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Schemes ou. Operational amplifier? It's very simple
Good day to all. In the last article I talked about nutrition. In this article I will talk about the use Op-amp in linear circuits.
Voltage repeater
The first circuit I'll talk about is a unity gain amplifier circuit (unit amplifier) or so-called. The circuit of this amplifier is shown below
It is advisable to use dual potentiometers for inputs 1, 2 and 3 to adjust the left and right channels simultaneously. For operational amplifiers, we mean amplifiers that have the following characteristics. At this time, the electrical industry makes the entire range of operational amplifiers integrated design, so their use has reached the level of using common parts. Most of these op amps have two inputs.
Inverted Input - The input signal to this input appears at the 180° phase-reversed output. Without reverse input - the signal from this input will be converted to output in the same phase. For op amps we use the following circuits. Circuits for setting the operating rest point - resistors are used or compensated transistor connections are used.
Unity gain amplifier (voltage follower).
This circuit is a modification, the difference is that there is no feedback resistor and a resistor at the inverting input. Thus, the voltage from the output of the op-amp is completely supplied to the inverting input of the op-amp, and, therefore, the feedback transmission coefficient is equal to unity (β = 1).
Circuits for suppressing drift drives and differential components of differential inputs. Differential amplifiers are used at the inputs of operational amplifiers. This is a transistor connection that will allow you to output a signal to two inputs. The basic arrangement of a balanced differential amplifier is shown in Fig.
The main partition layout is different. increased. The operating points of the transistors are set so that both transistors are in a conducting state. For connections, we distinguish two scenarios. Skew indices are derived from English nouns. The nebulous quality of the differential amplifier is the so-called brakeness of the sensation signal, which is referred to, denoted in dB, and the value of H is greater the higher the gain.
As is known, the input impedance of a feedback op-amp is determined by the following expression
- where R BX is the input impedance of the op-amp without feedback,
Then for the voltage follower the input resistance will be
The output impedance of a closed loop op amp is given by
The resulting differential amplifier voltages are related. Basic connection of operational amplifiers. The basic operation of operational amplifiers is based on inverting and non-inverting connections. Inverting wiring is shown in Fig. It can be seen that at very high clipping, the switching of the inverting inputs is only return and input impedance and is not affected by unstable amplifiers. Feedback should always be directed to the inverting input to make it unbeatable.
The network connection is shown in Fig. We allow the input signal to return to the input. Again, we are rising out of balance between the voltages on both inputs. In non-inverse connections, the input voltage does not rotate and the height is always greater than or equal to. Application of operational amplifiers.
- where R BІX is the input impedance of the op-amp without feedback,
- β – transmission coefficient of the OS circuit,
- K is the gain of the op-amp without feedback.
Since the voltage follower has a feedback transmission coefficient equal to unity (β = 1), the output resistance will have the following form
Applications such as inversion and non-inversion amplifiers have been shown to benefit. Other options. Possible connection diagrams are shown in Fig. The advantage of these connections, which use both inverting and non-inverting connections, is that the reference source is limited DC, and the reverse voltage can be set to the desired output voltage level.
Possible connections are again shown in Fig. The amplifier regulates the output voltage so that a current equal to the input current always flows. In the first two connections it is also closely coupled to the reverse link circuit. Unfortunately, neither end of the lot should be grounded. This drawback is eliminated by the participation of a third party.
Example of calculating voltage follower parameters
For example, let's calculate a voltage follower on an op-amp, which at the required frequency has a gain K Y = 80 (38 dB), input resistance R BX = 500 kOhm, output resistance R BыX = 300 Ohm.
Summation is performed at one input. A ratio is applied to the voltage output. The behavior of the characteristic curve is shown in Fig. The connection diagram is shown in Fig. Transmission overridden. The transmission frequency is shown in Fig. These values are used to asymptotically approximate the ratio. The connection and transmission are shown in Fig.
