R.C.-generator is a generator of harmonic oscillations, in which, instead of an oscillatory system containing elements L And WITH, a resistive-capacitive circuit is used ( R.C.-circuit) with frequency selectivity.

The exclusion of inductors from the circuit makes it possible to significantly reduce the size and weight of the generator, especially at low frequencies, since as the frequency decreases, the dimensions of the inductors sharply increase. An important advantage R.C.-generators compared to L.C.- generators is the possibility of their manufacture using integrated technology. However R.C.- generators have low stability of the frequency of generated oscillations due to low quality factor R.C.-circuits, as well as poor oscillation shape due to poor filtering of higher harmonics in the output oscillation spectrum.

R.C.-generators can operate in a wide range of frequencies (from fractions of a hertz to tens of megahertz), but they have found application in communication equipment and measuring technology mainly at low frequencies.

Basic theory R.C.-generators were developed by Soviet scientists V.P. Aseev, K.F. Teodorchik, E.O. Saakov, V.G. Kriksunov and others.

R.C.- the generator usually includes a wideband amplifier made of a tube, transistor or integrated circuit and R.C.-chain feedback, which has selective properties and determines the oscillation frequency. The amplifier compensates for energy losses in passive elements and ensures that the amplitude conditions for self-excitation are met. The feedback circuit ensures that the phase condition of self-excitation is fulfilled at only one frequency. By type of feedback circuit R.C.-generators are divided into two groups:

with zero phase shift in the feedback circuit;

with a phase shift in the feedback circuit by 180.

To improve the shape of the generated oscillations in R.C. generators use elements that have nonlinearity, which limit the increase in the amplitude of oscillations. The parameters of such an element change depending on the amplitude of the oscillations, and not on their instantaneous values ​​(a thermistor, the resistance of which depends on the degree of heating by the current passing through it). With this limitation, the shape of the oscillations does not change; they remain harmonic even in stationary mode.

Let's consider both types R.C.-autogenerators.

Self-oscillator with a 180 phase shift in the feedback circuit.

Such a self-generator is also called a self-generator with a three-link chain. R.C..

In the diagrams R.C.-oscillators with a phase shift of 180 use amplifiers in the feedback circuit to invert the phase of the input voltage. Such an amplifier can, for example, be an operational amplifier with an inverting input, a single-stage amplifier, or a multi-stage amplifier with an odd number of inverting stages.

In order for the phase balance equation to be satisfied, the feedback circuit must provide a phase shift OS = 180.

To substantiate the structure of the feedback circuit, we reproduce the phase-frequency characteristics of the simplest R.C.-links (Fig. 3,4).

Rice. Option 3 R.C.-link and its phase response

Rice. 4 Option R.C.-link and its phase response

From the graphs it is clear that one of the simplest R.C.-link introduces a phase shift not exceeding 90. Therefore, a phase shift of 180 can be achieved by cascading connection of three elementary R.C.-links (Fig. 5).

Rice. 5 Circuits and phase response of three-element R.C.-chains

Elements R.C.- the circuits are designed so as to obtain a phase shift of 180 at the generation frequency. One of the generator options with a three-link circuit R.C. shown in Figure 6

Rice. 6 Generator with three-link chain R.C.

The generator consists of a resistive transistor amplifier and a feedback circuit. A single-stage amplifier with a common emitter produces a phase shift between the voltage on the collector and the base K = 180. Therefore, to achieve phase balance, the feedback circuit must provide OS = 180 at the frequency of the generated oscillations.

Let us analyze the feedback circuit, for which we will compile a system of equations using the loop current method.

Solving the resulting system with respect to the feedback coefficient, we obtain the expression

From the expression it follows that the phase shift 180 is obtained in the case when it is a real and negative value, i.e.

therefore, generation is possible at a frequency

At this frequency the modulus of the feedback coefficient

This means that to excite self-oscillations, the amplifier coefficient must be greater than 29.

The output voltage of the generator is usually taken from the collector of the transistor. To obtain harmonic oscillations, a thermistor is included in the emitter circuit R T with positive temperature coefficient of resistance. As the oscillation amplitude increases, the resistance R T increases and the depth of negative feedback in the amplifier for alternating current increases, respectively, the gain decreases. When a stationary oscillation mode occurs ( TO= 1), the amplifier remains linear and no distortion of the collector current shape occurs.

