 # How to judge whether the operational amplifier is good or bad?

The ideal operational amplifier has the characteristics of “virtual short” and “virtual break”. These two characteristics are very helpful for analyzing linear operational amplifier circuits. In order to make sure the linear circuit operates, the operational amplifier must work with negative feedback in the close-loop state (if there is no negative feedback, the operational amplifier under open-loop amplification becomes a comparator). Therefore, to judge the quality of the device, we should first distinguish whether the operational amplifier in the circuit is used as an amplifier or as a comparator.

No matter what types the amplifiers are, they all have a feedback resistor, and they can be detected in the circuit. The resistance value between the output terminal and the invert output terminal can be detected by a multimeter. If the value is too big, such as several MΩ, it may be used as a comparator. If the value is too small (0Ω~several dozen kΩ), and the resistor is connected between the output terminal and the invert input terminal, it must be used as a comparator.

According to the principle of “virtual short” of the operational amplifier, if the op-amp operates as a linear circuit, the voltage of its non-inverting terminal and invert terminal must be the same. Even there is a difference, it must be the degree of mv. Of course, in some high-input resistance circuits, the inner resistance of the multimeter will affect the voltage test, but the voltage value will not change beyond 0.2V. If the value difference is 0.5V, the component must be broken.

If the operational amplifier is used as a comparator, the voltage of the invert input terminal and the non-inverting input terminal can not be the same. When the non-inverting voltage is big than the invert input terminal, the output voltage is close to the positive maximum. When the non-inverting voltage is smaller than the inverted voltage, the output voltage is close to 0V or negative maximum. If the voltage detected does not comply with the rule, the component must be broken.