Voltmeter to make an operational amplifier. AC rms voltmeter. Automotive voltmeter on LM3914 chip

High accuracy of HF voltage measurements (up to the third or fourth digit) is, in fact, not needed in amateur radio practice. The quality component is more important (the presence of a signal is sufficient high level- the more, the better). Typically, when measuring an RF signal at the output of a local oscillator (oscillator), this value does not exceed 1.5 - 2 volts, and the circuit itself is adjusted to resonance according to the maximum RF voltage value. When adjusted in the IF paths, the signal increases step by step from units to hundreds of millivolts.

For such measurements, tube voltmeters (type VK 7-9, V 7-15, etc.) with measurement ranges of 1 -3V are still often offered. High input resistance and low input capacitance in such devices are the determining factor, and the error is up to 5-10% and is determined by the accuracy of the dial measuring head used. Measurements of the same parameters can be carried out using homemade pointer instruments, the circuits of which are made using field-effect transistors. For example, in B. Stepanov’s HF millivoltmeter (2), the input capacitance is only 3 pF, the resistance in various subranges (from 3 mV to 1000 mV) even in the worst case does not exceed 100 kOhm with an error of +/- 10% (determined by the head used and instrumentation error for calibration). At the same time, the measured HF voltage has an upper limit of the frequency range of 30 MHz without an obvious frequency error, which is quite acceptable in amateur radio practice.

Because modern digital instruments are still expensive for most radio amateurs; last year in the Radio magazine B. Stepanov (3) proposed using an RF probe for a cheap digital multimeter like M-832 with detailed description its schemes and methods of application. Meanwhile, without spending any money at all, you can successfully use pointer HF millivoltmeters, while freeing up the main digital multimeter for parallel measurements of current or resistance in the circuit being developed...

In terms of circuit design, the proposed device is very simple, and the minimum components used can be found “in the box” of almost every radio amateur. Actually, there is nothing new in the scheme. The use of op amps for such purposes is described in detail in the amateur radio literature of the 80-90s (1, 4). The widely used microcircuit K544UD2A (or UD2B, UD1A, B) with field-effect transistors at the input (and therefore with high input resistance) was used. You can use any operational amplifiers of other series with field switches at the input and in a typical connection, for example, K140UD8A. Specifications millivoltmeter-voltmeter correspond to those given above, since the basis of the device was B. Stepanov’s circuit (2).

In voltmeter mode, the gain of the op-amp is 1 (100% OOS) and the voltage is measured with a microammeter up to 100 μA with additional resistances (R12 - R17). They, in fact, determine the subranges of the device in voltmeter mode. When the OOS decreases (switch S2 turns on resistors R6 - R8) Kus. increases, and accordingly the sensitivity of the operational amplifier increases, which allows it to be used in millivoltmeter mode.

Feature The proposed development is the ability to operate the device in two modes - voltmeter DC with limits from 0.1 to 1000 V, and a millivoltmeter with upper limits of subranges of 12.5, 25, 50 mV. In this case, the same divider (X1, X100) is used in two modes, so that, for example, in the 25 mV subrange (0.025 V) using the X100 multiplier, a voltage of 2.5 V can be measured. To switch subranges of the device, one multi-position two-board switch is used.

Using an external RF probe on a GD507A germanium diode, you can measure RF voltage in the same subranges with a frequency of up to 30 MHz.

Diodes VD1, VD2 protect the pointer measuring device from overloads during operation. Another feature microammeter protection at transition processes problems that arise when turning the device on and off, when the device needle goes off scale and may even bend, is to use a relay to turn off the microammeter and close the op-amp output to the load resistor (relays P1, C7 and R11). In this case (when the device is turned on), charging C7 requires a fraction of a second, so the relay operates with a delay and the microammeter is connected to the output of the op-amp a fraction of a second later. When the device is turned off, C7 is discharged through the indicator lamp very quickly, the relay is de-energized and breaks the microammeter connection circuit before the op-amp power supply circuits are completely de-energized. The protection of the op-amp itself is carried out by switching on the inputs R9 and C1. Capacitors C2, C3 are blocking and prevent excitation of the op-amp. Balancing of the device (“setting 0”) is carried out by a variable resistor R10 in the 0.1 V subrange (it is also possible in more sensitive subranges, but when the remote probe is turned on, the influence of hands increases). Capacitors are preferably of the K73-xx type, but if they are not available, you can also take ceramic ones 47 - 68N. The remote probe probe uses a KSO capacitor for an operating voltage of at least 1000V.

