Checking the performance of operational amplifiers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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In amateur radio practice, it is often necessary to use op-amps extracted from old structures or printed circuit boards. As practice shows, it is not superfluous to check the microcircuits purchased on the radio market.
First test method based on the use of an op-amp as a voltage follower. Consider it on the example of the simplest op-amp with internal correction LM358N.
The connection of external pins is shown in Fig.1, and in Fig.2 - the testing scheme. To install the op-amp, a DIP-8 socket is used, but DIP-14 / I6 can also be used. All the details are attached to the socket with the shortest possible conclusions. Since one LM358N package contains two op-amps, the first one is checked first (pins 1, 2, 3). and then the second one (5, 6, 7). Capacitor C3 is mounted directly on the socket. Next, the test circuit in Fig. 2 is assembled, and power is supplied to it. Resistor R2 is used if the power supply used does not have protection current adjustment.
If it is, then R2 is not installed, but the PSU protection current is turned on for the value of the short-circuit current. 10...20 mA. A DC voltmeter PV with a limit of 20 V is connected to the output of the op-amp. In some cases, the elements R1, CI, C2 can be omitted. After switching on, we translate SA1 from one position to another and observe the voltmeter. If the op-amp is working, then in position "1" of the switch, the voltmeter should show almost the supply voltage, and in position "0" - close to zero.
Second test method is based on the switching circuit of the OS as a comparator, i.e. comparison of two voltages (Fig. 3). The installation of this circuit has the same requirements as the previous one. Using R1, the voltage is set to several volts, which is controlled by a high-resistance voltmeter PV1. Approximately the same voltage must be set with resistor R2, also controlled by high-resistance PV2.
The voltage at the output of the op-amp is controlled by a PV3 voltmeter, and for a serviceable op-amp, it will change abruptly from practically supplying to almost zero with a slight movement of the R1 engine in one direction or another. The values of the resistors R1, R2 can be chosen in the range from 10 kΩ to 1 MΩ, but they must be the same. Of course, it is not at all necessary to use three voltmeters in the considered circuit, it can be one, connected alternately at three points.
In conclusion, we note that the second scheme is more universal, since allows you to test OA that does not contain a built-in correction ("anti-excitation"), without installing the latter by external elements.
Author: V.Artemenko, UT5UDJ, Kyiv
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