Operational amplifier: switching circuits, operating principle. Amplifier circuit on op amp of noninverting amplifier. Scheme of the amplifier of a voltage of a direct current on the operational amplifier

The article will consider the standard schemeamplifier on the operational amplifier, as well as examples of different modes of operation of this device. To date, no control device can do without the use of operational amplifiers. These are truly universal devices that allow you to perform various functions with a signal. About how it works and what specifically allows you to make this device, you'll find out later.

Inverting Amplifiers The circuit of the inverting amplifier at the op amp is sufficientsimple, you can see it in the image. It is based on the operational amplifier (its inclusion schemes are discussed in this article). In addition, here:

1. On the resistor R1 voltage drop is present, in its value it is the same as the input one.
2. On the resistor R2 there is also a voltage drop - it is the same as the output voltage.

In this case, the ratio of the output voltage tothe resistance R2 is equal in value to the ratio of the input to R1, but back to it by the sign. Knowing the resistance and voltage, you can calculate the gain. To do this, it is necessary to divide the output voltage into an input voltage. At the same time, the operational amplifier (its switching circuits can be any) can have the same gain regardless of type.

Feedback work Now we need to disassemble one more in detailthe key point is the work of feedback. Suppose there is some voltage at the input. For simplicity of calculations, let's take its value equal to 1 V. Let us also assume that R1 = 10 kΩ, R2 = 100 kOhm.

And now suppose that some kind ofan unforeseen situation, because of which the voltage was set at 0 V at the output of the cascade. Then an interesting picture is observed: two resistances begin to work in pairs, together they create a voltage divider. At the output of the inverting cascade, it is maintained at the level of 0.91 V. In this case, the op-amp allows to fix the mismatch by the inputs, and at the output there is a decrease in the voltage. Therefore, it is very simple to design a circuit on operational amplifiers that implements the function of a signal amplifier from a sensor, for example.

And this change will continue until the very samepores until a stable value of 10 V is established at the output. It is at this instant that the potentials on the inputs of the operational amplifier will be equal. And they will be the same as the potential of the earth. On the other hand, if the voltage continues to decrease on the output of the device, and it will be less than -10 V, the potential at the input will be lower than that of the ground. The consequence of this is that the voltage begins to increase at the output.

This scheme has a big drawback -the input impedance is very small, in particular for amplifiers with a large value of the voltage gain, in the event that the feedback loop is closed. And the structure considered further is devoid of all these shortcomings.

Non-Inverting Amplifier The figure shows the circuit of the non-inverting amplifier on the operational amplifier. After analyzing it, we can draw several conclusions:

1. The voltage value UA is equal to the input voltage.
2. The voltage UA, which is equal to the ratio of the product of the output voltage and R1 to the sum of the resistances R1 and R2, is removed from the divider.
3. In the case where UA is equal in value to the input voltage, the gain is equal to the ratio of the output voltage to the input voltage (or it is possible to add unity to the resistance ratio R2 and R1).

This construction is called non-invertingamplifier, it has an almost infinite input impedance. For example, for the operational amplifiers 411 series, its value is 1012 Ohm, minimum. And for operational amplifiers on bipolar semiconductor transistors, as a rule, more than 108 Ohm. But the output impedance of the cascade, as well as in the previously considered circuit, is very small - the fraction of the Ohm. And this should be taken into account when calculating circuits on operational amplifiers.

AC Amplifier Circuit Both schemes, considered in the article earlier, workon direct current. But if the alternating current acts as a connection between the input signal source and the amplifier, it is necessary to provide ground for the current at the input of the device. And you need to pay attention to the fact that the current value is extremely small in magnitude.

In the event that amplification of signals occursAC, it is necessary to reduce the signal gain constant to unity. This is especially true in cases where the voltage gain is very high. Due to this, it is possible to significantly reduce the effect of the shear stress that is applied to the input of the device.

The second example of a circuit for working with an alternating voltage In this circuit, at a level of -3 dB, you can see a matching frequency of 17 Hz. On it at the capacitor the impedance is at the level of two kilo. Therefore, the capacitor must be large enough.

To build an ac amplifier,It is necessary to use a non-inverting type of circuit on operational amplifiers. And it should have a sufficiently large voltage gain. But the capacitor can be too big, so it's best to stop using it. True, it is necessary to correctly select the shear stress, equating it to zero in value. And you can apply the T-shaped divider and increase the resistances of both resistors in the circuit.

Which scheme is preferable to use Most developers give theirpreference for non-inverting amplifiers, since they have a very high impedance at the input. Neglecting schemes of the inverting type are neglected. But the latter has a huge advantage - it is not exacting to the operational amplifier itself, which is its "heart".

In addition, the characteristics, in fact, hemuch better. And with the help of imaginary grounding, you can combine all signals without special effort, and they will not exert any influence on each other. It can be used in the designs and the circuit of the direct current amplifier on the operational amplifier. Everything depends on needs.

And the most recent is the case, if the whole scheme,discussed here, is connected to the stable output of another operational amplifier. In this case, the value of the impedance at the input does not play a significant role - at least 1 kOhm, at least 10, though infinity. In this case, the first stage always performs its function with respect to the next.

