Op Amp Wien-Bridge Oscillator

Posted on Feb 6, 2014

The opamp Wien-bridge oscillator provides a nice view into classic oscillator design using feedback analysis. Feedback analysis reveals if your circuit is stable (well behaved) or unstable (may oscillate). When designing amplifiers (especially high-speed ones), the trick is to avoid the conditions that make the circuit oscillate. When designing os

Op Amp Wien-Bridge Oscillator
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cillators, you strive to achieve those conditions in a predictable way. Feedback analysis simply means opening the circuit and injecting an AC signal VTEST at one end of the circuit. Then, by looking at the magnitude (gain) and phase (time-shift) of signal as it travels around the opened loop, you can tell whether you`ve got an amplifier or an oscillator on your hands. When wearing your oscillator design hat, the idea is to pick components that will make the open-loop analysis meet the conditions for oscillation at your chosen frequency: The conditions are as follows: Open the loop of the Wien Bridge Oscillator at the op amp`s output. This is a good point because of the relatively low impedance of the output terminal. Likewise, VTEST also has a low output impedance. Therefore, opening the loop here does not significantly alter the circuit`s behavior. Two basic sections form the Wien-bridge oscillator: an RC tuning network and an amplifier. Both are necessary to achieve the conditions for oscillations. The RC network is characterized by a center frequency where R=R1=R2 and C=C1=C2. At the center frequency, two interesting things occur at V(3). First, the phase shift goes through 0 degrees. And second, the magnitude reaches a peak of 1/3 V/V. CIRCUIT ANALYSIS Run a SPICE simulation of the open-loop circuit OPWEIN_OL. CIR. Add trace VM(3) to see what the Magnitude looks like at the center frequency. For R1=R2=10k and...

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