Self-oscillator with zero phase shift in the feedback circuit.

A characteristic feature of the circuits R.C.-oscillators with zero phase shift in the feedback circuit is the use of amplifiers in them that do not invert the phase of the input signal. Such an amplifier can, for example, be an operational amplifier with a non-inverting input or a multi-stage amplifier with an even number of inverting stages. Let's consider some possible options for feedback circuits that provide zero phase shift (Fig. 7).

Rice. 7 Options for feedback circuits providing zero phase shift

They consist of two links, one of which represents RС-link with a positive phase shift, and the second – with a negative phase shift. As a result of adding the phase response at a certain frequency (generation frequency), a phase shift equal to zero can be obtained.

In practice, the phase-balance bridge, or in other words the Wien bridge (Fig. 7c), the use of which is shown in the diagram, is most often used as a selective circuit with zero phase shift R.C.-oscillator with zero phase shift, made on an operational amplifier (Fig. 8).

Rice. 8 R.C.-generator with zero phase shift in the OS circuit

In this circuit, the voltage from the output of the amplifier is supplied to its non-inverting input through a feedback circuit formed by the elements of the Wien bridge R 1 C 1 and R 2 C 2. Resistive circuit R.R. T forms another feedback - negative, which is designed to limit the increase in the amplitude of oscillations and maintain their harmonic form. The negative feedback voltage is applied to the inverting input of the operational amplifier. Thermistor R T must have a negative temperature coefficient of resistance.

Feedback circuit gain

must be a real and positive quantity, and this is possible if the equality

From here the frequency of the generated oscillations is determined. If R 1 = R 2 =R, C 1 = C 2 = C, That

The amplitude condition for self-excitation at frequency 0 requires the fulfillment of the inequality

If there is equality R 1 = R 2 = R And C 1 = C 2 = C gain TO > 3.

The oscillation frequency can be changed by changing the resistances R or capacitor capacities WITH, included in the Wien bridge, and the amplitude of oscillations is regulated by resistance R.

Main advantage R.C.- generators in front L.C.-generators is that the former are easier to implement for low frequencies. For example, if in a generator circuit with zero phase shift in the feedback circuit (Fig. 8) R 1 = R 2 = 1 MOhm, C 1 = C 2 = 1 µF, then the generated frequency

.

To get the same frequency in L.C.-generator, inductance would be required L= 10 16 Hn at WITH= 1 µF, which is difficult to implement.

IN R.C.-generators can be done by simultaneously changing the values ​​of the capacitors WITH 1 and WITH 2, obtain a wider frequency tuning range than is the case in L.C.-generators. For L.C.-generators

while for R.C.- generators, with WITH 1 = WITH 2

To the disadvantages R.C.-generators should be attributed to the fact that on relatively high frequencies they are more difficult to implement than L.C.-generators. Indeed, the value of the capacitance cannot be reduced below the installation capacitance, and a decrease in the resistance of the resistors leads to a drop in the gain, which makes it difficult to satisfy the amplitude condition of self-excitation.

Listed advantages and disadvantages R.C.-generators led to their use in the low-frequency range with a large frequency overlap coefficient.

Harmonic oscillation generator called a device that creates an alternating sinusoidal voltage in the absence of input signals. Generator circuits always use positive feedback.

Oscillations are called free(or their own), if they are accomplished due to the initially perfect energy with the subsequent absence external influences to an oscillatory system (a system that oscillates). The simplest type of oscillations are harmonic oscillations - oscillations in which the oscillating quantity changes over time according to the law of sine (cosine).

Generators are an integral part of many measuring instruments and the most important blocks of automatic systems.

There are analog and digital generators. For analog harmonic generators, an important problem is the automatic stabilization of the output voltage amplitude. If the circuit does not include automatic stabilization devices, stable operation of the generator will be impossible. In this case, after the occurrence of oscillations, the amplitude of the output voltage will begin to constantly increase, and this will lead to the fact that the active element of the generator (for example, an operational amplifier) ​​will enter saturation mode. As a result, the output voltage will differ from harmonic. Automatic amplitude stabilization schemes are quite complex.

