How to test an operational amplifier with a multimeter. Operational Amplifier Test Methods

In amateur radio practice, it is often necessary to use op-amps extracted from old designs or printed circuit boards. As practice shows, it is not superfluous to check microcircuits purchased on the radio market.
First method testing is based on the use of an op-amp as a voltage follower. Let's consider it using the example of the simplest op-amp with internal correction LM358N.

Connection of external pins is shown in Fig. 1 and in Fig. 2 - testing scheme. To install the op-amp, a DIP-8 socket is used, but DIP-14/I6 can also be used. All parts are attached to the socket with the shortest leads possible. Since one LM358N package contains two op-amps, the first one is checked initially (pins 1, 2, 3). and then the second (5, 6, 7). The SZ capacitor is mounted directly on the socket. Next, assemble the test circuit in Fig. 2 and supply power to it. Resistor R2 is used if the power supply used does not have protection current regulation.

TO ENLARGE (REDUCE) THE DIAGRAM, CLICK ON THE PICTURE

If it is, then R2 is not installed, but the power supply protection current is switched on to the importance of the short-circuit current. 10...20 mA. A voltmeter is connected to the op-amp output DC voltage PV with a limit of 20 V. In some cases, elements R1, CI, C2 may not be installed. After turning on, we move SA1 from one position to another and observe the voltmeter. If the op-amp is working properly, then in position “1” of the switch the voltmeter should show almost the supply voltage, and in position “O” - close to zero.
Second method testing is based on the switching circuit of the op-amp as a comparator, i.e. comparison of two voltages (Fig. 3). The installation requirements for this circuit are the same as the previous one. Using R1, a voltage of several volts is set, which is controlled by a high-resistance voltmeter PV1. Approximately the same voltage must be set with resistor R2, which is also controlled by high-resistance PV2.

TO ENLARGE (REDUCE) THE DIAGRAM, CLICK ON THE PICTURE

The voltage at the output of the op-amp is controlled by a voltmeter PV3, and for a working 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 resistors R1, R2 can be chosen in any range from 10 kOhm to 1 MOhm, 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, because allows you to test op-amps that do not contain built-in correction (“anti-excitation”), without installing the latter with external elements.

Vladislav Artemenko, UT5UDJ, Kyiv

Operational amplifier (op-amp) English. Operational Amplifier (OpAmp), popularly known as an operational amplifier, is a direct current amplifier (DCA) with a very high gain. The phrase "DC amplifier" does not mean that the operational amplifier can only amplify D.C.. This means, starting from a frequency of zero Hertz, and this is direct current.

The term “operational” has been strengthened for a long time, since the first samples of op-amps were used for various mathematical operations such as integration, differentiation, summation, etc. The gain factor of an op-amp depends on its type, purpose, structure and can exceed 1 million!

Operational amplifier circuit

In the diagrams, the operational amplifier is designated like this:

or so

Most often, op-amps on diagrams are indicated without power pins

An input with a plus sign is called non-inverting, and an input with a minus sign is called inverting. Do not confuse these two signs with power polarity! They do NOT say that it is necessary to supply a signal with negative polarity to the inverting input, but to a non-inverting signal with positive polarity, and then you will understand why.

Op amp power supply

If the power pins are not indicated, then it is assumed that the op-amp receives bipolar power supply +E and -E Volts. It is also labeled +U and -U, V CC and V EE, Vc and V E. Most often it is +15 and -15 Volts. Bipolar nutrition is also called bipolar nutrition. How do you understand this - bipolar nutrition?

Let's imagine a battery

I think you all know that a battery has a “plus” and a “minus”. In this case, the “minus” of the battery is taken as zero, and the batteries are counted relative to zero. In our case, the battery voltage is 1.5 Volts.

Let's take another such battery and connect them in series:

So, our total voltage will be 3 Volts, if we take the minus of the first battery as zero.

But what if you take the minus of the second battery to zero and measure all the voltages relative to it?

This is where we got bipolar power supply.

Ideal and real operational amplifier model

In order to understand the essence of the operation of the op-amp, consider it perfect And real models.

