Maintenance skills of printed circuit board: Test method of operational amplifier

Whether an operational amplifier is good or not? This is really not an easy question for quite a lot of electronics repairmen to answer in judgment. Ideal operational amplifier has the features of virtual short and virtual fault, which are very useful for analyzing the operational amplifier circuit under linear application. To guarantee its linear application, operational amplifier must work under closed loop (negative feedback) only. If there is no negative feedback, the operational amplifier under open-loop amplification will work as a comparator. To judge whether an operational amplifier works, it’s better to first determine whether it is used as an amplifier or a comparator in the circuit. 

No matter the kind of amplifier, there will be a feedback resistor Rf. During service, we can check this feedback resistor in circuits by detecting the resistance between output terminal and reverse input terminal with a multimeter. If the detected resistance is too big, for example several MΩ or above, we can generally determine that the amplifier is used as comparator, but if this value is very small, for example 0Ω to tens of kΩ, check whether there exists resistance between output terminal and reverse input terminal and if yes, we can determine that the amplifier is used as amplifier. 

According to the principle of “virtual short” of amplifier, if the operational amplifier works normally, the voltage of its codirectional input terminal must be equal to that of the reverse input terminal; even if some difference exists, the difference will be at mv level. Of course, in some high input impedance circuits, the internal resistance of multimeter may have a little impacts on voltage test, but the value of impact does not exceed 0.2V generally; if the difference is over 0.5V, it can be certain that amplifier is damaged. If the device is used as comparator, it’s allowed that the voltage of codirectional input terminal differs from that of reverse input terminal. 

If codirectional voltage > reverse voltage, output voltage is close to positive maximum value. 

If codirectional voltage < reverse voltage, output voltage is close to 0V or negative maximum value (depending on power supply type - single power supply or dual power supplies).

If the detected voltage does not conform to this rule, the device must be damaged!

In this way, you do not have to use the substitution method; you can judge whether an operational amplifier is good or not without having to remove the chip of printed circuit board.