Structural generator circuit is shown in the figure below:

IE is a source of energy,

UE - amplifier,

POS - positive feedback circuit,

OOS - negative feedback circuit,

FC - oscillation former (LC circuit or phasing RC circuit).

By method of obtaining oscillations generators are divided into two groups: generators with external stimulation and generators with self-excitation. An externally excited generator is a power amplifier, the input of which is supplied electrical signals from the source of vibrations. Self-excited generators contain oscillation formers; such generators are often called autogenerators .

The principle of operation of a self-generator.

It is based on automatic replenishment of the energy expended by the oscillation driver.

In this case, the following must be observed:

-amplitude balance rule- the product of the gain and the feedback coefficient must be equal to 1.

-phase balance rule- it means that oscillations occur at a very specific frequency at which the phases coincide.

If both conditions are met, oscillations arise smoothly or abruptly and are automatically maintained with a given range. With a large phase shift, the oscillations will cancel each other out and subsequently disappear completely.

There are many types of sine wave generator circuits. Generators for frequencies from several tens of kilohertz and above contain LC circuits , and generators for low frequencies, as a rule, RC filters .

Circuits of LC harmonic generators.

In generators with LC circuits Inductive coils and capacitors with high quality factor are used. A self-oscillator - an oscillation former - is one or more amplification stages with positive frequency-dependent feedback circuits; Feedback circuits contain oscillatory circuits. Various options for switching on the oscillatory circuit relative to the electrodes of the electronic device are possible: only at the input, only at the output, or simultaneously in several sections of the circuit. Based on the methods of connecting LC elements to the electrodes of the amplifying elements, a distinction is made between transformer coupling and the so-called three-point coupling - inductive or capacitive. A self-generator with transformer coupling is shown in Fig. 1.

Rice. 1. Autogenerator-former of sinusoidal oscillations with transformer coupling.

The oscillatory circuit, consisting of a coil Lk and a capacitor C, is the collector load of the transistor V1. The inductive coupling between the output and input of the amplifier is provided by the coil Lb connected to the base of the transistor. Elements R1, R2, Re, Se are designed to provide the required operating mode for DC and its thermal stabilization.

Thanks to capacitor C1, which has low resistance at the generation frequency, a circuit is created for the alternating current component between the base and emitter of the transistor. The dots indicate the beginnings of the windings Lb and Lk, since it is necessary to comply with the phase balance condition. Phase balance condition observed if the influx of energy occurs synchronously with a change in the sign of the voltage on the circuit; for example, in a cascade with a transistor connected according to an OE circuit, the phases of the input and output signals are mutually shifted by 180° C. Therefore, the ends of the coil Lb must be connected so that the input and output oscillations are in phase. Amplitude balance condition is that losses in the circuit and load are continuously replenished by the power source.

Rice. 1a. Autogenerator operation. Transient processes.

Anti-generator operation(Fig. 1a) begins when the Ek source is turned on. The initial current pulse excites oscillations in the LcC circuit with a frequency , which could stop due to thermal energy losses in the active resistance of the coil and capacitor. But since there is an inductive coupling between the coils Lb and Lk with a mutual induction coefficient M, in the base circuit there will be alternating current , coinciding in phase with the current of the collector circuit (the phase balance condition is ensured by the rational inclusion of the ends of the winding Lb). The amplified oscillations are transmitted from the circuit again to the base circuit, and the amplitude of the oscillations gradually increases, reaching a given value.

Rice. 2. Generators of sinusoidal oscillations based on an oscillatory circuit assembled using a three-point inductive (a) and capacitive (b) circuit.

Autogenerator assembled according to three-point scheme, shown in Fig. 2, a. The oscillatory circuit, consisting of a sectioned coil Lk and a capacitor Sk, is the load of transistor V1. The Lk coil is divided into two parts: one of its terminals is connected to the collector, the second to the base of the transistor; energy is supplied to one of the middle turns of this coil. This connection ensures phase balance and is very simple and reliable. The DC operating mode of the transistor and its thermal stabilization are carried out using the same elements as in the transformer generator circuit (see Fig. 1). The capacitive three-point circuit (Fig. 2,b) contains two capacitors in the capacitive branch of the oscillatory circuit, the middle point between which is connected to the emitter of transistor V1. The oscillatory circuit is connected in series between the energy source and the UE. The voltages on the capacitors have opposite polarity relative to the common point, which ensures that the phase balance condition is met.

