Operational amplifier

Very often, a signal (for example from a sensor) needs to be amplified, inverted (flipped), biased, several signals need to be added or load isolation done; all these tasks are handled by an operational amplifier, which takes second place after transistors. The first operational amplifiers were used for the mathematical operations of differentiation (dividing complex into simple) and integration (summing from simple to complex), hence the name operational
Операционный усилитель LM358.jpg
Операционный усилитель LM358.jpg

Operating principle of an operational amplifier

At its core, an operational amplifier is a DC amplifier that has two inputs and one output. The power supply of the operational amplifier can be either bipolar (for example +15 volts and -15 volts) or unipolar (+5 v and GND). The operating principle of an amplifier is very similar to a relay (for ease of explanation), which switches the voltage one way or the other, depending on the voltage at its control contacts.

The operational amplifier compares the voltage at the + (non-inverting) input and at the - (inverting) input, and depending on who supplies the + or - voltage more to the output, but from the power circuit. Those. if we power the op-amp from +5 volts, and +0.01v is applied to the + input, +0.00999V is applied to the input, then at the output we will get almost 5 volts, while the input current I is many times less than the output current, which is due to very high resistance between inputs and low resistance between output and + or - power supply. Of course, in fact there will be a drop, but the increase can be hundreds and thousands of times.

Designation on diagrams

In the diagrams, operational amplifiers are designated in the form of a triangle, which has two inputs signed + and -, which should not be confused with power supply + and -, since this is the designation for the investing and non-inverting input, op-amps are also often designated without power, since this is only complicates the circuit.

If the power supply is not indicated, then bipolar power supply +V and -V is implied; in addition, there may be a designation of both +E and -E, but the designation Vcc and Vee is much more interesting. If we remember transistors, then we had three contacts:

The designation Vcc and Vee, as you already guessed, takes its origin from there, where Vcc is + power, since the collector is on the plus side, and Vee is a derivative of Emitter and - power, this designation is used in circuits using bipolar transistors. Circuits based on field-effect transistors are no less popular, but as we remember, the designation there is slightly different (D-drain (drain +), S- source (source -), G-gate (gate)). In such microcircuits, power supply is designated as Vdd for positive and Vss for negative
Обозначение операционного усилителя на схемах.jpg

Voltage repeater

The simplest thing that can be done on an operational amplifier is a voltage follower, with this circuit we close the inverting input (-) to the amplifier output, in fact we repeat the voltage that is supplied to the non-inverting input, i.e. if +2 volts came to it, then at the output we will also get +2 volts and so on up to the maximum, which is determined by the power supply of the microcircuit, if we are powered by +9 volts, then at the output we can get a maximum of (almost) 9 v. Why then do we need a whole microcircuit, you tell me, if you can just make a jumper, it’s all about the current strength and matching the input and output resistances, also with the help of such a circuit you can control large loads or decouple the power part of the circuit from the percisine one (which has high accuracy) .

Inverting amplifier

If, instead of a jumper between the investing input and the output of the amplifier, we install a voltage divider on two resistors, 3K and 1K, then we will get a three-fold amplification of the signal and its inversion, i.e. if we apply +1 volt, we get -3 volts at the output, and if we apply -2 volts, then we get +6 volts at the output, and so on up to the maximum supply voltage, and if we apply a 40 Hz sine wave to the input, then at the output we get an inverted (mirrored) the same sinusoid, but whose amplitude will be three times higher.
Инвертированный OY.jpg

Schmitt trigger

It is also very often necessary to exclude all kinds of interference and make a binary conclusion, yes or no. For such purposes, you can assemble a Schmitt trigger circuit using an operational amplifier. As you can see from the figure, we have the signal we need at 40 hertz, we impose interference in the form of 1000 hertz on it, all this is connected into a circuit using three resistors and at the output we see a clear rectangular meander, which tells us where the sinusoid is now, above or below zero, respectively, at the output we get +5 or 0 (1 true or 0 false), which we will need in the future for microcontrollers.
Триггер шпитта.jpg
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