Sine wave generating circuit

Crystal oscillator

Circuit principle: The TR1 junction FET acts as a voltage-controlled varistor here. Together with R3 and R4, it forms the negative feedback loop of the venturi oscillator. The larger the resistance of TR1, the stronger the negative feedback. D2, D3, R8, R9, R10 and IC(2/2) perform full-wave rectification on the output oscillating voltage, and generate a negative rectified output voltage at pin 1 of the IC. After D1 and R7, C4 filter, a negative DC is obtained. The voltage, which is approximately equal to the amplitude of the oscillating output. This negative voltage is applied to the G pole of TR1, which controls the resistance between the DS poles of TR1. As the amplitude of the oscillation output increases, the G-pole voltage of TR1 is more negative, the resistance between the DS poles of TR1 becomes larger, and the negative feedback is enhanced, so that the oscillation amplitude is reduced. Through the above automatic adjustment, the oscillation amplitude is kept stable, and the amplifier is prevented from entering the nonlinear region, thereby obtaining a good sinusoidal waveform.

A common measure of stability for Venturi oscillators is the addition of diodes to the negative feedback loop (see figure below):

The goal is also to increase the negative feedback as the output amplitude increases, but due to the nonlinearity of the diode, the output waveform is slightly distorted. The negative feedback loop of this circuit is provided without nonlinear components, so that a high quality sinusoidal waveform can be obtained.