The article will consider the standard amplifier circuit on an operational amplifier, as well as examples of various modes of operation of this device. To date, no control device is complete 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 will learn further.
Inverting amplifiers
The circuit of the inverting amplifier on the op-amp is quite simple, you can see it in the image. It is based on an operational amplifier (its inclusion circuits are discussed in this article). Other than that, here:
- On the resistor R1, a voltage drop is present, in its value it is the same as the input.
- There is also a voltage drop on resistor R2 - it is the same as the output voltage .
In this case, the ratio of the output voltage to the resistance R2 is equal in value to the ratio of the input to R1, but vice versa. Knowing the values of resistance and voltage, you can calculate the gain. To do this, it is necessary to divide the output voltage into the input voltage. At the same time, the operational amplifier (it can have any switching circuits) can have the same gain, regardless of type.
Feedback work
Now you need to analyze in more detail one key point - the work of feedback. Suppose there is some voltage at the input. For simplicity of calculations, we take its value equal to 1 V. Assume also that R1 = 10 kOhm, R2 = 100 kOhm.
Now suppose that some unforeseen situation arose, due to which the voltage at the output of the cascade was set to 0 V. Next, 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. At the same time, the op-amp allows you to fix the mismatch in the inputs, and the voltage decreases at the output. Therefore, it is very simple to design a circuit on operational amplifiers that implements the function of an amplifier of a signal from a sensor, for example.
And this change will continue until the very time, until a stable value of 10 V is established at the output. It is precisely at this moment that the potentials at the inputs of the operational amplifier turn out to be equal. And they will be the same as the potential of the earth. On the other hand, if the voltage at the output of the device continues to decrease, and it is less than -10 V, the potential at the input will become lower than at the ground. The consequence of this is that the voltage begins to increase at the output.
Such a circuit has a big drawback - the input impedance is very small, especially for amplifiers with a large voltage gain value, if the feedback circuit is closed. And the design discussed further is devoid of all these shortcomings.
Non-inverting amplifier
The figure shows a diagram of a non-inverting amplifier on an operational amplifier. After analyzing it, we can draw several conclusions:
- The voltage value UA is equal to the input.
- The voltage UA is removed from the divider, which is equal to the ratio of the product of the output voltage and R1 to the sum of the resistances R1 and R2.
- In the case when UA is equal to the input voltage in value, the gain is equal to the ratio of the output voltage to the input (or one can be added to the ratio of the resistances R2 and R1).
This design is called a non-inverting amplifier, it has an almost infinite input impedance. For example, for 411 series operational amplifiers, its value is 1012 Ohms, the minimum. And for operational amplifiers based on bipolar semiconductor transistors, as a rule, more than 108 ohms. But the output impedance of the cascade, as well as in the previously considered circuit, is very small - the fraction of ohms. And this must be taken into account when calculating circuits on operational amplifiers.
AC amplifier circuit
Both circuits considered in the article earlier operate on direct current. But if alternating current acts as the connection between the input signal source and the amplifier, then it is necessary to provide grounding 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 case when amplification of AC signals occurs, it is necessary to reduce the gain of the DC signal to unity. This is especially true for cases where the voltage gain is very large. Due to this, it is possible to significantly reduce the effect of shear stress, which is brought to the input of the device.
The second example of a circuit for working with alternating voltage
In this scheme, at a level of -3 dB, you can see the correspondence to a frequency of 17 Hz. On it at the capacitor, the impedance is at the level of two kilo-ohms. 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 may be too large, so it is best to abandon its use. True, it is necessary to choose the right shear stress, equating it to zero in value. And you can apply a T-shaped divider and increase the resistance values of both resistors in the circuit.
Which scheme is preferable to use
Most developers prefer non-inverting amplifiers, since they have a very high input impedance. And neglect the circuits of the inverting type. But the latter has a huge advantage - it is not demanding on the operational amplifier itself, which is its “heart”.
In addition, the characteristics, in fact, he is much better. And with the help of imaginary grounding, it is possible to combine all signals without special effort, and they will not exert any influence on each other. Can be used in designs and the circuit of a DC amplifier on an operational amplifier. It all depends on the needs.
And the most recent is the case if the entire circuit discussed here is connected to the stable output of another operational amplifier. In this case, the impedance value at the input does not play a significant role - at least 1 kOhm, at least 10, at least infinity. In this case, the first cascade always performs its function with respect to the next.
Repeater circuit
The repeater operates on an operational amplifier similar to an emitter built on a bipolar transistor. And performs similar functions. In fact, this is a non-inverting amplifier in which the first resistor is infinitely large, and the second is zero. In this case, the gain is equal to unity.