Circuits of RC harmonic oscillation generators.

RC oscillators used to generate infra-low and low frequency oscillations (from fractions of a hertz to several tens of kilohertz); RC generators can produce oscillations at higher frequencies, but low-frequency oscillations are more stable.

Rice. 3. Autogenerators of sinusoidal oscillations with a target of L-shaped RC links (a) and bridge type (b).

An RC oscillator consists of an amplifier (single- or multi-stage) and a frequency-dependent feedback circuit. Feedback circuits are made in the form of “ladder” (Fig. 3, a) or bridge (Fig. 3, b) RC circuits.

RC oscillator with multi-link The RC feedback circuit is shown in Fig. 3, a. Three series-connected phasing evens R1C1-R3C3, connected between the output and input of the amplifier stage, form a positive feedback circuit with filtering properties. It supports the oscillatory process only at one specific frequency; Without RC elements, a single stage amplifier would have negative voltage feedback. Phase balance condition The result is that each of the RC links rotates the phase of the signal by an angle of 60°, and the total shift angle is 180°. The amplitude balance condition is satisfied by choosing the appropriate stage gain.

Autogenerator with RC filter bridge type shown in Fig. 3, b. Two arms of the bridge - links R1C1 and R2C2 - are connected to the non-inverting input of amplifier 2 (the number inside the triangle indicates the number of stages). These links form the PIC chain. Another diagonal is connected to the inverting input of the same amplifier, composed of nonlinear elements R3 and r, which creates an OOS circuit. In this circuit, the bridge has a selective property and the phase balance condition is ensured at one frequency (at which the output signal of the bridge is in phase with the input). Frequency adjustment in this self-oscillator is simple and convenient, and is possible in a very wide frequency range. It is carried out by changing either the resistances of both resistors or the capacitances of both capacitors of the bridge.

A common drawback of all generators is the sensitivity of the generated frequency to changes in supply voltages, temperature, and “aging” of circuit elements.

The most widespread are two types of phase-shifting chains: the so-called ladder chains (Figure 3, a, b) and the Wien bridge (Figure 3, c).

Rice. 3. Three-link RС circuits (a, b) and Wien bridge diagram (c)

Ladder chains represent a series connection of usually three R.C. links, each of which with identical elements ( R 1 = R 2 = R 3 = R And C 1 = C 2 = C 3 = C ) provides a phase shift of the signal by 60°. As a result, the output voltage will be shifted in relation to the input voltage by 180°. Depending on which of the chain elements is final, they are named either WITH -parallel (Figure 3,a), or R -parallel (Figure 3, b). To excite oscillations, the amplifier must also have a phase shift of 180°, i.e. it must be inverting. The ladder circuit must be connected to the inverting input of the amplifier.

The generator frequency is determined by the time constant R.C. chains. The frequency of generated sinusoidal oscillations for these circuits, provided R 1 = R 2 = R 3 = R And C 1 = C 2 = C 3 = C calculated using the following formulas:

For circuit WITH -parallel

for the circuit R -parallel

To ensure amplitude balance, the gain of the amplifier must be equal to or exceed the attenuation introduced by the phase-shifting chain through which the output voltage is supplied to the amplifier input. Calculations show that for the above circuits the attenuation is equal to 210. Consequently, circuits using three-link phase-shifting chains with identical links can generate sinusoidal oscillations with a frequency only if the amplifier gain exceeds 210. Wien bridge (chain) (Figure 3 ,c) consists of two RС links The first link consists of a series connection R And WITH and has resistance

The second link consists of parallel connection the same R And WITH and has resistance

The transmission coefficient of the positive feedback link is determined by the expression

from where after substitution Z1 And Z2 , we'll find

If the condition is met

then the phase shift will be zero, and .