Settings millivoltmeter-voltmeter is carried out in the following sequence. First, set up the voltage divider. Operating mode – voltmeter. Trimmer resistor R16 (10V subrange) is set to maximum resistance. At resistance R9, monitoring with an exemplary digital voltmeter, set the voltage from a stabilized power source of 10 V (position S1 - X1, S3 - 10 V). Then in position S1 - X100, using trimming resistors R1 and R4, use a standard voltmeter to set 0.1V. In this case, in position S3 - 0.1V, the microammeter needle should be set to the last mark of the instrument scale. The ratio is 100/1 (the voltage across the resistor R9 - X1 is 10V to X100 - 0.1V, when the position of the arrow of the device being adjusted is at the last scale mark in the S3 sub-range - 0.1V) is checked and adjusted several times. In this case, a mandatory condition: when switching S1, the reference voltage of 10V cannot be changed.

Next. In measurement mode DC voltage in the position of the divider switch S1 - X1 and the subrange switch S3 - 10V, use the variable resistor R16 to set the microammeter needle to the last division. The result (at 10 V at the input) should be the same instrument readings on the 0.1 V - X100 subrange and the 10 V - X1 subrange.

The method for setting the voltmeter in the 0.3V, 1V, 3V and 10V subranges is the same. In this case, the positions of the resistor motors R1, R4 in the divider cannot be changed.

Operating mode: millivoltmeter. At the entrance 5th century. In position S3 - 50 mV, divider S1 - X100 with resistor R8 set the arrow to the last scale division. We check the voltmeter readings: in the 10V X1 or 0.1V X100 subrange, the needle should be in the middle of the scale - 5V.

The adjustment method for the 12.5mV and 25mV subranges is the same as for the 50mV subrange. The input is supplied with 1.25V and 2.5V respectively at X 100. The readings are checked in voltmeter mode X100 - 0.1V, X1 - 3V, X1 - 10V. It should be noted that when the microammeter needle is in the left sector of the instrument scale, the measurement error increases.

Peculiarity This method of calibrating the device: it does not require a standard power source of 12 - 100 mV and a voltmeter with a lower measurement limit of less than 0.1 V.

When calibrating the device in the RF voltage measurement mode with a remote probe for the 12.5, 25, 50 mV subranges (if necessary), you can build correction graphs or tables.

The device is mounted mounted in metal case. Its dimensions depend on the size of the measuring head used and the power supply transformer. For example, I have a bipolar power supply unit assembled on a transformer from an imported tape recorder ( primary winding at 110v), The stabilizer is best assembled on MS 7812 and 7912 (or LM317), but it can be simpler - parametric, on two zener diodes. The design of the remote RF probe and the features of working with it are described in detail in (2, 3).

Literature used:

1. B. Stepanov. Measurement of low RF voltages. J. “Radio”, No. 7, 12 – 1980, p.55, p.28.
2. B. Stepanov. High frequency millivoltmeter. Journal “Radio”, No. 8 – 1984, p.57.
3. B. Stepanov. HF head to digital voltmeter. Journal "Radio", No. 8, 2006, p.58.
4. M. Dorofeev. Volt-ohmmeter on op-amp. Journal "Radio", No. 12, 1983, p. 30.

Vasily Kononenko (RA0CCN).

HF voltmeter with linear scale
Robert AKOPOV (UN7RX), Zhezkazgan, Karaganda region, Kazakhstan

One of the necessary devices in the arsenal of a shortwave radio amateur is, of course, a high-frequency voltmeter. Unlike a low-frequency multimeter or, for example, a compact LCD oscilloscope, such a device is rarely found on sale, and the cost of a new branded one is quite high. Therefore, when there was a need for such a device, it was built with a dial milliammeter as an indicator, which, unlike a digital one, allows you to easily and clearly evaluate changes in readings quantitatively, and not by comparing results. This is especially important when setting up devices where the amplitude of the measured signal is constantly changing. At the same time, the measurement accuracy of the device when using a certain circuitry is quite acceptable.

