[Repost] Analysis of the causes of op amp self-excitation

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[Repost] Analysis of the causes of op amp self-excitation [Copy link]

In my personal experience, the most effective method is:

1. Is the resistance of the amplifier circuit too large? For example, I have seen a circuit that amplifies 10 times, and the resistors are 100k and 1M. (Even textbooks say so). The intrinsic circuit current is too small, the anti-interference ability is weak, and the output signal is more easily coupled in.

2. When the output trace is too long, it is not only easy to self-excite, but also the signal is easily interfered. It is best to use shielding. If there is no condition, you can consider adding a small resistor in series with the output. Adding a small resistor in series does not completely solve the phase problem. It also solves the abnormal operation of the op amp caused by excessive capacitive load.

3. The amplification factor should not be too large. If the output signal is too strong and the input signal is too weak, it is easy to couple.

4. Transimpedance parallel capacitors, although theoretically effective, but the capacitor affects the output phase consistency, the capacitor discrete type is too large. In the scene that emphasizes phase consistency, the phase impact needs to be considered.

5. Grounding, an eternal topic.

Reasons for op amp oscillation and self-excitation:

1. The loop gain is greater than 1

2. The phase difference between the signals before and after feedback is more than 360 degrees, which means that positive feedback can be formed.

Refer to "Principles of Automatic Control" and "Circuit Design Based on Operational Amplifiers and Analog Integrated Circuits"

The main reason for self-oscillation is that the integrated operational amplifier is composed of multiple stages of DC amplifiers. Since the output of each stage of the amplifier and the input of the next stage of the amplifier have output impedance, input impedance and distributed capacitance, there is an RC phase shift network between the stages. When the signal passes through each RC network, an additional phase shift will be generated. In addition, the external bias resistor and input capacitance of the operational amplifier, the output resistance of the operational amplifier and the capacitive load feedback capacitance, as well as the common internal resistance of the power supply of the multi-stage operational amplifier, and even the distributed inductance on the power supply line, poor grounding and other couplings can all form additional phase shifts. As a result, the signal output by the operational amplifier is superimposed through the negative feedback loop to increase the additional phase shift to 180 degrees, and if the feedback amount is large enough, the negative feedback will eventually turn into positive feedback, thereby causing oscillation.

To be more specific,

1. The op amp may be caused by distributed capacitance and inductance

2. The op amp drives a capacitive load. 3. It may be caused by too deep feedback. Solution: 1. In-loop compensation. The feedback resistor of the op amp is connected in parallel with the feedback capacitor: 51, 51)]The small capacitor is called a phase-shifting capacitor. It is used to prevent the op amp from self-excitation. The value is usually from a few picofarads to tens of picofarads or hundreds of picofarads, depending on the operating frequency and the model of the op amp.

To put it simply, the larger the added capacitance, the narrower the bandwidth.

Prevent oscillation Rf and the input capacitance and stray capacitance of the op amp form a pole. If the pole is within the frequency range of the op amp, it may cause the op amp to oscillate. After adding Cf, Cf and Rf produce a zero point to offset the pole. Generally, the value of Cf>Ci, where Ci is the input capacitance of the op amp and the total stray capacitance of the input pin.

2.Out-of-loop compensation method, connect a small resistor in series at the output end of the op amp and then connect it to the next stage. It can be between a dozen ohms and several dozen ohms. The specific value is related to the input capacitance of the next stage circuit. You can try different resistance values to obtain stable output. "]1. The power supply should be stable. It is best to connect 0.1uf, 10uf and other capacitors in parallel

2. The amplification factor should not be too large, and the number of amplification levels should not exceed four levels

Before the experiment or test, if an oscilloscope is connected to the output of the op amp, sometimes a waveform with a higher frequency and close to a sine wave can be seen, and occasionally low-frequency oscillations may occur. Different methods can be used to solve the problem according to the principle of oscillation:

(1) Is the feedback polarity connected incorrectly or is the negative feedback too strong? If the negative feedback is connected incorrectly as positive feedback, oscillation is very likely to occur. In addition, the stronger the negative feedback, the more likely it is to self-excite.

