26-09-2013, 03:23 PM
To verify the characteristics of Wein Bridge Oscillator
INTRODUCTION
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 oscillators, you strive to achieve those
conditions in a predictable way.
FEEDBACK ANALYSIS
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.
OPENING THE LOOP
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.
NON-INVERTING AMPLIFIER
The RC network falls short of the oscillation conditions in that the gain is only 1/3 V/V. How is
the gain of 1V/V around the loop to be achieved? As you might have guessed, the non-inverting
amplifier provides the needed gain. How much? A gain of 3 V/V makes the total gain 1/3 x 3 = 1
V/V. Setting the correct op amp gain is critical. Not enough - oscillations will cease. Too much
– oscillation amplitude will grow until the output saturates.