Low distortion Wien bridge sine wave oscillator circuit

Function of the circuit

The Wien bridge circuit has been used as a sine wave generator circuit and can be used when a low-distortion waveform is required in the low-frequency range. By changing the resistor RO or the capacitor CO, an oscillation frequency of several hundred kilohertz or less can be obtained.

How the Circuit Works

The oscillation principle is that when the phase shift in the loop is an integer multiple of 0 degrees or 360 degrees, and the loop gain is greater than 1, the circuit will oscillate. If the oscillation increases, the circuit will saturate, so an amplitude stabilization circuit is required.

The attenuation of the Wien bridge circuit when it resonates is 1/3. In order to start oscillation, the voltage gain of the feedback amplifier A1 must be greater than 3.

The oscillation frequency FO of the parameter-free Wien bridge circuit is determined by FO=1/2πCO.RO. The capacity of capacitor CO should ensure that the base reactance XO is between 1K and several kilohms. After determining the capacity of CO, the resistance value of RO can be calculated according to RO=1/2πFO.CO.

The amplitude stabilization circuit uses the junction FET drain-source resistance to be controlled by voltage and has a variable resistance value. If the drain voltage of the FET increases, the waveform distortion will also increase, so the inverting amplifier A2 is used to amplify the oscillation by about 3 times, and then A1 reduces the level to 1/3. And add local feedback (R3, R4) between the drain and gate.

The amplitude control loop uses OP amplifier A3 to compare and integrate the reference voltage generated by the Zener diode with the average value of the rectified current output by A2 by D1, and controls the gate voltage of the FET. In order to offset the temperature coefficient of the rectifier diode, a compensation diode D2 is added to the reference voltage circuit.

Capacitor C2 is used to determine the integration time constant. Smaller capacitance means faster response, but the rectifier circuit will produce pulsation, increasing filter distortion. The function of resistor R5 is to make the integration circuit produce advance compensation, which can speed up the response, but if its resistance is larger, there will be residual pulsation.

Component Selection

In the Wien bridge oscillator circuit, it is difficult to achieve ultra-low frequency oscillation, because the lower the frequency, the longer the stabilization time. On the other hand, the upper limit of the oscillation frequency is limited by the phase characteristics of the OP amplifier A1. If the oscillation frequency is required to reach hundreds of kilohertz, a video OP amplifier should be selected, and the resistance of RO should be as small as possible under the condition that the driving capacity allows. If the capacity of CO is too small, it will be affected by the parasitic capacitance of the circuit or the input capacitance of the OP amplifier, and the circuit will be unstable.

Adjustment

In order to reduce the distortion rate, the ripple of the rectifier circuit must be suppressed. The method is to use R2 to reduce the variable range of the FET. If there is an error in each arm of the Wien bridge, the attenuation will not be 3, so the adjustable range of the gain must be designed to be slightly larger.

The response time of the amplitude control circuit depends on the variation range of C2, R5 and FET. If the capacity of C2 is increased, the stabilization time will be extended. If the lead compensation of R5 is increased, it can stabilize quickly, but the distortion rate will increase.

To check the stability, you can observe the oscillation waveform after the power is turned on, or you can ground the non-inverting input of the OP amplifier A1 and understand the stability by observing the rising characteristics after the oscillation stops.