There is a typo in the diagram in the magazine: R9 should have a resistance of 4.7 MOhm

RF voltmeters can be divided into three groups. The first ones were built on the basis broadband amplifier with the inclusion of a diode rectifier in the negative feedback circuit. The amplifier ensures the operation of the rectifier element in the linear section of the current-voltage characteristic. The devices of the second group use a simple detector with a high-resistance direct current amplifier (DCA). The scale of such an HF voltmeter is nonlinear at the lower measurement limits, which requires the use of special calibration tables or individual calibration of the device. An attempt to linearize the scale to some extent and shift the sensitivity threshold down by passing a small current through the diode does not solve the problem. Before the start of the linear section of the current-voltage characteristic, these voltmeters are, in fact, indicators. Nevertheless, such devices, both in the form of complete structures and attachments to digital multimeters, are very popular, as evidenced by numerous publications in magazines and the Internet.
The third group of devices uses scale linearization when a linearizing element is included in the OS circuit of the UPT to provide the necessary change in gain depending on the amplitude of the input signal. Such solutions are often used in professional equipment components, for example, in broadband high-linear instrumentation amplifiers with AGC, or AGC components of broadband RF generators. It is on this principle that the described device is built, the circuit of which, with minor changes, is borrowed from.
Despite its obvious simplicity, the HF voltmeter has very good parameters and, naturally, a linear scale, which eliminates problems with calibration.
The measured voltage range is from 10 mV to 20 V. The operating frequency band is 100 Hz...75 MHz. Input resistance is at least 1 MOhm with an input capacitance of no more than several picofarads, which is determined by the design of the detector head. The measurement error is no worse than 5%.
The linearizing unit is made on the DA1 chip. Diode VD2 in the negative feedback circuit helps to increase the gain of this stage of the amplifier at low input voltages. The decrease in the detector output voltage is compensated; as a result, the device readings acquire a linear dependence. Capacitors C4, C5 prevent self-excitation of the UPT and reduce possible interference. Variable resistor R10 is used to set the arrow measuring instrument PA1 to the zero mark of the scale before taking measurements. In this case, the input of the detector head must be closed. The device's power supply has no special features. It is made on two stabilizers and provides a bipolar voltage of 2x12 V to power operational amplifiers (the network transformer is not shown in the diagram, but is included in the assembly kit).

All parts of the device, with the exception of parts of the measuring probe, are mounted on two printed circuit boards ah from one-sided foil fiberglass. Below is a photograph of the UPT board, power board and test probe.

Milliammeter RA1 - M42100, with a full needle deflection current of 1 mA. Switch SA1 - PGZ-8PZN. Variable resistor R10 is SP2-2, all trimming resistors are imported multi-turn ones, for example 3296W. Resistors of non-standard values ​​R2, R5 and R11 can be made up of two connected in series. Operational amplifiers can be replaced with others, with a high input impedance and preferably with internal correction (so as not to complicate the circuit). All permanent capacitors are ceramic. Capacitor SZ is mounted directly on the input connector XW1.
The D311A diode in the RF rectifier was selected based on the optimality of the maximum permissible RF voltage and rectification efficiency at the upper measured frequency limit.
A few words about the design of the measuring probe of the device. The probe body is made of fiberglass in the form of a tube, on top of which a copper foil screen is placed.

Inside the case there is a board made of foil fiberglass on which the probe parts are mounted. A ring made of a strip of tinned foil approximately in the middle of the body is intended to provide contact with the common wire of a removable divider, which can be screwed on in place of the probe tip.
Setting up the device begins with balancing op-amp DA2. To do this, switch SA1 is set to the “5 V” position, the input of the measuring probe is closed, and the arrow of the PA1 device is set to the zero scale mark using trimming resistor R13. Then the device is switched to the “10 mV” position, the same voltage is applied to its input, and resistor R16 is used to set the arrow of device PA1 to the last scale division. Next, a voltage of 5 mV is applied to the input of the voltmeter; the arrow of the device should be approximately in the middle of the scale. Linearity of readings is achieved by selecting resistor R3. Even better linearity can be achieved by selecting resistor R12, but it should be borne in mind that this will affect the gain of the UPT. Next, the device is calibrated on all subranges using the appropriate trimming resistors. As a reference voltage when calibrating the voltmeter, the author used an Agilent 8648A generator (with a load equivalent of 50 Ohms connected to its output), which has a digital output signal level meter.
The entire article from Radio No. 2 magazine, 2011 can be downloaded from here
LITERATURE:
1. Prokofiev I., Millivoltmeter-Q-meter. - Radio, 1982, No. 7, p. 31.
2. Stepanov B., HF head for a digital multimeter. - Radio, 2006, No. 8, p. 58, 59.
3. Stepanov B., RF voltmeter on a Schottky diode. - Radio, 2008, No. 1, p. 61, 62.
4. Pugach A., High-frequency millivoltmeter with a linear scale. - Radio, 1992, No. 7, p. 39.