The transmission has three elements: the first has the integration sign, the second is proportional, and the third is the output. Participation used in this way has two disadvantages. Large-scale integration of an integral element at low frequencies causes low-frequency wiring instability.
The input resistance of the voltage follower will be
The output resistance of the voltage follower will be
Disadvantages of the simplest voltage follower circuit
Due to the fact that the gain of an open-circuit op-amp changes with frequency (as the frequency increases, the gain decreases), therefore, the input and output resistances also depend on frequency (as the frequency increases, the input resistance decreases and the output resistance increases).
The high frequency of the derivatized cell with increasing frequency leads to the need for intensified shaking. A log amplifier outputs output voltages that are not matched by the logarithm of the input voltage. The logarithmic characteristic is obtained using semiconductor diodes or transistors.
For the output signal we obtain the relation. The connection is used when the input signal varies by more than ten years and we require the output signal to be within the range. The diode input operates over a range of about 3 input channels. If the input signal changes logarithmically, the output signal changes linearly.
If the input signal has a sufficiently large DC component and a significant amplitude swing, then a situation may arise where the common-mode input voltage limit is exceeded. To eliminate this problem, the signal must be supplied to the non-inverting input through a decoupling capacitor, and a resistor must be connected between the non-inverting input and ground, but this resistor will affect the input impedance of the repeater.
If we require action on decadal lines, the transistor is used as an exponential element. The main connection is shown in the figure. Here you can write the relation for the transistor current and for the voltage output. When using transistors, the dynamic range can be up to 9 decibels of input voltage. However, the op amp must have low drift and low input currents to take full advantage of the range. In practice, these amplifiers are complemented by circuits that suppress oscillations in the generator circuit and thermal dependence compensation circuits.
Another way to improve the parameters of a voltage follower, which is recommended by op-amp manufacturers, is to include resistors with the same resistance in the OS circuit and between the non-inverting input and ground. In this case, the gain of the op-amp will also be equal to unity, but the input and output resistance will depend on external resistors, and not on the parameters of the op-amp.
The nonlinear characteristic can be realized by the circuit shown in FIG. When designing a circuit, we approximate the theoretical nonlinear dependence with a line of linear arrays, the number of which is specified up to approximation. The numbers are marked to help understand the three strands. The amplifier's gain is reduced to a value, and the voltage output that is connected to the input current and the common feedback resistor will begin to rise more slowly. The advantage of the connection is that there is a very low temperature dependence, the disadvantage is that in case of demand for action over the rules, the number of diodes increases very much.
The most effective way to improve the parameters of a single amplifier is a circuit in which, after the voltage follower circuit, a power amplifier is connected that provides a large output current. In this case, the voltage gain will be approximately unity, and the feedback current is determined by the characteristics of the power amplifier (the input and output resistance are multiplied by the gains of both amplifiers).
Logarithms are used to implement multipliers by creating the logarithm of one object as a pair of two logarithms of the input signals, and by receiving the output signal from the logger, we will get the required file. Another procedure is to help the quadrators who implement the equation.
Exponential amplifiers are used to delocalize a malignant signal or to perform exponential types of functions. An operational amplifier is a component with two inputs and one output. It stands out as a triangle with the tip exiting and the entrances on the opposite side, with the fact that the feed legs are not usually drawn as that is of course the case.
Non-inverting amplifier
After analyzing the voltage follower, which, in fact, is a non-inverting amplifier with a gain equal to unity, we will move on to considering the circuit of a non-inverting amplifier with an arbitrary gain. This type of amplifier is characterized by having high input and low output impedance, the amplifier circuit is shown below
It is usually a small integrated circuit with complex internal connections. It has the following features. We can use this to compare which of two inputs has more high voltage. But if we want to boost the signal, the gain is usually too large and we don't know it exactly.
Feedback means that we will somehow connect the output signal to the input. If the feedback is positive, it increases the gain frequency and usually causes the circuit to become unstable, which then switches to the high or low limit position and remains there. If the positive connection is delayed, it may cause the circuit to flash.