In this case, the frequency of the generator can be determined by the formula

Thus, the Wien bridge at the “quasi-resonance” frequency does not create a phase shift and has an attenuation equal to 1/3. Therefore, a Wien bridge must be included in the positive feedback circuit of an amplifier whose open-circuit gain is OS must be at least 3. The use of single-stage amplifier circuits in this case is impossible. In common emitter or common source stages, the phase shift between the input and output signals is 180° , which precludes their use, because in this case, the phase balance condition is violated. Circuits with a common collector or common source, although they do not reverse the phases of the signal, have a voltage gain coefficient of less than unity, as a result of which it is impossible to satisfy the amplitude balance condition. Amplifier stages with a common base or common gate have a very low input resistance, which, when feedback is introduced, shunts its output, reducing its transmission coefficient. Therefore, fulfilling the balance condition turns out to be very difficult. Therefore, when building a generator using discrete elements, a two-stage amplifier is used.

The easiest way to build a generator is on a Wien bridge using an operational amplifier. There's a chain in it POS generated by the Wien bridge can be connected to a direct, non-inverting input, and the desired gain can be set by a resistive divider in the circuit OOC, connected to the inverting input (Figure 4).

Rice. 4. Generator based OU

Resistor ratio in a circuit OOC, ensuring the fulfillment of the amplitude balance condition must meet the relation since the gain for the signal supplied to the non-inverting input is one greater than the ratio of the indicated resistors.

RC generators belong to the class of self-oscillating systems

relaxation type. The main elements of such a generator are

amplifier and aperiodic links made up of resistors and

capacitors. Not having an oscillatory circuit, such

generators, however, make it possible to obtain oscillations close in shape to

harmonic. However, with strong system regeneration, when using

essentially nonlinear areas of the amplifier characteristics, oscillation shape,

due to the absence of an oscillatory circuit, it is greatly distorted. That's why

the generator must operate when the threshold is slightly exceeded

self-excitation.

The main advantages of RC-type generators are simplicity and

small dimensions. These advantages are especially pronounced when

generating low frequencies. To generate frequencies of the order of 100 Hz in

LC generators (Thomson generators) would require very large

inductance and capacitance values

The previous chapter discussed LC self-oscillators. They are used at high frequencies. If it is necessary to generate low frequencies, the use of LC generators becomes difficult. Why? Everything is very simple. Since the formula for determining the frequency of oscillation generation looks like this:

then it is easy to see that to reduce the frequency it is necessary to increase the capacitance and inductance of the circuit. And an increase in capacitance and inductance directly leads to an increase overall dimensions. In other words, the dimensions of the contour will be gigantic. And with frequency stabilization, things will be even worse.

Therefore, they came up with RC self-oscillators, which we will consider here.

The simplest RC generator is the so-called circuit with a three-phase phasing circuit, which is also called a circuit with reactive elements of the same sign. It is shown in Fig. 1.

Rice. 1 - RC self-oscillator with phase-shifting chain

From the diagram it is clear that this is just an amplifier, between the output and input of which a circuit is connected that reverses the phase of the signal by 180º. This circuit is called a phase-shifting circuit. The phase-shifting chain consists of elements C1R1, C2R2, C3R3. Using one chain of rezik and conder, you can obtain a phase shift of no more than 90º. In reality, the shift turns out to be close to 60º. Therefore, to obtain a phase shift of 180º, three chains have to be installed. From the output of the last RC circuit, the signal is supplied to the base of the transistor.

Operation begins the moment the power source is turned on. The resulting collector current pulse contains a wide and continuous spectrum of frequencies, which will necessarily contain the required generation frequency. In this case, the oscillations of the frequency to which the phase-shifting circuit is tuned will become undamped. For oscillations of other frequencies, the self-excitation conditions will not be met and they, accordingly, quickly decay. The oscillation frequency is determined by the formula:

In this case, the following condition must be met:

R1=R2=R3=R
C1=C2=C3=C

Such generators can only operate at a fixed frequency.

In addition to the considered generator using a phase-shifting circuit, there is another interesting, by the way, the most common, option. Let's look at fig. 2.

Rice. 2 - Passive RC bandpass filter with frequency-independent divider

So, this very structure is the so-called Wien-Robinson bridge, although the most common name is simply Wien Bridge. Some more literate people write Wien's bridge with two "n".