Cost of printed circuit boards (probe, main board and power supply board) with mask and markings: 80 UAH

High accuracy of HF voltage measurements (up to the third or fourth digit) is, in fact, not needed in amateur radio practice. The quality component is more important (the presence of a sufficiently high level signal - the more, the better). Typically, when measuring an RF signal at the output of a local oscillator (oscillator), this value does not exceed 1.5 - 2 volts, and the circuit itself is adjusted to resonance according to the maximum RF voltage value. When adjusted in the IF paths, the signal increases step by step from units to hundreds of millivolts.

When setting up local oscillators and IF paths, tube voltmeters (such as VK 7-9, V7-15, etc.) with measurement ranges of 1 - 3 V are still often used. High input resistance and low input capacitance in such devices are the determining factor, and the error is up to 5-10% and is determined by the accuracy of the dial measuring head used. Measurements of the same parameters can be carried out using homemade pointer instruments, the circuits of which are made on microcircuits with field effect transistors at the entrance. For example, in B. Stepanov’s HF millivoltmeter (2), the input capacitance is only 3 pF, the resistance in various subranges (from 3 mV to 1000 mV) even in the worst case does not exceed 100 kOhm with an error of +/- 10% (determined by the head used and instrumentation error for calibration). At the same time, the measured HF voltage has an upper limit of the frequency range of 30 MHz without an obvious frequency error, which is quite acceptable in amateur radio practice.

In terms of circuit design, the proposed device is very simple, and the minimum components used can be found “in the box” of almost every radio amateur. Actually, there is nothing new in the scheme. The use of op amps for such purposes is described in detail in the amateur radio literature of the 80-90s (1, 4). The widely used microcircuit K544UD2A (or UD2B, UD1A, B) with field-effect transistors at the input (and therefore with high input resistance) was used. You can use any operational amplifiers of other series with field switches at the input and in a typical connection, for example, K140UD8A. The technical characteristics of the millivoltmeter-voltmeter correspond to those given above, since the basis of the device was B. Stepanov’s circuit (2).

In voltmeter mode, the op-amp gain is 1 (100% OOS) and the voltage is measured with a microammeter up to 100 μA with additional resistances (R12 - R17). They, in fact, determine the subranges of the device in voltmeter mode. When the OOS decreases (switch S2 turns on resistors R6 - R8) Kus. increases, and accordingly the sensitivity of the operational amplifier increases, which allows it to be used in millivoltmeter mode.

A feature of the proposed development is the ability to operate the device in two modes - a DC voltmeter with limits from 0.1 to 1000 V, and a millivoltmeter with upper limits of subranges of 12.5, 25, 50 mV. In this case, the same divider (X1, X100) is used in two modes, so that, for example, in the 25 mV subrange (0.025 V) using the X100 multiplier, a voltage of 2.5 V can be measured. To switch subranges of the device, one multi-position two-board switch is used.

Using an external RF probe on a GD507A germanium diode, you can measure RF voltage in the same subranges with a frequency of up to 30 MHz.
Diodes VD1, VD2 protect the pointer measuring device from overloads during operation.
Another feature of protecting a microammeter during transient processes that occur when turning the device on and off, when the arrow of the device goes off scale and may even bend, is the use of a relay to turn off the microammeter and short circuit the output of the op-amp to the load resistor (relays P1, C7 and R11). In this case (when the device is turned on), charging C7 requires a fraction of a second, so the relay operates with a delay and the microammeter is connected to the output of the op-amp a fraction of a second later. When the device is turned off, C7 is discharged through the indicator lamp very quickly, the relay is de-energized and breaks the microammeter connection circuit before the op-amp power supply circuits are completely de-energized. The protection of the op-amp itself is carried out by switching on the inputs R9 and C1. Capacitors C2, C3 are blocking and prevent excitation of the op-amp.

