There are many reasons for the self-excitation of the op amp.

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There are many reasons for the self-excitation of the op amp. [Copy link]

1. Insufficient compensation. For example, in order to improve the high-frequency response of op amps such as OP37 during design, the compensation amount is small, and self-excitation will occur when the feedback is deep. By measuring the BODE diagram of its open-loop response, it can be seen that with the increase of frequency, the open-loop gain of the op amp increases. The gain will decrease. If the phase lag exceeds 180 degrees before the gain drops to 0db, the closed loop will inevitably self-excite. 2. Power supply feedback self-excitement. From the analysis of the internal structure of the operational amplifier, it is a multi-stage amplifier circuit. Generally, operational amplifiers are composed of more than 3 stages of circuits. The front stage completes the high gain. The second stage completes the phase compensation function and the final stage realizes power amplification. If the internal resistance of the power supply to the operational amplifier is large, the power consumption of the final stage will cause power supply fluctuations. This fluctuation will affect the operation of the previous stage circuit and be amplified by the previous stage, causing greater fluctuations in the subsequent stage circuit. This vicious cycle will lead to self-excitation. 3. External interference. To be exact, this is not self-excitation, but the phenomenon is similar to self-excitation. The output generates a signal that has nothing to do with the input. Because we are in an environment covered by electromagnetic waves, there are 50Hz and 100Hz power frequency interference, hundreds of Hz medium wave broadcast interference, and several MHz short wave interference. Interference, TV broadcast and FM broadcast interference of tens to hundreds of Hz, wireless communication interference around 1GHz, etc. If the circuit design is not well shielded, interference will naturally be introduced into the circuit and amplified. If the circuit has self-excitation, we should first determine what caused it. The first kind of self-excitation occurs when the operational amplifier is used in a closed loop and the gain is low. Generally, it only occurs when the gain is less than 10. In fact, this kind of self-excitation is the easiest to solve. Just choose the right operational amplifier. For some high-speed operational amplifiers, the manufacturer's manual will indicate the lowest closed-loop gain. SimSun]gain. On the contrary, the latter two situations all occur under high gain conditions, which is very important and can accurately determine the cause of self-excitation. Relatively speaking, the latter two self-excitations are more difficult to solve. I am not modest to say thatOnly with a certain amount of analog circuit design experience can the above situation be avoided. The basic principle is to increase the area of the ground line as much as possible, and to add high-frequency decoupling capacitors near the power supply pins of the operational amplifier, and to use high-frequency shielding and other methods to eliminate self-excitation and reduce interference.