Practical application of operational amplifiers. Part one. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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So - operational amplifier. Further we will call it OU, otherwise it is very lazy to write it completely every time.

On schematic diagrams, most often, it is indicated as follows:

The figure shows the three most important conclusions of the op-amp - two inputs and an output. Of course, there are also power pins and sometimes frequency correction pins, although the latter is becoming less common - most modern op-amps have it built-in. The two inputs of the op-amp - Inverting and Non-inverting are named so for their inherent properties. If we apply a signal to the Inverting input, then at the output we will get an inverted signal, that is, a 180-degree phase-shifted signal - a mirror image; if we apply a signal to the Non-inverting input, then at the output we will get a phase-changed signal.

As well as the main conclusions, there are also three main properties of the op-amp - you can call them TriO (or OOO - as you like): Very high input resistance, Very high gain (10000 or more), Very low output resistance. Another very important parameter of the op amp is called the output voltage slew rate (slew rate in bourgeois). It actually denotes the speed of this op-amp - how quickly it can change the output voltage when it changes at the input.

This parameter is measured in volts per second (V/s).

This parameter is important primarily for comrades designing ultrasonic frequencies, because if the op-amp is not fast enough, then it will not keep up with the input voltage at high frequencies and fair non-linear distortion will occur. Most modern general purpose op amps have a slew rate of 10V/μs or more. For high-speed op-amps, this parameter can reach a value of 1000V / μs.

You can evaluate whether this or that op-amp is suitable for your purposes in terms of signal slew rate using the formula:

where, fmax is the frequency of the sinusoidal signal, Vmax is the slew rate of the signal, Vout is the maximum output voltage.

Well, let's not pull the cat by the tail anymore - let's get down to the main task of this opus - where, in fact, these cool things can be stuck and what can be obtained from it.

The first scheme for switching on the OS - inverting amplifier.

The most popular and common op-amp amplifier circuit. The input signal is applied to the inverting input, and the non-inverting input is connected to ground.

The gain is determined by the ratio of resistors R1 and R2 and is calculated by the formula:

Why "minus"? Because, as we remember, in an inverting amplifier, the phase of the output signal is "mirror" to the phase of the input.

The input resistance is determined by the resistor R1. If its resistance is, for example, 100 kOhm, then the input impedance of the amplifier will be 100 kOhm.

The following scheme is inverting amplifier with increased input impedance.

The previous circuit is good for everyone, with the exception of one nuance - the ratio of input resistance and gain may not be suitable for the implementation of any specific project. After all, what happens - let's say we need an amplifier with K = 100. Then, based on the fact that the values ​​​​of the resistors should be within reasonable limits, we take R2 \u1d 1MΩ, and R10 \u10d XNUMXkΩ. That is, the input impedance of the amplifier will be XNUMX kOhm, which in some cases is not enough.

In these very cases, the following scheme can be applied:

In this case, the gain is calculated using the following formula:

That is, with the same gain, the resistance R1 can be increased, and hence the input impedance of the amplifier can also be increased.

Let's go further - non-inverting amplifier.

It looks like this:

The gain factor is defined as follows:

In this case, as you can see, there are no minuses - the phase of the signal at the input and at the output is the same.

The main difference from the inverting amplifier is the increased input resistance, which can reach 10 MΩ and higher.

If, when implementing this circuit in practical designs, it is necessary to provide for DC decoupling from the previous stages - to install a separating capacitor, then you need to connect a resistor with a resistance of about 100 kOhm between the input of the op-amp and the common wire, as shown in the figure.

If this is not done, then the op-amp will be overexcited and you will not get anything sensible from it. Well, except for half the power output.

Variable Gain Amplifier.

Let's take R1=R2=R3=R. And we introduce some variable A, which can take values ​​from 1 to 0, depending on the rotation of the variable resistor R3.

Then the gain can be defined as follows:

K=2A-1

The input resistance is practically independent of the position of the variable resistor slider.

So, we figured out the amplifiers - then we have according to plan - filters.

Publication: radiokot.ru

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