Balancing of the device (“setting 0”) is carried out by a variable resistor R10 in the 0.1 V subrange (it is also possible in more sensitive subranges, but when the remote probe is turned on, the influence of hands increases). Capacitors are preferably of the K73-xx type, but if they are not available, you can also take ceramic ones 47 - 68N. The remote probe probe uses a KSO capacitor for an operating voltage of at least 1000V.

Setting up the millivoltmeter-voltmeter is carried out in the following sequence. First, set up the voltage divider. Operating mode - voltmeter. Trimmer resistor R16 (10V subrange) is set to maximum resistance. At resistance R9, monitoring with an exemplary digital voltmeter, set the voltage from a stabilized power source of 10 V (position S1 - X1, S3 - 10 V). Then in position S1 - X100, using trimming resistors R1 and R4, use a standard voltmeter to set 0.1V. In this case, in position S3 - 0.1V, the microammeter needle should be set to the last mark of the instrument scale. The ratio is 100/1 (the voltage across the resistor R9 - X1 is 10V to X100 - 0.1V, when the position of the needle of the device being adjusted is at the last scale mark in the S3 sub-range - 0.1V) is checked and adjusted several times. In this case, a mandatory condition: when switching S1, the reference voltage of 10V cannot be changed.

Next. In the DC voltage measurement mode, in the position of the divider switch S1 - X1 and the subrange switch S3 - 10V, the variable resistor R16 sets the microammeter needle to the last division. The result (at 10 V at the input) should be the same instrument readings on the 0.1 V - X100 subrange and the 10 V - X1 subrange.

The method for setting the voltmeter in the 0.3V, 1V, 3V and 10V subranges is the same. In this case, the positions of the resistor motors R1, R4 in the divider cannot be changed.

Operating mode - millivoltmeter. At the entrance 5th century. In position S3 - 50 mV, divider S1 - X100 with resistor R8 set the arrow to the last scale division. We check the voltmeter readings: in the 10V X1 or 0.1V X100 subrange, the needle should be in the middle of the scale - 5V.

The adjustment method for the 12.5mV and 25mV subranges is the same as for the 50mV subrange. The input is supplied with 1.25V and 2.5V respectively at X 100. The readings are checked in voltmeter mode X100 - 0.1V, X1 - 3V, X1 - 10V. It should be noted that when the microammeter needle is in the left sector of the instrument scale, the measurement error increases.

The peculiarity of this method of calibrating the device: it does not require a standard power source of 12 - 100 mV and a voltmeter with a lower measurement limit of less than 0.1 V.

When calibrating the device in the RF voltage measurement mode with a remote probe for the 12.5, 25, 50 mV subranges (if necessary), you can build correction graphs or tables.

The device is mounted mounted in a metal case. Its dimensions depend on the size of the measuring head used and the power supply transformer. In the above circuit, a bipolar power supply unit operates, assembled on a transformer from an imported tape recorder (primary winding at 110V). The stabilizer is best assembled on MS 7812 and 7912 (or two LM317), but it can be simpler - parametric, on two zener diodes. The design of the remote RF probe and the features of working with it are described in detail in (2, 3).

Literature used:

1. B. Stepanov. Measurement of low RF voltages. J. "Radio", No. 7, 12 - 1980, p.55, p.28.
2. B. Stepanov. High frequency millivoltmeter. Journal "Radio", No. 8 - 1984, p.57.
3. B. Stepanov. RF head for digital voltmeter. Journal "Radio", No. 8, 2006, p.58.
4. M. Dorofeev. Volt-ohmmeter on op-amp. Journal "Radio", No. 12, 1983, p. 30.

In the practice of a radio amateur, there are times when it is necessary to simultaneously measure the constant component of the signal and the variable one. Usually in this case they use an oscilloscope, but what if you don’t have an oscilloscope? If there is no need to accurately determine the waveform of the alternating component, you can use two voltmeters, one for measuring direct voltage, the other for alternating voltage, connecting them to one point.

In this case, two devices are required, using one universal one (with a “variable-constant” switch) is not convenient, it is impossible to simultaneously observe the tribal and constant components, it takes time to switch, and in some cases it is desirable to see the change in both components.