1) the ideal op-amp is infinitely large.

In real op-amps, the value of the input resistance depends on the purpose of the op-amp (universal, video, precision, etc.), the type of transistors used and the circuit design of the input stage and can range from hundreds of ohms to tens of megohms. A typical value for general purpose op amps is a few megohms.

2) The second rule follows from the first rule. Since the input impedance of an ideal op-amp is infinitely large, the input impedance will be zero.

In fact, this assumption is quite valid for op-amps with input currents, whose input currents can be less than picoamps. But there is also an op-amp with an input. Here the input current can already be tens of microamps.

3) The output impedance of an ideal op-amp is zero.

This means that the voltage at the output of the op-amp will not change when the load current changes. In real general use op amps it is tens of ohms (usually 50 ohms).
In addition, the output impedance depends on the signal frequency.

4) The gain in an ideal op-amp is infinitely large. In reality, it is limited by the internal circuitry of the op-amp, and output voltage limited by supply voltage.

5) Since the gain is infinitely large, therefore, the voltage difference between the inputs of an ideal op-amp is zero. Otherwise, even if the potential of one input is greater or less than at least the charge of one electron, then the output will have an infinitely large potential.

6) The gain in an ideal op-amp does not depend on the signal frequency and is constant at all frequencies. In real op-amps this condition is satisfied only for low frequencies to any cutoff frequency, which is individual for each op-amp. Typically, the cutoff frequency is taken to be a gain drop of 3 dB or up to 0.7 of the gain at zero frequency (DC).

The circuit of the simplest op-amp using transistors looks something like this:

Operating principle of an operational amplifier

Let's look at how op-amps work

The operating principle of the op-amp is very simple. It compares the two voltages and produces a negative or positive supply potential at the output. It all depends on which input has the greatest potential. If the potential at the NON-inverting input U1 is greater than at the inverting U2, then the output will be +Upit, but if the potential at the inverting input U2 is greater than at the NON-inverting U1, then the output will be -Upit. That's the whole principle ;-).

Let's look at this principle in the Proteus simulator. To do this, we will select the simplest and most common operational amplifier LM358 (analogs 1040UD1, 1053UD2, 1401UD5) and assemble a primitive circuit showing the principle of operation

Let's apply 2 Volts to the non-inverting input, and 1 Volt to the inverting input. Since the potential is greater at the non-inverting input, therefore, at the output we should get +Upit. We got 13.5 Volts, which is close to this value

But why not 15 Volts? The internal circuitry of the op-amp itself is to blame for everything. The maximum value of the op-amp may not always be equal to the positive or negative supply voltage. It can deviate from 0.5 to 1.5 Volts depending on the type of op-amp.

But, as they say, every family has its blacks, and therefore op-amps have long appeared on the market that can produce an acceptable supply voltage at the output, that is, in our case, these are values close to +15 and -15 Volts. This feature is called Rail-to-Rail, which is literally translated from English. “from rail to rail”, and in the language of electronics “from one power bus to the other”.

Let's now apply a potential greater to the inverting input than to the non-inverting input. We apply 2 Volts to the inverting one, and 1 Volt to the non-inverting one:

As you can see, in this moment the output went to -Upit, since the potential at the inverting input was greater than at the non-inverting input.

So as not to download again software package Proteus, you can simulate the operation of an ideal op-amp online using the Falstad program. To do this, select the Circuits—Op-Amps—>OpAmp tab. As a result, the following diagram will appear on your screen:

On the right control panel you will see sliders for adding voltage to the inputs of the op-amp and you can already visually see what happens at the output of the op-amp when the voltage at the inputs changes.

So, we have considered the case when the voltage at the inputs may differ. But what happens if they are equal? What will Proteus show us in this case? Hmm, showed +Upit.

What will Falstad show? Zero Volt.

Who to believe? No one! In real life, it is impossible to do this in order to drive absolutely equal voltages to the two inputs. Therefore, such a state of the op-amp will be unstable and the output values can take values of either -E Volt or +E Volt.