Non-inverting amplifier circuit.
This circuit is one of the standard operational amplifier circuits and contains op-amp DA1, bias resistor R1 and feedback resistor R2. The operational amplifier in this circuit is covered by serial voltage feedback, the transmission coefficient of the feedback circuit is
In the simplest case, we will connect the circuit as follows. The input voltage will then be equal to one tenth of the output. When we input a voltage of 0.5V at the input, the op-amp will increase the output voltage until the input voltage reaches the same level.
But this requires the output to be ten times the input voltage! Thus, the amplifier “forcibly” increases the voltage 10 times. We will simply calculate the gain in the return circuit using the formula. The op-amp can also be connected so that the amplified signal has an inverted value. Against the previous connection, it is enough to replace the ground and signal input.
Then the input resistance of the non-inverting amplifier will be
R BX.O-Amp – input impedance of the op-amp with the OS circuit open,
K op-amp – op-amp gain when the feedback circuit is open.
The output impedance of a non-inverting amplifier can be calculated from the following expression
Since zero voltage is present at the input, the output must always have the opposite voltage to bias it. We will calculate the wiring inversion gain as follows. Positive feedback with negative feedback resistors as an integration article. The circuit alternately discharges and charges the capacitor. We can use rectangular and triangular waveforms as output. The voltages are plotted over time.
The following 4 amplifier amplifiers have 3 outputs that generate triangle, square and sine wave signals at selectable frequencies. The red part is a voltage divider, giving the op amps half the supply voltage as the center against which they amplify the AC signal. The yellow part is an oscillator in which square and triangle waveform is generated in nature. In the purple portion of the triangle waveform, we maintain the amplitude but increase the current so that possible sampling does not interfere with the oscillator frequency. The green section uses the diode's voltage drop to preserve the steep edges of the triangle for small deflections and attenuate the signal for large deflections. The blue part is a classic amplifier. This triangle signals a sine wave! . A useful connection will occur when we connect the output to the input only and the signal is introduced to the input.
R OUT.O-Amp – output resistance of the op-amp when the OS circuit is open.
Non-inverting amplifier gain
In this type of amplifier there is a certain level of bias voltage UCM at the input, so this circuit can be used where the level of bias voltage at the input does not have a significant effect. The input bias voltage level will be
Although the circuit will not be amplified, it will allow current to be drawn from the output without the need for input flow. In this way, we can measure voltage even in cases where the circuit behavior and minimum current consumption may change. When we replace the feedback resistor with a capacitor, the output of the amplifier will be the integral of the input signal. In other words, the input-output ratio will be the same as the ratio of driving speed.
Op amps for everyone. . An operational amplifier is used to implement electronic integrated circuits, such as adding or subtracting voltages and currents. In real systems, the input resistance is very high, on the order of megaohms, and the voltage gain is on the order of tens of decibels. Due to the high input impedance, the operational amplifier does not load the signal source. Because it has a high voltage amplitude, the linear range is small, so this type of system is designed to operate with external feedback, typically negative feedback.
Example of calculation of a non-inverting amplifier
Let's calculate a non-inverting amplifier that should provide a gain K = 10. We use the K157UD2 as an op-amp, which has the following parameters: gain (at a frequency of 1 kHz) K = 1800 (65 dB), input resistance R BX.O-Amp = 500 kOhm, output resistance R BУХ.ОУ = 300 Ohm, bias voltage U CM = 10 mV, input current I ВХ ≤ 500 nA. The input signal has a level U IN = 40 mV.
Non-inverting adder
Continuing the topic of non-inverting amplifiers, I’ll tell you about a non-inverting adder, which performs the function of adding input signals and is used as linear signal mixers (mixers), for example, when signals from several sources need to be combined and fed to the input of a power amplifier. The non-inverting adder circuit is shown below
This circuit is a non-inverting amplifier with two inputs and consists of op-amp DA1, current-limiting input resistors R1 and R2, bias resistor R3 and feedback resistor R4.