In such a situation, the device described below may be useful. It contains two in one body electronic voltmeter, AC and DC, having one common power source and one common wire, and two independent dial indicators and inputs.

Both inputs of such a voltmeter can be connected to one point and simultaneously observe the change in the direct and alternating components, or use a direct current voltmeter to measure any control voltage or mode of operation of the cascade (for example, bias voltage), and at the same time observe the level of the output alternating signal at using a voltmeter AC switched on at the output of the device.

The device has the following parameters: range of measured DC voltages - from 1 mV to 1000V, range of measured AC voltages - from 1 mV to 100V, input resistance of the DC voltage measurement input - 10 MΩ, input resistance of the measurement input AC voltage- 1 MOhm, power consumption from the network 1 W, cutoff frequency of the measured alternating voltage - 100 kHz with an error of no more than 1% and 1 MHz with an error of no more than 10%.

The circuit diagram is shown in Figure 1. The DC voltmeter is made using operational amplifier A1. Here, when switching measurement limits, two methods are used simultaneously: firstly, the input voltage is divided using a two-stage divider on resistors R1 R2, and secondly, the gain of the operational amplifier itself is changed by changing the OOS depth by switching resistors R7-R9.

When measuring a voltage of less than 1 V (within the limits of 0.01, 0.1, 1 V), the input signal is not divided, and only the gain of op-amp A1 changes; when measuring a voltage of more than 1 V (within the limits of 10, 100, 1000 V), the input the signal is divided into 1000 by resistors R1 R2, and the choice of these limits is also made by changing the gain of the op-amp.

The input circuit, consisting of resistor R3 and bidirectional zener diode V1, is designed to protect the input of the operational amplifier from overload caused by mistakenly incorrectly switching on the measurement limit. The resistor and zener diode are a parametric stabilizer that prevents the input voltage from being greater than 6.2 V.

Microammeter PV1, on the scale of which the DC voltage is measured, is included in the OOS circuit of the op-amp between its inverting input and output, its resistance, together with the resistance of resistors R7-R9, creates an output voltage divider, and accordingly changing the lower arm of this divider (when switching resistors) changes and the depth of the feedback, therefore the gain also changes. This design of the circuit for selecting measurement limits made it possible to minimize the number of high-resistance resistors.

Pre-installation dial indicator to the zero position (before starting the measurement) is done by balancing the operational amplifier using a variable resistor R5. Resistors R4 and R6 limit the balancing limits and increase the accuracy of zero setting. To set zero, the limit switch S1 must be set to position "0", while input circuit voltmeter is short-circuited.

The alternating voltage is measured by a voltmeter on operational amplifier A2. The same circuit is used here with a two-stage input divider and a three-stage change in the op-amp gain. The difference is that the input divider has frequency correction on capacitors C2 and C3. This is necessary to ensure reliable measurements over a wide range of input frequencies.

Resistor R12 and Zener diode V2 serve to protect the input from overload if the measurement limit is incorrectly selected; they work in exactly the same way as in a DC voltmeter.

The PV2 indicator is the same as in a DC voltmeter, but here it serves to measure alternating voltage and is connected through a bridge rectifier on diodes V3-V6, resistor R16 is used to accurately set the sensitivity of the microammeter, to preserve the existing scale calibration.

The op-amp gain factors are also switched by changing the depth of the feedback loop by changing the division coefficient of the circuit consisting of a microammeter and one of the resistors R17-R19, connected between the inverse input and the output of op-amp A2.

Setting the zero of the measuring device is done by balancing the operational amplifier using a variable resistor R14; resistors R13 and R15 limit the limits of balancing, making it more accurate.

The power supply is made using a simple transformer circuit with a bridge rectifier and a parametric bipolar stabilizer based on zener diodes V7 and V8 (op-amps consume a small current, and the use of transistor stabilizers that provide a large output current is not required).

They often started asking me questions about analog electronics. Did the session take the students for granted? ;) Okay, it’s high time for a little educational activity. In particular, on the operation of operational amplifiers. What is it, what is it eaten with and how to calculate it.

What is this
Operational amplifier This is an amplifier with two inputs, nevi... hmm... high signal amplification and one output. Those. we have U out = K*U in and K ideally equals infinity. In practice, of course, the numbers are more modest. Let's say 1,000,000. But even such numbers blow your mind when you try to apply them directly. Therefore, as in kindergarten, one Christmas tree, two, three, many Christmas trees - we have a lot of gain here;) And that’s it.