Let's serve sine wave with an amplitude of 1 Volt and a frequency of 1 kilohertz to the non-inverting input, and put the inverting input to ground, that is, to zero.

Let's see what we have on the virtual oscilloscope:

What can be said in this case? When the sinusoidal signal is in the negative region, we have -Upit at the output of the op-amp, and when the sinusoidal signal is in the positive region, then we have +Upit at the output. Also note that the voltage at the output of the op-amp cannot change its value suddenly. Therefore, the op-amp has such a parameter as the rate of rise of the output voltage V Uout .

This parameter shows how quickly the op-amp's output voltage can change when operating in pulsed circuits. Measured in Volts/sec. Well, as you understand, the higher the value of this parameter, the better the op-amp behaves in pulsed circuits. For LM358 this parameter is 0.6 V/µs.

With input from Jeer

Hi all. Today I bring to your attention a short note on purchasing the OPA627U.
Wandering around ebay and asking the price for high-quality op-amps, I came across fairly cheap OPA627U (2 pcs/lot), in used condition.
Since this is a popular and at the same time expensive op-amp, the Chinese do not hesitate to counterfeit it. Here's an example of a situation like this:

In this regard, it is scary to take expensive components in such places, be it an op-amp or, for example, a powerful driver for Mosfet (tested from my own negative experience).

At the same time, sellers either sell op-amps for next to nothing (99% of them are fake) or very expensively (then what’s the point of buying from them if the offline price is about the same?). It’s better to remain silent about Aliexpress... Although you will win the dispute, you will waste time.

The price for a new op-amp, in reliable stores, is about $25 apiece: here, two for $6.5 (delivery fee is $4).

The subject attracted me because it seemed to be used, and at the same time the seller had quite a lot of orders without negative reviews.
The seller sends two op-amps at once, which is very convenient. Apparently, he is already running out of them.

So, what did they send?(sorry for the poor photo quality) :

As far as you can see, the op-amp is really used, at least soldered (by the way, it’s difficult to notice by eye), but in very good condition. As far as I understand, the year of manufacture is 2000.

OS check.

In search of information about checking the originality of such op-amps, I came across the following topic from vegalab:

Probably the most correct way to check here would be to test for the declared noise using an oscilloscope (as far as I understand, taking into account power supply noise). Unfortunately, I don't have such an opportunity yet.
As a result, I checked the resistance between legs 1 and 5 of the microcircuit, on each op-amp, this is what happened:

As you can see, the resistance is around 50 kOhm, supposedly original).

I checked the op-amp data, it works fine. I won’t write about audio tests so as not to stir up controversy, and I haven’t had time to seriously test them yet, I just checked their performance.

In addition, I’m still waiting for adapters for them (to DIP8): in order to drive this vaunted op-amp in various tests, specifically when listening to music.

I hope this note will help those who were looking for this op-amp for reasonable money, since the subject is similar to the original.

I'm planning to buy +13 Add to favorites I liked the review +26 +42

There is a wide variety of these microcircuits, and they are incompatible with each other in terms of pin locations. These microcircuits can be checked by setting the operating mode, which can be done on a stand specially assembled for a particular case, where the microcircuit is connected using a universal contact socket, or the test can be carried out already as part of a circuit assembled on them. The second is more convenient, as it requires less time.

Now about the verification itself. First of all, you need to measure the levels of supply voltages, voltages at the inputs of the microcircuit, as well as at the output (with a digital voltmeter). Usually, if the values of the negative feedback resistors are known, then by calculating the gain, you can draw conclusions about what should be at the output and with what sign, of course, if it is a linear amplifier.

Doubts may arise when checking more complex circuits (integrators, autogenerators, etc.). In this case, you can use another method. As you know, any operational amplifier can easily be made to work in comparator mode. To do this, we can temporarily apply alternately to the direct and inverse inputs of the microcircuit from external source a small voltage through a current-limiting resistor (Fig. 6.17). The voltage at the output of the “op-amp” must be monitored with a digital voltmeter or oscilloscope (if normal operation we will see the output switching).

