The opamp Wien-bridge oscillator provides a nice view into classic oscillator design using feedback analysis. Feedback analysis reveals if your circuit is stable or unstable. When designing amplifiers, 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.
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.
What’s needed is a mechanism to guarantee oscillations will start (GAIN > 3), yet, limit the gain (GAIN=3) at steady state. Enter our heros – D1, D2 and R12. The circuit adjusts its gain depending on the signal level. For small signals, the diodes do not conduct and the gain is set by
For larger signals, the voltage across R12 is big enough to make D1 and D2 conduct. The shunt resistance of the conducting diodes effectively reduces the R12 resistance, consequently, reducing the overall gain to GAIN=3.
CIRCUIT ANALYSIS Run a simulation of OPWIEN_OL.CIR. View the AC output of the op amp VM(4). For R10=10k, R11=18k and R12=5k, the op amp gain is (1 + (18+5)/10) = 3.3 V/V. This should make the overall open-loop gain equal to 1/3 x 3.3 = 1.1 V/V. Does the peak at VM(4) reach this expected gain?
It’s time to close the loop and try out the Wien-Bridge Oscillator. Run a simulation of closed-loop circuit OPWIEN.CIR and plot the Transient Analysis at V(4). How much time does it take for the amplitude to stabilize?
HANDS-ON DESIGN Design the circuit with a different oscillation frequency. Calculate the values for R and C. (Example: For fo = 10kHz, choose R1=R2=10k and calculate C1=C2=1/(2ð x R1 x fo) = 1.6nF.) Test drive your oscillator. If there’s too little or too many sinewaves on the plot, adjust the total time of the Transient Analysis to another value like 5 ms by modifying the .TRAN statement to look like
.TRAN 0.05MS 5MS
If there’s no input signal to an oscillator, what starts the oscillations? Current source IS injects a pulse into the RC network to jump start the oscillations. In a real circuit, the large transient at power up will kick the circuit into action.
CIRCUIT INSIGHT What happens if there’s not enough gain around the loop? Reduce R12 to 1k making the total loop gain less than 1. Run a simulation. The circuit rings briefly, but there’s not enough gain to sustain oscillations.
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