And there are two entrances. And one of them is direct, and the other is inverse.

Moreover, the inputs are high-impedance. Those. their input impedance is infinity in the ideal case and VERY high in the real case. The count there goes into hundreds of MegaOhms, or even gigaohms. Those. it measures the voltage at the input, but has minimal effect on it. And we can assume that no current flows in the op-amp.

The output voltage in this case is calculated as:

U out =(U 2 -U 1)*K

Obviously, if the voltage at the direct input is greater than at the inverse input, then the output is plus infinity. Otherwise it will be minus infinity.

Of course, in a real circuit there will not be infinity plus and minus, and they will be replaced by the highest and lowest possible supply voltage of the amplifier. And we will get:

Comparator
A device that allows you to compare two analog signals and make a verdict - which signal is larger. Already interesting. You can come up with a lot of applications for it. By the way, the same comparator is built into most of I showed microcontrollers and how to use them using the example of AVR in articles about creating . The comparator is also great for creating .

But the matter is not limited to one comparator, because if you introduce feedback, then a lot can be done from the op-amp.

Feedback
If we take a signal from the output and send it straight to the input, then feedback will arise.

Positive Feedback
Let’s take and drive the signal directly from the output into the direct input.

• The voltage U1 is greater than zero - the output is -15 volts
• The voltage U1 is less than zero - the output is +15 volts

What happens if the voltage is zero? In theory, the output should be zero. But in reality, the voltage will NEVER be zero. After all, even if the charge of the right one outweighs the charge of the left one by one electron, then this is already enough to drive the potential to the output at an infinite gain. And at the output all hell will begin - the signal jumps here and there at the speed of random disturbances induced at the inputs of the comparator.

To solve this problem, hysteresis is introduced. Those. a kind of gap between switching from one state to another. To do this, positive feedback is introduced, like this:

We assume that at this moment there is +10 volts at the inverse input. The output from the op-amp is minus 15 volts. At the direct input it is no longer zero, but a small part of the output voltage from the divider. Approximately -1.4 volts Now, until the voltage at the inverse input drops below -1.4 volts, the op-amp output will not change its voltage. And as soon as the voltage drops below -1.4, the output of the op-amp will sharply jump to +15 and there will already be a bias of +1.4 volts at the direct input.

And in order to change the voltage at the output of the comparator, the U1 signal will need to increase by as much as 2.8 volts to reach the upper level of +1.4.

A kind of gap appears where there is no sensitivity, between 1.4 and -1.4 volts. The width of the gap is controlled by the ratios of resistors in R1 and R2. The threshold voltage is calculated as Uout/(R1+R2) * R1 Let's say 1 to 100 will give +/-0.14 volts.

But still, op-amps are more often used in negative feedback mode.

Negative Feedback
Okay, let's put it another way:

In case of negative feedback The op-amp has an interesting property. It will always try to adjust its output voltage so that the voltages at the inputs are equal, resulting in a zero difference.
Until I read this in the great book from comrades Horowitz and Hill, I could not get into the work of the OU. But it turned out to be simple.

Repeater
And we got a repeater. Those. at the input U 1, at the inverse input U out = U 1. Well, it turns out that U out = U 1.

The question is, why do we need such happiness? It was possible to directly connect the wire and no op-amp would be needed!

It is possible, but not always. Let's imagine this situation: there is a sensor made in the form of a resistive divider:

The lower resistance changes its value, the distribution of output voltages from the divider changes. And we need to take readings from it with a voltmeter. But the voltmeter has its own internal resistance, albeit large, but it will change the readings from the sensor. Moreover, what if we don’t want a voltmeter, but want the light bulb to change brightness? There’s no way to connect a light bulb here anymore! Therefore, we buffer the output with an operational amplifier. Its input resistance is huge and its influence will be minimal, and the output can provide quite a noticeable current (tens of milliamps, or even hundreds), which is quite enough to operate the light bulb.
In general, you can find applications for a repeater. Especially in precision analog circuits. Or where the circuitry of one stage can affect the operation of another, in order to separate them.

