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It’s difficult to judge the quality of operational amplifiers for quite a lot of electronic repairers, not only because of their educational level (there are many undergraduates undergraduates. The same is true for graduate students whose tutors are learning inverter control!), I would like to discuss with you here, and I hope it will be helpful to everyone.
The ideal operational amplifier has the characteristics of "virtual short" and "virtual break", these two characteristics are very useful for analyzing the linear application of the operational amplifier circuit. In order to ensure linear application, the op amp must work in a closed loop (negative feedback). If there is no negative feedback, the op amp under open-loop amplification becomes a comparator. If you want to judge the quality of the device, you should first distinguish whether the device is used as an amplifier or as a comparator in the PCB board.
No matter what type of amplifier, there is a feedback resistance Rf. You can check this feedback resistance from the circuit during maintenance. Use a multimeter to check the resistance between the output terminal and the reverse input terminal. MΩ or more, you can be sure that the device is used as a comparator. If the resistance is less than 0Ω to several tens of kΩ, then check whether there is a resistance connected between the output terminal and the reverse input terminal. For amplifier.
According to the principle of virtual shortness of the amplifier, that is to say, if the operational amplifier works normally, the voltage at the same direction input terminal and the reverse input terminal must be equal, and even if there is a difference, it is at the mv level. Of course, in some high input impedance circuits, a multimeter The internal resistance will affect the voltage test a bit, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier will undoubtedly be broken! (FLUKE179 multimeter used here)
The above is the introduction of the judging method of operational amplifier. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.
If the device is used as a comparator, the same direction input terminal and the reverse input terminal are allowed to be different, the same direction voltage> reverse voltage, the output voltage is close to the positive maximum value; the same direction voltage
In this way, it is not necessary to use the substitution method, and it is possible to judge whether the operational amplifier is good or bad without removing the chip on the PCB board.