(2) If a capacitive load is connected to the output end, the capacitive load increases the phase shift of the circuit, making it more prone to self-excitation. Another RC link can be used to compensate for the phase shift. If the compensation is done well, the self-oscillation will be eliminated.

(3) The stray capacitance of the wiring is too large. When the input circuit is high impedance, the lag link composed of the stray capacitance and resistance between the wiring and the ground or between the wiring will make the component unstable. For this reason, a capacitor CF can be connected in parallel across Rf (feedback resistor), or an RC branch can be connected in parallel at the input end of the op amp. Both of these links are of the nature of lead correction, that is, the phase lead effect they produce may offset the phase lag effect of the stray capacitance mentioned above, thereby stabilizing the op amp.

(4) Insufficient bypass measures for power supply wiring. The power supply lead wire not only has a certain resistance, but also a certain inductance and distributed capacitance. Therefore, when many op amps are connected to the same power line, these factors will affect each other. The solution is to connect a few tens of uF electrolytic capacitor and a 0.01uF ceramic capacitor in parallel between the positive and negative power terminals on the printed circuit board socket and the ground. If the op amp is used as a broadband amplifier, a capacitor with low inductance must be selected.

The old man in the research institute used to tell me: analog electronics is an art; some people also say: analog electronics is a magic trick that is difficult to grasp; in fact, I think: analog electronics is a gambling technique, just buying big or small. Discover the rules in the hands-on process, combine theory with practice, and develop in a spiral.

"](3) The stray capacitance of the wiring is too large. When the input circuit is high impedance, the lag link composed of the stray capacitance and resistance between the wiring and the ground or between the wiring will make the component unstable. For this reason, a capacitor CF can be connected in parallel across Rf (feedback resistor), or an RC branch can be connected in parallel at the input end of the op amp. Both of these links are of the nature of lead correction, that is, the phase lead effect they produce may offset the phase lag effect of the stray capacitance mentioned above, thereby stabilizing the op amp.

(4) Insufficient bypass measures for power supply wiring. The power supply lead wire not only has a certain resistance, but also a certain inductance and distributed capacitance. Therefore, when many op amps are connected to the same power line, these factors will affect each other. The solution is to connect a few tens of uF electrolytic capacitor and a 0.01uF ceramic capacitor in parallel between the positive and negative power terminals on the printed circuit board socket and the ground. If the op amp is used as a broadband amplifier, a capacitor with low inductance must be selected.

The old man in the research institute used to tell me: analog electronics is an art; some people also say: analog electronics is a magic trick that is difficult to grasp; in fact, I think: analog electronics is a gambling technique, just buying big or small. Discover the rules in the hands-on process, combine theory with practice, and develop in a spiral.

"](3) The stray capacitance of the wiring is too large. When the input circuit is high impedance, the lag link composed of the stray capacitance and resistance between the wiring and the ground or between the wiring will make the component unstable. For this reason, a capacitor CF can be connected in parallel across Rf (feedback resistor), or an RC branch can be connected in parallel at the input end of the op amp. Both of these links are of the nature of lead correction, that is, the phase lead effect they produce may offset the phase lag effect of the stray capacitance mentioned above, thereby stabilizing the op amp.

(4) Insufficient bypass measures for power supply wiring. The power supply lead wire not only has a certain resistance, but also a certain inductance and distributed capacitance. Therefore, when many op amps are connected to the same power line, these factors will affect each other. The solution is to connect a few tens of uF electrolytic capacitor and a 0.01uF ceramic capacitor in parallel between the positive and negative power terminals on the printed circuit board socket and the ground. If the op amp is used as a broadband amplifier, a capacitor with low inductance must be selected.

The old man in the research institute used to tell me: analog electronics is an art; some people also say: analog electronics is a magic trick that is difficult to grasp; in fact, I think: analog electronics is a gambling technique, just buying big or small. Discover the rules in the hands-on process, combine theory with practice, and develop in a spiral.

懒猫爱飞

It is difficult to master op amps. I am still learning. Thanks for sharing!