There are special types of operational amplifiers that are used in technology only for repeater circuits. They have significantly better characteristics - as a rule, this is high speed. As an example, such operational amplifiers as OPA633, LM310, TL068. The latter has a case, like a transistor, as well as three conclusions. Very often, such amplifiers are simply called buffers. The fact is that they have the properties of an insulator (very large input impedance and extremely low output). Approximately according to this principle, a current amplifier circuit is built on an operational amplifier.
Active mode
In fact, this is such a mode of operation in which the outputs and inputs of the operational amplifier are not overloaded. If a very large signal is applied to the input of the circuit, then it simply starts cutting at the output by 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 op amp inputs. In this case, the magnitude cannot be greater than the supply voltage of the amplifier stage.
Most of the circuits on operational amplifiers are calculated in such a way that this range is less than the supply voltage by 2 V. But it all depends on which particular amplifier circuit is used on the operational amplifier. There is the same restriction on the stability of a current source based on an operational amplifier.
Suppose there is a voltage drop in the source with a floating load. If the current has a normal direction of movement, you can meet a strange load at first glance. For example, several polarity batteries. This design can be applied in order to obtain a direct charge current.
Some precautions
A simple voltage amplifier on an operational amplifier (any circuit can be selected) can be made literally "on the knee". But you will need to consider some features. Be sure to make sure that the feedback in the circuit is negative. This also suggests that it is unacceptable to confuse the non-inverting and inverting inputs of the amplifier. In addition, a DC feedback loop must be present. Otherwise, the operational amplifier will quickly go into saturation mode.
Most operational amplifiers have a very small input differential voltage. In this case, the maximum difference between non-inverting and inverting inputs can be limited to 5 V for any connection to the power source. If this condition is neglected, rather large currents will appear at the input, which will lead to the fact that all the characteristics of the circuit will deteriorate.
The worst part is the physical destruction of the operational amplifier itself. As a result, the amplifier circuit on the operational amplifier stops working completely.
Should be considered
And, of course, you need to talk about the rules that should be followed in order to ensure stable and durable operation of the operational amplifier.
Most importantly, the op-amp has a very high voltage gain. And if the voltage between the inputs changes by a fraction of millivolts, at the output its value can change significantly. Therefore, it is important to know: for an operational amplifier, the output tries to ensure that the voltage difference between the inputs is close (ideally equal) to zero.
The second rule - the current consumption of the operational amplifier is extremely small, literally nanoamperes. If field-effect transistors are installed at the inputs, then it is calculated by picoamperes. From this we can conclude that the inputs do not consume current, regardless of which operational amplifier is used, the circuit - the principle of operation remains the same.
But you should not think that the op amp really constantly changes the voltage at the inputs. Physically, this is almost impossible to implement, since there would be no compliance with the second rule. Thanks to the operational amplifier, the status of all inputs is evaluated. Using the external feedback circuit, voltage is transmitted to the input from the output. The result - between the inputs of the operational amplifier, the voltage difference is at zero.
Feedback concept
This is a common concept, and it is already used in a broad sense in all areas of technology. In any control system, there is feedback that compares the output signal and the setpoint (reference). Depending on what value is current - there is an adjustment in the right direction. Moreover, the control system can be anything, even a car that rides on the road.
The driver presses the brakes, and the feedback here is the beginning of the slowdown. Having drawn an analogy with such a simple example, we can better understand feedback in electronic circuits. A negative feedback would be if the car accelerated by depressing the brake pedal.
In electronics, feedback refers to the process during which a signal is transmitted from output to input. In this case, the signal is also extinguished at the input. On the one hand, this is not a very reasonable idea, because it may seem from the side that the gain will decrease significantly. Such reviews, by the way, were received by the founders of feedback development in electronics. But it is worthwhile to understand in more detail its effect on operational amplifiers - consider practical schemes. And it will become clear that it really slightly reduces the gain, but it allows you to slightly improve the remaining parameters:
- Smooth frequency response (leads them to the necessary).
- Allows you to predict the behavior of the amplifier.
- Able to eliminate non-linearity and signal distortion.
The deeper the feedback (we are talking about negative), the less influence the characteristics of an open-source OS have on the amplifier. The result - all of its parameters depend only on what properties the circuit has.
It is worth paying attention to the fact that all operational amplifiers operate in a mode with very deep feedback. And the voltage gain (with its open loop) can reach even several million. Therefore, the amplifier circuit on an operational amplifier is extremely demanding to comply with all parameters for power and input signal level.