At low to moderate frequencies the Wien bridge oscillator (Figure 1) is a useful source of low-distortion sinusoidal signals. The idea is to make a feedback amplifier
with 0◦ phase shift at the desired output frequency, then adjust the loop gain so that a self-sustaining oscillation just barely takes place.
For equal-value R’s and C’s as shown, the voltage gain from the non-inverting input to op-amp output should be exactly +3.00. With less gain the oscillation will cease, and with more gain the output will saturate. The distortion is low if the amplitude of oscillation remains within the linear region of the amplifier, i.e., it must not be allowed to go into a full-swing oscillation.
Without some trick to control the gain, that is exactly what will happen, with the amplifier’s output increasing until the effective gain is reduced to 3.0 because of saturation. The tricks involve some sort of long-time-constant gain-setting
feedback, as we will see.
In the 1st circuit (Figure 1-A), an incandescent lamp is used as a variable-resistance feedback element. As the output level rises, the lamp heats slightly, increasing its resistance and therefore reducing the non-inverting gain. The circuit shown has less than 0.003% harmonic distortion for audio frequencies above 1 kHz; see LTC App. Note 5 (12/84) and App. Note 43 (6/90) for more details.
In the 2nd circuit (Figure 1-B), an amplitude discriminator consisting of the biased divider and diode charges a long-time constant RC; this voltage adjusts the ac gain by varying the resistance of the FET, which behaves like a voltage variable
resistance for small applied voltages.
Note the long time constant used (2 seconds); this is essential to avoid distortion, because fast feedback will distort the wave by attempting to control the amplitude within the time of one cycle.
It has been claimed that, with careful design, Wienbridge oscillators can be built with distortion “well into the parts-per-billion range.” Tricks to do this, which include the use of cascaded op-amps (for high loop gain, therefore low distortion), and cancellation of remaining harmonic distortion, are described in Linear Technology Magazine, Feb. 1994, pp. 26–28.
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