LC generators, RC generators. Harmonic oscillation generators Operating principle of an rc oscillator

A generator is a self-oscillating system that generates electric current pulses, in which the transistor plays the role of a switching element. Initially, from the moment of its invention, the transistor was positioned as an amplifying element. The presentation of the first transistor took place in 1947. The presentation of the field-effect transistor occurred a little later - in 1953. In pulse generators it plays the role of a switch and only in alternating current generators does it realize its amplifying properties, while simultaneously participating in the creation of positive feedback to support the oscillatory process.

A visual illustration of frequency range division

Classification

Transistor generators have several classifications:

• by frequency range of the output signal;
• by type of output signal;
• according to the operating principle.

The frequency range is a subjective value, but for standardization the following division of the frequency range is accepted:

• from 30 Hz to 300 kHz – low frequency (LF);
• from 300 kHz to 3 MHz – medium frequency (MF);
• from 3 MHz to 300 MHz – high frequency (HF);
• above 300 MHz – ultra-high frequency (microwave).

This is the division of the frequency range in the field of radio waves. There is an audio frequency range (AF) - from 16 Hz to 22 kHz. Thus, wanting to emphasize the frequency range of the generator, it is called, for example, an HF or LF generator. The frequencies of the sound range, in turn, are also divided into HF, MF and LF.

According to the type of output signal, generators can be:

• sinusoidal – for generating sinusoidal signals;
• functional – for self-oscillation of signals of a special shape. A special case is a rectangular pulse generator;
• noise generators are generators of a wide range of frequencies, in which, in a given frequency range, the signal spectrum is uniform from the lower to the upper section of the frequency response.

According to the operating principle of generators:

• RC generators;
• LC generators;
• Blocking generators are short pulse generators.

Due to fundamental limitations, RC oscillators are usually used in the low-frequency and audio ranges, and LC oscillators in the high-frequency range.

Generator circuitry

RC and LC sinusoidal generators

The most simple way to implement a transistor generator is in a capacitive three-point circuit - the Colpitts generator (Fig. below).

Transistor oscillator circuit (Colpitts oscillator)

In the Colpitts circuit, elements (C1), (C2), (L) are frequency-setting. The remaining elements are standard transistor wiring to ensure the required DC operating mode. A generator assembled according to an inductive three-point circuit—the Hartley generator—has the same simple circuit design (Fig. below).

Three-point inductively coupled generator circuit (Hartley generator)

In this circuit, the generator frequency is determined by a parallel circuit, which includes elements (C), (La), (Lb). The capacitor (C) is necessary to create positive AC feedback.

The practical implementation of such a generator is more difficult, since it requires the presence of an inductance with a tap.

Both self-oscillation generators are primarily used in the mid and high frequency ranges as carrier frequency generators, in frequency-setting local oscillator circuits, and so on. Radio receiver regenerators are also based on oscillator generators. This application requires high frequency stability, so the circuit is almost always supplemented with a quartz oscillation resonator.

The master current generator based on a quartz resonator has self-oscillations with a very high accuracy of setting the frequency value of the RF generator. Billions of a percent are far from the limit. Radio regenerators use only quartz frequency stabilization.

The operation of generators in the region of low-frequency current and audio frequency is associated with difficulties in realizing high inductance values. To be more precise, in the dimensions of the required inductor.

The Pierce generator circuit is a modification of the Colpitts circuit, implemented without the use of inductance (Fig. below).

Pierce generator circuit without the use of inductance

In the Pierce circuit, the inductance is replaced by a quartz resonator, which eliminates the time-consuming and bulky inductor and, at the same time, limits the upper range of oscillations.

The capacitor (C3) does not allow the DC component of the base bias of the transistor to pass to the quartz resonator. Such a generator can generate oscillations up to 25 MHz, including audio frequency.

The operation of all of the above generators is based on the resonant properties of an oscillatory system composed of capacitance and inductance. Accordingly, the oscillation frequency is determined by the ratings of these elements.

RC current generators use the principle of phase shift in a resistive-capacitive circuit. The most commonly used circuit is a phase-shifting chain (Fig. below).

RC generator circuit with phase-shifting chain

Elements (R1), (R2), (C1), (C2), (C3) perform a phase shift to obtain the positive feedback necessary for the occurrence of self-oscillations. Generation occurs at frequencies for which the phase shift is optimal (180 degrees). The phase-shifting circuit introduces a strong attenuation of the signal, so such a circuit has increased requirements for the gain of the transistor. A circuit with a Wien bridge is less demanding on transistor parameters (Fig. below).

RC generator circuit with Wien bridge

The double T-shaped Wien bridge consists of elements (C1), (C2), (R3) and (R1), (R2), (C3) and is a narrow-band notch filter tuned to the oscillation frequency. For all other frequencies, the transistor is covered by a deep negative connection.

Functional current generators

Functional generators are designed to generate a sequence of pulses of a certain shape (the shape is described by a certain function - hence the name). The most common generators are rectangular (if the ratio of the pulse duration to the oscillation period is ½, then this sequence is called a “meander”), triangular and sawtooth pulses. The simplest rectangular pulse generator is a multivibrator, which is presented as the first circuit for beginner radio amateurs to assemble with their own hands (Fig. below).

