Low distortion oscillator

Radio, №7, 1983

To stabilize the amplitude of oscillation in Wien bridge oscillator circuits sometimes are used two zener diodes in anti-parallel connection in negative feedback circuit. It allows to get a stable amplitude of oscillation, but it's hard to get a low distortion oscillation because of the difference in parameters of zener diodes.

A wien bridge oscillator with one zener diode circuit diagram

Fig. 1.

The Figure 1 shows a circuit diagram of the Wien bridge oscillator with only one zener diode V3, connected in the arm of bridge circuit made of diodes V1, V2, V4, V5. The total harmonic distortion of this circuit does not exceed 0.1%.

Low distortion oscillator circuit diagram

Fig. 2. A1 - 741, A2 - CA3086

To reduce harmonics further (THD < 0.04), a transistor array can be used in this circuit. All transistors in this array have the same parameters. Four transistors are connected as diodes. The circuit diagram of the Wien bridge oscillator based on the transistor array CA3086 is shown in Figure 2. The transistor V6 (emitter follower) is used to improve load-driving capability and it makes the oscillations independent of load changes.

Use potentiometer R4 to adjust the output voltage to 3..4 volts. The frequency of the oscillator circuit depends on Wien bridge parameters. If the two R1 and R2 arms are the same, and C1=C2, then the frequency can be found with the following formula:

F = 1/(2*π*R*C)

Where C= C1 = C2; R = R1 = R2; π = 3.141

With the resistor and capacitor values used in this circuit (R = 100K, C = 1.5nF), the frequency of the oscillations is about 1 kHz.

Wireless World, 1982, 9 Vol. 87, 1548