Identify Whether an Op Amp is Bad or Not

by Hugh S. Electronic Parts

1 Introduction

Operational amplifier is the most common simulator, which is widely used in signal transformation and processing, ADC sampling front-end, power supply circuit and so on. Therefore, in the process of use, how to judge its quality is very important. This paper will introduce how to judge the quality of operational amplifier and what to pay attention to in the process of use to prevent its damage.

2 Virtual Short and Virtual Break

The ideal operational amplifier has the characteristics of "virtual short" and "virtual break". These two characteristics are very useful for analyzing linear operational amplifier circuits. In order to ensure linear application, the op amp must work with negative feedback in the closed loop state (if there is no negative feedback, the op amp under open loop amplification becomes a comparator). Therefore, to judge the quality of the device, we should first distinguish whether the op amp is used as an amplifier or as a comparator in circuits.

No matter what type of amplifier, there is a feedback resistance Rf. You can check this feedback resistance from the circuit. For example, use a multimeter to check the resistance between the output terminal and the reverse input terminal. If it is too large, such as a few MΩ or more, then it is probably certain that the op amp is used as a comparator. If the resistance is small (0Ω to tens of kΩ), then check to see if there is a resistor connected between the output terminal and the inverting input terminal. If so, it must be used as an amplifier use.

According to the principle of virtual short, if the op amp works linearly, the voltage at the non-inverting input terminal and the inverting input terminal must be equal, and even if there is a difference, it is mv level. In addition, in some circuits with high input impedance, the internal resistance of the multimeter will affect the voltage test result, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier undoubtedly be broken.

If the op amp is used as a comparator, the voltages of the non-inverting input terminal and the inverting input terminal are allowed to be unequal. When the non-inverting voltage large than the inverting voltage, the output voltage is close to the maximum forward value. If, instead, then the output voltage is close to 0V or the negative maximum value (consider dual power supply or single power supply). If the detected result is not met one of above-mentioned rules, the device must be damaged.

In this way, it is not necessary to use the substitution method, or to remove the chip on the circuit board to judge the quality of the operational amplifier.

3 Op Amp ICs Checking Rules

In many instruments and meters, there are integrated operational amplifiers for small signal amplification. It is more prone to damage than other integrated blocks and components. Many integrated blocks in more complex instruments are directly soldered on the PCB, then it is inserted into the socket, and the distance between the circuit boards is also very close. So it is very difficult to directly measure the voltage. Here are some simple and practical methods for judging integrated op amp blocks.

1) Touch the temperature of the integrated op amp block after powering on for a period of time. If the temperature is greater than 50°C, you should suspect whether it is damaged.

2) If possible, measure the DC current and it should be within a few milliamperes. Otherwise it is damaged.

3) The integrated op amp has a single op amp and several op amps. You should be familiar with its power supply and pins. There are also certain rules to find out from the relevant manuals. The resistance between the input pins of the integrated operational amplifier should be relatively large (generally greater than 10M ohms). When the measured value is small, observe that whether there is an infinite diode at the input end, otherwise it will be damaged.

4) The input voltage for online measurement should be relatively small, generally below 0.1V. Touch the input terminal with your hand, the voltage should change at the output terminal, or touch the two ends of the feedback resistor (equivalent to a resistor in parallel), and the output will change accordingly. Of course, it is also possible to directly connect the corresponding feedback resistors in parallel.

4 Op Amp Using Tips

In order to prevent damage to the op amp, the following points should be noted:

1) The input voltage cannot exceed the rated value.

2) Do not connect capacitors directly at the output of the op amp.

3) Do not connect capacitors in parallel in the feedback loop of the amplifier circuit.

4) Pay attention to the output swing of the op amp.

5) Pay attention to the wiring of the feedback loop.

6) Pay attention to the filtering of power supply.