents the op-amp from reaching its saturation limits, ensuring the output remains a stable sinusoid with a fixed amplitude.
Thus, the correct option is (A).Continue learning with Success Tracker
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