A Wein bridge oscillator can be used to produce sine waves with very low distortion level. The Wein bridge oscillator produces zero phase shift at only one frequency (f = ½ ℼRC) which will be the oscillation frequency. In the configuration shown in Figure 3-35, stable oscillation can occur only if the loop gain remains at unity at the oscillation frequency.
The circuit achieves this control by using the positive temperature coefficient of a small lamp to regulate gain(Rf/RLAMP) as the oscillator attempts to vary its output. This is a classic technique for achieving low distortion that has been used by numerous circuit designers for about 40 years. The smooth limiting action of the bulb, in combination with the Wein network’s near-ideal characteristics, yields very high performance. In this circuit a 741 op amp is used with ± 9 V power supplies. The tungsten lamp is a type # 327 miniature which has a standard bayonet base. This lamp is rated at 28.0 V and 40 rnA. For mass production of these oscillators, the lamps are burned in for a predetermined number of hours to stabilize the characteristics of the filament.
The oscillator shown here has four frequency bands covering about 15 Hz to 150 kHz. The frequency is continuously variable within each frequency range with ganged 20-kΩ potentiometers. The oscillator draws only about 4.0 rnA from the 9-V batteries. Its output is from 4 to 5 V with a 10-kΩ load and the Rf (feedback resistor) is set at about 5% below the point of clipping. As shown, the center arm of the 5-kΩ output potentiometer is the output terminal. It should be noted that if you couple the oscillator to a de type circuit, a capacitor should be inserted in series with the output lead.