The left side of this design is a passive RC bandpass filter, at point A the output voltage is removed. The right side is nothing more than a frequency-independent divider. It is generally accepted that R1=R2=R, C1=C2=C. Then the resonant frequency will be determined by the following expression:

In this case, the gain modulus is maximum and equal to 1/3, and the phase shift is zero. If the ratio of the divider is equal to the transmission ratio bandpass filter, then at the resonant frequency the voltage between points A and B will be zero, and the phase response at the resonant frequency makes a jump from -90º to +90º. In general, the following condition must be met:

Of course, everything, as usual, is considered in ideal or near-ideal cases. Well, in reality, as always, the situation is a little worse. Since each real element of the Wien bridge has a certain spread of parameters, even a slight failure to comply with the condition R3 = 2R4 will either lead to an increase in the amplitude of the oscillations until the amplifier is saturated, or to a damping of the oscillations or their complete impossibility.

To make it completely clear, we will insert an amplification stage into the Wien bridge. For simplicity, let's plug in an operational amplifier (op-amp).

Rice. 3 - The simplest generator with Vina bridge

In general, it will not be possible to use this scheme in this way, since in any case there will be a scatter in the bridge parameters. Therefore, instead of resistor R4, some kind of nonlinear or controlled resistance is introduced. For example, a nonlinear resistor, controlled resistance using transistors, both field-effect and bipolar, and other crap. Very often, the R4 resistor in the bridge is replaced with a micro-power incandescent lamp, the dynamic resistance of which increases with increasing current amplitude. The filament has a fairly large thermal inertia, and at frequencies of several hundred hertz it practically does not affect the operation of the circuit within one period.

Generators with a Wien bridge have one good property: if the resistors R1 and R2 are replaced with a variable one, but only a dual one, then it will be possible to regulate the generation frequency within certain limits. It is possible to divide the condensers C1 and C2 into sections, then it will be possible to switch ranges, and use a dual variable resistor to smoothly regulate the frequency in the ranges. For those in the tank, a nearly practical Wien bridge generator circuit is shown in Figure 4.

Rice. 4 - RC generator with Wien bridge

So, the Wien bridge is formed by conders C1-C8, a double rezik R1 and resonators R2R3. Switch SA1 selects the range, knob R1 allows smooth adjustment in the selected range. Op-amp DA2 is a voltage follower for matching with the load.

Currently, the main types of electronic sine wave generators are LC oscillators, crystal oscillators, and RC oscillators.
LC generators use an oscillating circuit consisting of a capacitor and an inductor, connected either in parallel or in series, the parameters of which determine the oscillation frequency. LC generators are used mainly in the radio frequency range. At low (sound) frequencies, it is more convenient to use RC generators, in which a resistive-capacitive circuit is used to set the oscillation frequency.

LC sine wave generators.

The main types of LC oscillators are the Hartley oscillator and the Colpitts oscillator.

Hartley generator.

In the Hartley generator, or as this circuit is also called - inductive three-point The positive feedback necessary for the occurrence of oscillations is taken from the tap of the inductor (L1 - L2) of the oscillatory circuit.

Colpitts generator.

In a Colpitts generator (three-point capacitive) positive feedback is removed from the midpoint of the composite capacitance (C1 - C2) of the oscillatory circuit. The Colpitts generator is more stable than the Hartley generator and is more commonly used. When high stability is required, crystal oscillators are used.

Quartz is a material capable of converting mechanical energy into electrical energy and vice versa. If an alternating voltage is applied to a quartz crystal, it will begin to oscillate in time with its frequency. Each crystal has its own resonant frequency, depending on its size and structure. The closer the frequency of the applied voltage is to the resonant frequency, the higher the intensity of the oscillations. To make a quartz resonator, metal electrodes are applied to a crystalline quartz plate.

Hartley crystal oscillator circuit with parallel feedback.

Quartz is connected in series to the feedback circuit. If the frequency of the oscillating circuit deviates from the frequency of the quartz, the wave resistance (impedance) of the quartz increases, reducing the amount of feedback to the oscillating circuit. The oscillating circuit returns to the quartz frequency.

Pierce generator.

A very popular circuit because it does not use inductors.

The upper limit of quartz resonance is 25 MHz. If a stable oscillator is needed at a higher frequency, a Butler circuit is used. The oscillating circuit is tuned to the quartz frequency or to the frequency of one of its odd harmonics (third or fifth).

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