Operational Amplifier PCB Board Repairing can be categoried as below steps:
1st of all, judging the quality of op amps is quite difficult for quite a few electronic maintainers. Ideal op amps have “virtual short” and “virtual break” characteristics. These two characteristics are very useful for analyzing operational amplifier circuits that are used linearly. To ensure linear operation, the op amp must work in a closed loop (negative feedback). Without 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 a comparator in the circuit.
Nno matter what type of amplifier, there is a feedback resistance Rf, then we can check this feedback resistance from the circuit during circuit board maintenance, and use a multimeter to check the resistance between the output and the reverse input If the value is too large, such as a few MΩ, then we can be sure that the device is used as a comparator. If the resistance is smaller than 0Ω to tens of kΩ, then check if there is a resistor connected to the output and the reverse input. Between the ends, some of them must be used as amplifiers.
According to the principle of the amplifier’s virtual short, that is to say, if this op amp works normally, its voltage at the same input and reverse input must be equal, even if there is a difference, it is of mv level. Of course, in some high input impedance circuits, The internal resistance will have a little impact on the voltage test, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier will definitely be bad! (I use the FLUKE179 multimeter)
If the device is used as a comparator, the non-inverting input terminal and the inverting input terminal are allowed to be different. In the same direction voltage> reverse voltage, the output voltage is close to the maximum positive value; in the same direction voltage <reverse voltage, the output voltage is close to 0V or negative maximum (depending on dual or single supply). If it is detected that the voltage does not meet this rule, the device will definitely be bad! This way you don’t have to use the replacement method, you can judge the quality of the operational amplifier without removing the chip on the circuit board.