For this circuit, the basic relationships correspond to the circuit of a simple non-inverting amplifier, taking into account the fact that the input voltage in the circuit corresponds to the average voltage of the input pins
And the resistance of the resistors must meet the following condition
The gains for different channels are determined by the following expression
R N – input resistor resistance,
K N – gain of the corresponding amplification channel.
The main disadvantage of the non-inverting adder circuit is the absence of a zero potential point, so the gains of the various inputs are not independent. This disadvantage manifests itself in cases where the internal resistance of the input voltage sources or only one of them is known approximately or changes during operation.
Theory is good, but theory without practice is just hot air.
The inverting amplifier is one of the simplest and most commonly used analog circuits. With just two resistors, we can set the gain we need. Nothing prevents us from making the coefficient less than 1, thereby weakening the input signal.
Often another R3 is added to the circuit, the resistance of which is equal to the sum of R1 and R2.
To understand how an inverting amplifier works, let's simulate simple diagram. We have a voltage of 4V at the input, the resistance of the resistors is R1=1k and R2=2k. It would be possible, of course, to substitute all this into the formula and immediately calculate the result, but let's see exactly how this scheme works.
Let's start with a reminder of the basic operating principles of an op-amp:
Rule No. 1 - the operational amplifier influences its output on the input through OOS (negative feedback), as a result of which the voltage at both inputs, both inverting (-) and non-inverting (+), is equalized.
Please note that the non-inverting input (+) is connected to ground, that is, it has a voltage of 0V. In accordance with rule No. 1, the inverting input (-) should also be 0V.
So, we know the voltage at the terminals of resistor R1 and its resistance is 1k. Thus, with the help we can perform a calculation and calculate how much current flows through resistor R1:
IR1 = UR1/R1 = (4V-0V)/1k = 4mA.
Rule No. 2 - amplifier inputs do not consume current
Thus, the current flowing through R1 continues to flow through R2!
Let's use Ohm's law again and calculate what voltage drop occurs across resistor R2. We know its resistance and we know what current is passing through it, therefore:
UR2 = IR2R2 = 4mA *2k = 8V.
It turns out that we have 8V at the output? Not certainly in that way. Let me remind you that this is an inverting amplifier, that is, if we apply a positive voltage to the input and remove a negative voltage at the output. How does this happen?
This is due to the fact that the feedback is installed at the inverting input (-), and to equalize the voltage at the input, the amplifier reduces the potential at the output. Connections of resistors can be considered as simple, therefore, in order for the potential at the point of their connection to be equal to zero, the output must be minus 8 volts: Uout. = -(R2/R1)*Uin.
There is another catch associated with rule 3:
Rule No. 3 - the voltages at the inputs and outputs must be in the range between the positive and negative supply voltage of the op-amp.
That is, we need to check that the voltages we calculated can actually be obtained through the amplifier. Beginners often think that an amplifier works as a source of free energy and produces voltage out of nothing. But we must remember that the amplifier also needs power to operate.
Classic amplifiers operate on voltages of -15V and +15V. In such a situation, our -8V, which we calculated, is the real voltage, since it is in this range.
However, modern amplifiers often operate at voltages of 5V or lower. In such a situation, there is no chance that the amplifier will give us minus 8V at the output. Therefore, when designing circuits, always remember that theoretical calculations always need to be supported by reality and physical capabilities.
It should be noted that the inverting amplifier has one drawback. We already know what does not load the signal source, since the amplifier inputs have very high resistance, and consume so little current that in most cases it can be ignored (rule #2).
The inverting amplifier has an input resistance equal to the resistance of resistor R1, in practice it ranges from 1k...1M. For comparison, an amplifier with field-effect transistor inputs has a resistance of the order of hundreds of megaohms and even gigaohms! Therefore, sometimes it may be advisable to install a voltage follower in front of the amplifier.