Amplifier
Now let’s do a feint with our ears - take our feedback and connect it to the ground through a voltage divider:

Now half of the output voltage is supplied to the inverse input. But the amplifier still needs to equalize the voltages at its inputs. What will he have to do? That's right - raise the voltage at your output twice as high as before in order to compensate for the resulting divider.

Now there will be U 1 on the straight line. On inverse U out /2 = U 1 or U out = 2*U 1.

If we put a divisor with a different ratio, the situation will change in the same way. So that you don’t have to turn the voltage divider formula in your mind, I’ll give it right away:

U out = U 1 *(1+R 1 /R 2)

It is mnemonic to remember what is divided into what is very simple:

It turns out that the input signal goes through a chain of resistors R 2, R 1 in U out. In this case, the direct input of the amplifier is set to zero. Let us remember the habits of the op-amp - it will try, by hook or by crook, to ensure that a voltage equal to the direct input is generated at its inverse input. Those. zero. The only way to do this is to lower the output voltage below zero so that a zero appears at point 1.

So. Let's imagine that U out =0. It's still zero. And the input voltage, for example, is 10 volts relative to U out. A divisor of R 1 and R 2 will divide it in half. Thus, at point 1 there are five volts.

Five volts is not zero and the op amp lowers its output until point 1 is zero. To do this, the output should become (-10) volts. In this case, relative to the input, the difference will be 20 volts, and the divider will provide us with exactly 0 at point 1. We have an inverter.

But we can also choose other resistors so that our divider produces different coefficients!
In general, the gain formula for such an amplifier will be as follows:

U out = - U in * R 1 / R 2

Well, a mnemonic picture for quick memorization xy from xy.

Let's say U 2 and U 1 are 10 volts each. Then at the 2nd point there will be 5 volts. And the output will have to become such that at the 1st point there is also 5 volts. That is, zero. So it turns out that 10 volts minus 10 volts equals zero. That's right :)

If U 1 becomes 20 volts, then the output will have to drop to -10 volts.
Do the math yourself - the difference between U 1 and U out will be 30 volts. The current through resistor R4 will be (U 1 -U out)/(R 3 +R 4) = 30/20000 = 0.0015A, and the voltage drop across resistor R 4 will be R 4 *I 4 = 10000 * 0.0015 = 15 volts . Subtract the 15 volt drop from the 20 input drop and get 5 volts.

Thus, our op-amp solved an arithmetic problem from 10 subtracted 20, resulting in -10 volts.

Moreover, the problem contains coefficients determined by resistors. It’s just that, for simplicity, I have chosen resistors of the same value and therefore all coefficients are equal to one. But in fact, if we take arbitrary resistors, then the dependence of the output on the input will be like this:

U out = U 2 *K 2 - U 1 *K 1

K 2 = ((R 3 +R 4) * R 6) / (R 6 +R 5)*R 4
K 1 = R 3 / R 4

The mnemonic technique for remembering the formula for calculating coefficients is as follows:
Right according to the scheme. The numerator of the fraction is at the top, so we add up the upper resistors in the current flow circuit and multiply by the lower one. The denominator is below, so we add up the lower resistors and multiply by the upper one.

Everything is simple here. Because point 1 is constantly reduced to 0, then we can assume that the currents flowing into it are always equal to U/R, and the currents entering node number 1 are summed up. The ratio of the input resistor to the feedback resistor determines the weight of the incoming current.

There can be as many branches as you like, but I only drew two.

U out = -1(R 3 *U 1 /R 1 + R 3 *U 2 /R 2)

Resistors at the input (R 1, R 2) determine the amount of current, and therefore the total weight of the incoming signal. If you make all the resistors equal, like mine, then the weight will be the same, and the multiplication factor of each term will be equal to 1. And U out = -1(U 1 +U 2)

Non-inverting adder
Everything is a little more complicated here, but it’s similar.

Uout = U 1 *K 1 + U 2 *K 2

K 1 = R 5 / R 1
K 2 = R 5 / R 2

Moreover, the resistors in the feedback must be such that the equation R 3 / R 4 = K 1 + K 2 is observed

In general, you can do any math using operational amplifiers, add, multiply, divide, calculate derivatives and integrals. And almost instantly. Analog computers are made using op-amps. I even saw one of these on the fifth floor of SUSU - a fool the size of half a room. Several metal cabinets. The program is typed by connecting different blocks with wires :)