How to Judge a Operational Amplifier
For a considerable number of electronic repairers, it is not easy to judge if an operational amplifier is good or bad. Ideal op amps have "short" and "imaginary" characteristics that are useful for analyzing linear op amp circuits. To ensure linearity, the op amp must be operated 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 identify the device in the circuit is used as an amplifier or comparator.
No matter what type of amplifier, there is a feedback resistor Rf. We can check this feedback resistor from the circuit during maintenance and check the resistance between the output and the inverting input with a multimeter. If the resistance is extremely large like several MΩ above, we probably can be sure that the device is used as a comparator. If this resistance is small, about 0Ω to tens of kΩ, you need to check whether the resistor is connected between the output and the inverting input, if so, it is used as an amplifier.
According to the principle of short virtual amplifier, which means that if the op amp is working properly, the same input and reverse input voltage must be equal, even if the difference is mv level. Of course, in some high input impedance circuits, the internal resistance of the multimeter will have a slight effect on the voltage test, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, it must used as the amplifier.
If the device is used as a comparator, then allow the voltage of same input and reverse input are different. In the case of the same voltage> reverse voltage, the output voltage approaches the positive maximum; if the same voltage< reverse voltage, the output voltage approaches 0V or negative maximum (depending on the dual power supply or the single power supply). If the detected voltage does not comply with this rule, then the device is broken undoubtedly.