Rice. 6.17. Principle of testing operational amplifiers

An oscilloscope is more convenient for carrying out such measurements, since it makes it possible to detect not only changes in output levels, but also the presence of unintended self-excitation of cascades (self-generation).

Source: Radio amateurs: useful diagrams. Book 6. - M / SOLON-Press, 2005. 240 p.

In table 1.4 and 1.5 present the electrical characteristics of solar modules and batteries. Table 1.4. Electrical characteristics solar modules of domestic production FSM-50 50 21 2.95 10720 FSM-55 55 21 3.15 1028x450x28…….

In a working element, during continuity between the power circuits there should be an infinitely large resistance, and between the control electrode and one of the terminals (the cathode of the thyristor) there should be a small resistance (from 30 to .......

To quickly tighten small screws in hard-to-reach places (and not lose them when unscrewing), you will need a screwdriver with a magnet. It is easy to make such a screwdriver from an ordinary one. It is enough to wrap 100-200...... around the screwdriver rod.

Be careful to protect yourself from electric shock or disconnection of connectors and parts. washing machine when troubleshooting. Observe electrical safety measures! First of all, check the connections of each connector. If you are replacing PWB…….

remote control(Remote control) 90% of remote controls are occupied by two types of defects: 1) some buttons do not work (usually those that are frequently pressed). In this case, you need to cut out a piece of foil and…….

When setting up various circuits using operational amplifiers (op-amps), there is a need to first, before installing it on the board, check the operation of the op-amp according to the pass/fail principle. As can be seen from the diagram shown in Fig. 1, the op-amp under test is connected by a voltage follower, the non-inverting input of which is supplied with voltage from the output of the amplifier through a three-link chain RC quadripole with zero phase shift.

This generator produces oscillations close in shape to rectangular. Since the output current of a widely used op-amp is usually insufficient for the LEDs to glow brightly, a current amplifier is switched on at the output of the op-amp, made according to a two-pin circuit using transistors with different types conductivity. When the op-amp is working, the LEDs light up alternately. If one of the LEDs is on, then the amplifier is unusable.
To power the circuit, a bipolar power supply with a rated voltage for most types of op amps is used.
In this circuit, you can check the operational amplifiers KR140UD608 (K140UD6), KR140UD708 (K140UD7), K140UD18, K544UD1, as well as others, taking into account their pinout and, if necessary, frequency correction circuits.
The testing circuit can be mounted on a printed circuit board made of single-sided foil fiberglass with dimensions of 55x42.5 mm and a thickness of 1.5 mm. The location of conductive paths on the board and radio elements is shown in Fig. 2. Passive elements of any type, for example, capacitors C1.C3 type K73-17, capacitors C4, C5 type K10-17, resistors with a power of 0.125 or 0.25 W. LEDs of any type of visible radiation with any glow color. The brightness of the glow is set by selecting R4.
This circuit can be used as a “flasher” even with incandescent lamps.
To do this, if necessary, transistors VT1, VT2 are made as composites and the supply voltage is selected for the type of lamps used.
Literature RADIOAMATOR 6.2000

Similar articles

Random articles

05.10.2014
This preamplifier is simple and has good parameters. This circuit is based on the TCA5550, containing a dual amplifier and outputs for volume control and equalization, treble, bass, volume, balance. The circuit consumes very little current. Regulators must be located as close to the chip as possible to reduce interference, interference and noise. Element base R1-2-3-4=100 Kohms C3-4=100nF …
16.11.2014
The figure shows the circuit of a simple 2-watt amplifier (stereo). The circuit is easy to assemble and has a low cost. Supply voltage 12 V. Load resistance 8 Ohms. Amplifier Circuit Figure printed circuit board(stereo)
20.09.2014
Its meaning is different for different hard drive models. Unlike high-level formatting - creating partitions and file structures, low-level formatting means basic layout of disk surfaces. For early model hard drives that were supplied with clean surfaces, such formatting creates only information sectors and can be performed by the hard drive controller under the control of the appropriate program. ...