Practical method of making RC sine wave oscillator

1. Oscillator composition

The oscillation circuit is mainly composed of three parts: the basic amplifier circuit, the frequency selection network and the positive feedback network. The basic amplifier circuit is to make the circuit obtain an output of a certain amplitude; the frequency selection network is to determine the oscillation frequency of the circuit to ensure that the circuit produces a sinusoidal oscillation; the role of the positive feedback network is to introduce a positive feedback signal as an input signal in the oscillation circuit when there is no input signal. In the figure below, the circuit introduces positive feedback, the upper box is the amplifier circuit, the lower box is the feedback network, and the feedback polarity is positive (in the figure, the feedback amount is the net input amount.)

Sine wave generation conditions:

The principle of RC bridge sine wave oscillation circuit is as follows:

As shown in the figure below. Due to the difference between theory and practice, the circuit diagram shown in the figure above is not ideal in practice, such as low oscillation frequency and oscillation stop, especially when using LM324 to make an oscillator, the waveform is seriously distorted. So, how to make improvements?

2. Common problems and their solutions

1. Amplitude stabilization. Since Uo and Uf have a good linear relationship, in order to stabilize the amplitude of the output voltage, a nonlinear link is generally added to the circuit. Here, two reverse parallel diodes are connected in series in the loop. The characteristic that the dynamic resistance of the diode decreases when the current increases is used to add a nonlinear link, thereby stabilizing the output voltage.

2. "Spiky" distortion, the waveform is shown in Figure 4. This distortion is a difficult problem that cannot be avoided when using the integrated operational amplifier LM324 to make a sine wave oscillator. A simple and effective solution is to connect a resistor of appropriate resistance between the output terminal and the negative power supply Vee, which can improve the distortion of the output waveform. And the amplitude of the signal is basically stable as the frequency changes.

3. Clipping distortion, the waveform is shown in the figure below. The obvious feature of this distortion is that the top of the waveform becomes flat, and the amplitude of the waveform is large, close to the power supply voltage. The reason for this distortion is mostly that the feedback resistance value is too large, which makes the circuit gain too large, resulting in too large output voltage peak. In severe cases, as the feedback resistance value increases, the output waveform will become very much like a square wave. The solution to this distortion is to reduce the total resistance of the feedback network. However, excessive reduction will prevent the circuit from oscillating, so its size is very critical. If the resistance value is uncertain, a potentiometer can be used instead. By fine-tuning the potentiometer, the waveform can be adjusted to the best effect.

4. Vibration stop phenomenon. In actual production, due to the quality and precision problems of the components themselves, the production effect of the oscillator will be greatly reduced. In the circuit, we need to adjust the coaxial dual potentiometer to change the frequency of the output sine wave. As the name implies, the dual coaxial potentiometer is composed of two potentiometers, which achieve the purpose of synchronously adjusting the two resistance values ​​by adjusting the same axis. But in practice, we found that the two resistance values ​​of the dual coaxial potentiometer cannot always remain equal. Instead, there is a difference, and sometimes this difference is very large, up to several thousand ohms. The existence of the difference causes the oscillator to stop vibrating at high frequencies. In other words, the output signal of the oscillator cannot reach a higher frequency. In this case,

Of course, the problem can be solved by replacing the double coaxial potentiometer with better accuracy and quality. However, in order to save costs, it is found in practice that if two small resistors are connected in series with the two variable resistors of the double coaxial potentiometer, the oscillation stop problem can be well solved, thereby significantly improving the frequency of the oscillator!

After improving the schematic, we get the oscillator circuit shown in the figure below. You can compare it with the figure above to see the changes.

3. Circuit Improvement

1. Increase the output voltage amplitude. This part of the circuit is shown in the dotted box in the figure below. Since the gain bandwidth of LM324 is a certain value, it is about 1.5MHz according to the user manual. Therefore, if the gain is to be increased, the bandwidth will be reduced. So, how can we achieve both a high amplification factor and a sufficiently wide bandwidth? Here we can use a two-stage amplifier to achieve this, and both stages use inverting amplifiers. There are fewer components to be welded, and the production process is relatively simple. It should be noted that, first, the amplification factor of the pre-amplifier should generally be smaller than the amplification factor of the second stage, otherwise the above-mentioned clipping distortion is likely to occur. The author's two-stage amplification factors are designed to be about 18 and 48 times respectively. The total amplification factor is about 864 times. Second. The input resistance should be selected appropriately. It should not be too small (tens of ohms) or too large (several megohms). The general selection method is that the sum of the input resistance and the feedback resistance is tens of kiloohms or hundreds of kiloohms. Third. It is best to use a large capacitor between each stage of the operational amplifier, generally an electrolytic capacitor. Its function is to isolate DC and couple. In addition, the output signal of the oscillation circuit shown in Figure 6 is generally not directly connected to the amplifier, but a potentiometer is used to adjust the output signal amplitude first, and then connected to the amplifier. Because when the input signal of the amplifier is too large, the output signal may also be clipped and distorted. The specific connection method can refer to R' in the figure below.

2. Improve the load capacity. Since the output current of LM324 is limited, generally only tens of milliamperes. Under the condition of constant current. In order to increase the output power of the circuit, an effective way is to reduce the output impedance of the circuit. To reduce the output impedance, a simple way is to use a voltage follower. Because the characteristics of the voltage follower are large input impedance and small output impedance, it can play the role of impedance transformation and isolation. It is very easy to make a voltage follower with LM324.

3. Use a single power supply. LM324 is usually powered by a dual power supply, in which case its 4th pin is connected to the positive power supply (where the positive power supply voltage is +12V). The 11th pin is connected to the negative power supply Vee (-12V).

If you want to use a single power supply, the principle is: the place where the circuit was originally connected to the positive power supply is still connected to the positive power supply. The place where it was originally grounded is changed to 1/2VCC. The place where it was originally connected to the negative power supply is changed to ground. Taking this circuit as an example, the 4th pin of LM324 is still connected to +12V, and the 11th pin is grounded. All the places where it was originally grounded in the circuit are changed to +6V, of which +6v can be obtained by dividing the +12V voltage. In this way, the negative power supply is omitted, thus realizing single power supply.

4. Welding and debugging

Connect the various parts of the circuit to get the final complete circuit diagram, as shown in the figure below, in which the parameter values ​​of each component are marked.

In the figure below, R is a 20kΩ dual coaxial potentiometer. The input signal amplitude of the amplifier is adjusted, and the two R7s complete the voltage division, making the potential of point A +6V.

After soldering the circuit as shown below, check and power on, adjust the value of potentiometer R3 to be slightly larger than 1k, and adjust R' to a smaller value so that the output of the amplifier has no clipping distortion. The output from Uo is a sine wave with a frequency range of 362Hz to 102kHz, with no distortion or oscillation, and the amplitude is basically stable: the amplification factor is about 700 times, and the output voltage amplitude can be adjusted arbitrarily within the range of OV to 5V by adjusting potentiometer R14