Diode stabilization problem

shaorc

Diode stabilization problem [Copy link]

 

As shown in the figure, the Venn oscillator circuit

The amplifier gain circuit uses the nonlinear characteristics of diodes D1 and D2 to stabilize the output sinusoidal signal.

What is the principle? If one end of the diode is connected to the output, then when the branch is turned on, the other end of the diode is clamped at a difference of 0.7V?

maychang

Saying that "a diode turns on above 0.7V and turns off below 0.7V" is a poor approximation. The voltage-current characteristic curve of an actual diode is an exponential curve, as shown in the figure below.

shaorc

maychang posted on 2023-9-21 13:52 It is a poor approximation to say that "a diode is turned on above 0.7V and turned off below 0.7V". The voltage-current characteristic curve of an actual diode...

I didn't see the picture, but if the anti-parallel diode is removed, wouldn't the amplitude of the oscillation be limited by the supply voltage of the op amp?

maychang

The so-called resistance of a diode under a certain voltage or current is the red tangent of the black diode voltage-current characteristic curve in the figure. Obviously, the resistance of tangent 1 is larger, the resistance of tangent 2 is smaller than that of tangent 1, and the resistance of tangent 3 is even smaller.

maychang

In the first circuit, when the oscillation is not started, the voltage across R3 is zero, the resistance of D1D2 is very large, and the resistance value of R3 plus R2 is greater than twice that of R1, which meets the oscillation condition that the loop gain is greater than 3, so the circuit can oscillate. After oscillation, the output amplitude continues to increase. As the output amplitude increases, the voltage across R3 increases (note that it is AC), and the resistance of D1D2 decreases. When the resistance of D1D2 decreases to a certain extent, the resistance of R3 in parallel with the diode plus R2 is exactly equal to twice that of R1, the loop gain is exactly equal to 3, and the oscillation amplitude stabilizes and no longer increases.

maychang

shaorc posted on 2023-9-21 14:11 I didn't see the picture. If the anti-parallel diode is removed, won't the amplitude of the oscillation be limited by the supply voltage of the op amp?

If the anti-parallel D1D2 is removed, the circuit can oscillate, but the waveform is limited by the power supply voltage, and the top and bottom are cut off, becoming a trapezoidal wave with great distortion, which does not meet the requirements of sinusoidal.

maychang

shaorc posted on 2023-9-21 14:11 I didn't see the picture. If the anti-parallel diode is removed, won't the amplitude of the oscillation be limited by the supply voltage of the op amp?

I have written an article in this forum titled "Miscellaneous Talks on Sinusoidal Oscillation Circuits" , in which I discussed the problem of amplitude stabilization in sinusoidal oscillation circuits. You can refer to it.

maychang

shaorc posted on 2023-9-21 14:11 I didn't see the picture. If the anti-parallel diode is removed, won't the amplitude of the oscillation be limited by the supply voltage of the op amp?

That post is far more detailed than the above replies. It not only has text descriptions, but also waveform diagrams and comparison diagrams of the waveforms produced by different resistance values.

shaorc

maychang posted on 2023-9-21 14:42 I have written an article in this forum called "Miscellaneous Talks on Sine Oscillator Circuits", which discusses the problem of amplitude stabilization of sine wave oscillation circuits. You can refer to it.

thank you

led2015

Through the nonlinear characteristics of the diode, the output sinusoidal signal can be adjusted to a stable amplitude.

MrCU204

Nonlinearity, what is nonlinearity? !

Whether it is linear or not depends on the ratio of voltage to current, which is based on the instantaneous relationship rather than the average or effective value.

What the RC sine oscillator needs is ARC (automatic resistance control). This R is not allowed to be affected by the instantaneous value.

MrCU204

用二极管或稳压管反馈，不是线性反馈，但有类似于 贝克箝位 的「刹车」作用。

dyjccx

At the zero point, the diode is not conducting, high gain. The voltage exceeds the conduction voltage, and R3 is bypassed. Low gain, achieving limiting!

MrCU204

What I've been tinkering with recently is also related to this.

I found that placing a diode or a voltage regulator here also has a similar effect as "voltage stabilization".

That is, doing this will prevent the op amp from suddenly stopping operation and will not allow Vout to hit ±Vcc.