Repeater circuit Operates repeater on opampsimilar to the emitter, built on a bipolar transistor. And performs similar functions. In fact, it is a non-inverting amplifier, in which the resistance of the first resistor is infinitely large, and the second resistance is zero. The gain is unity.

There are special types of operational amplifiers,which are used in engineering only for repeater circuits. They have much better characteristics - as a rule, this is high speed. As an example, we can mention such operational amplifiers as OPA633, LM310, TL068. The latter has a body, like a transistor, and also three leads. Very often such amplifiers are called simply buffers. The fact is that they have the properties of an insulator (very large input impedance and extremely low output). Approximately by this principle, the circuit of the current amplifier on the operational amplifier is also constructed.

Active mode of operation In fact, this is a mode of operation in whichThe outputs and inputs of the operational amplifier are not overloaded. If a very large signal is applied to the input of the circuit, then at the output it will simply begin to cut according to the voltage level of the collector or emitter. But when the output voltage is fixed at the cutoff level - the voltage does not change at the inputs of the op-amp. In this case, the range can not be greater than the supply voltage of the amplifier stage.

Most of the circuits on the operational amplifiersis calculated in such a way that this range is less than the supply voltage by 2 V. But everything depends on which particular amplifier circuit is used on the operational amplifier. The same is the limitation on the stability of the current source based on the operational amplifier.

Suppose there is a floating load sourcea certain drop in voltage. If the current has a normal direction of motion, you can meet a strange load at first glance. For example, several re-polarized batteries. Such a design can be used to obtain a direct charge current.

Some precautions A simple voltage amplifier on the operatingamplifier (the circuit can be chosen any) can be made literally "on the knee". But you need to take into account some features. It is necessary to make sure that the feedback in the scheme is negative. This also means that it is unacceptable to confuse the non-inverting and inverting inputs of the amplifier. In addition, there should be a feedback loop for direct current. Otherwise, the operational amplifier will quickly switch to saturation mode.

Most op amps have an inputThe differential voltage is very small in value. In this case, the maximum difference of the non-inverting and inverting inputs can be limited to a value of 5 V for any power source connection. If this condition is ignored, rather large values ​​of currents appear at the input, which will lead to the fact that all characteristics of the circuit will deteriorate.

The most terrible thing in this is the physical destruction of the operational amplifier itself. As a result, the amplifier circuit on the operational amplifier ceases to work completely.

Should be considered And, of course, you need to talk about the rules that should be observed to ensure a stable and long-lasting operation of the operational amplifier.

Most importantly, the OS has a very highvoltage gain. And if the voltage between the inputs changes to a fraction of millivolts, the output may change significantly at the output. Therefore, it is important to know: in the operational amplifier, the output tries to strive to ensure that between the inputs the voltage difference is close (ideally equal) to zero.

The second rule is current consumption by operatingamplifier is extremely small, literally nanoamperes. If the inputs are field-effect transistors, then it is calculated by picoamperes. Hence it can be concluded that the inputs do not consume current, no matter what operational amplifier is used, the circuit - the principle of operation remains the same.

But do not think that the OS is reallyconstantly changing the input voltage. Physically, this is almost impossible, since there would be no correspondence with the second rule. Due to the operational amplifier, the state of all inputs is evaluated. By means of the circuit of the reverse external connection, the voltage is transferred to the input from the output. The result is that between the inputs of the operational amplifier the voltage difference is at the zero level.

The concept of feedback

This is a common concept, and it is alreadyIt is applied in wide senses in all fields of engineering. In any control system, there is a feedback that compares the output signal and the specified value (reference). Depending on what value is current - there is an adjustment in the right direction. And the control system can be anything, even a car that travels along the road.

The driver presses on the brakes, and the feedback here- the beginning of deceleration. By analogy with such a simple example, we can better deal with feedback in electronic circuits. And negative feedback is if the car accelerated when the brake pedal was depressed. In electronics, feedback is called a process,during which the signal is transmitted from the output to the input. In this case, the signal at the input is also canceled. On the one hand, this is not a very sensible idea, because it may seem from the side that the gain factor will decrease significantly. Such feedback, by the way, was received by the founders of the development of feedback in electronics. But it is worthwhile examining in more detail its effect on operational amplifiers - practical schemes to consider. And it becomes clear that it really does reduce the gain slightly, but it allows us to slightly improve the other parameters:

1. Smooth the frequency characteristics (leads them to the required).
2. Allows you to predict the behavior of the amplifier.
3. It is able to eliminate the non-linearity and distortion of the signal.

The deeper the feedback (it is aboutnegative), the less influence is exerted on the amplifier of the open-loop characteristic. The result - all its parameters depend only on what properties the circuit has.

It is worth paying attention to the fact that allThe operational amplifiers operate in a very deep feedback mode. A voltage gain factor (with its open loop) can reach even a few million. Therefore, the amplifier circuit on the operational amplifier is extremely demanding for observing all the parameters for power and the level of the input signal. 