Multivibrator circuit - rectangular pulse generator

A special feature of the multivibrator is that it can use almost any transistors. The duration of the pulses and pauses between them is determined by the values ​​of the capacitors and resistors in the base circuits of transistors (Rb1), Cb1) and (Rb2), (Cb2).

The frequency of self-oscillation of the current can vary from units of hertz to tens of kilohertz. HF self-oscillations cannot be realized on a multivibrator.

Generators of triangular (sawtooth) pulses, as a rule, are built on the basis of generators of rectangular pulses (master oscillator) by adding a correction chain (Fig. below).

Triangular pulse generator circuit

The shape of the pulses, close to triangular, is determined by the charge-discharge voltage on the plates of capacitor C.

Blocking generator

The purpose of blocking generators is to generate powerful current pulses with steep edges and low duty cycle. The duration of pauses between pulses is much longer than the duration of the pulses themselves. Blocking generators are used in pulse shapers and comparing devices, but the main area of ​​application is the master horizontal scan oscillator in information display devices based on cathode ray tubes. Blocking generators are also successfully used in power conversion devices.

Generators based on field-effect transistors

A feature of field-effect transistors is a very high input resistance, the order of which is comparable to the resistance of electronic tubes. The circuit solutions listed above are universal, they are simply adapted for the use of various types of active elements. Colpitts, Hartley and other generators, made on a field-effect transistor, differ only in the nominal values ​​of the elements.

Frequency-setting circuits have the same relationships. To generate HF oscillations, a simple generator made on a field-effect transistor using an inductive three-point circuit is somewhat preferable. The fact is that the field-effect transistor, having a high input resistance, has practically no shunting effect on the inductance, and, therefore, the high-frequency generator will operate more stable.

Noise generators

A feature of noise generators is the uniformity of the frequency response in a certain range, that is, the amplitude of oscillations of all frequencies included in a given range is the same. Noise generators are used in measuring equipment to evaluate the frequency characteristics of the path being tested. Audio noise generators are often supplemented with a frequency response corrector to adapt to subjective loudness for human hearing. This noise is called “gray”.

Video

There are still several areas in which the use of transistors is difficult. These are powerful microwave generators in radar applications, and where particularly powerful high-frequency pulses are required. Powerful microwave transistors have not yet been developed. In all other areas, the vast majority of oscillators are made entirely with transistors. There are several reasons for this. Firstly, the dimensions. Secondly, power consumption. Thirdly, reliability. On top of that, transistors, due to the nature of their structure, are very easy to miniaturize.

In this article we will look at the RC generator and the principle of its operation, we will consider in detail its circuits, including the operational amplifier.

Description and principle of operation

In amplifier tutorials, we have seen that a single-stage transistor amplifier can generate 180 o of phase shift between its output and input signals when connected in a class A type configuration.

In order for the oscillator to oscillate indefinitely, sufficient feedback of the correct phase must be provided, i.e. "positive feedback", and a transistor amplifier is used as an inverting stage to achieve this goal.

IN RC oscillator circuits the input is shifted 180 o through the amplifier stage and 180 o again through the second inverting stage, giving us "180 o + 180 o = 360 o " phase shift, which is actually 0 o , thereby giving us the required positive feedback. In other words, the phase shift of the feedback loop must be equal to “0”.

IN resistance-capacitance generator or just in the generator R.C. we exploit the fact that a phase shift occurs between the input to an RC network and the output from the same network, for example by using RC elements in the feedback branch.

RC phase circuit

The diagram on the left shows one resistor-capacitor network whose output voltage “leads” the input voltage by an angle of less than 90 o. An ideal single-pole RC circuit will produce exactly 90 o of phase shift, and since oscillation requires 180 o of phase shift, the design RC generator At least two single-pole ones must be used.

However, in reality it is difficult to obtain exactly 90° of phase shift, so more stages are used. The amount of actual phase shift in the circuit depends on the values ​​of the resistor and capacitor, and the selected oscillation frequency with phase angle (Φ) is given by:

Where: X C is the capacitance of the capacitor, R is the resistance of the resistor, and ƒ is the frequency.

In our simple example above, the values ​​of R and C were chosen so that at the desired frequency the output voltage leads the input voltage by an angle of about 60 o. The phase angle between each subsequent RC section is then increased by another 60 o, giving a phase difference between input and output of 180 o (3 x 60 o), as shown in the following phasor diagram.

Then, by connecting three such RC networks together in series, we can produce a full 180 o phase shift in the circuit at a selected frequency, and this forms the basis of a "phase shift oscillator", otherwise called RC generator .

We know that in an amplifier circuit using a bipolar transistor or an op-amp, it will produce a phase shift of 180 o between its input and output. If a three-stage phase-shift RC network is connected between this input and the output of the amplifier, the total phase shift required for regenerative feedback is 3 x 60 o + 180 o = 360 o , as shown below.

Three RC stages are cascaded to obtain the required slope for a stable oscillation frequency. The phase shift of the feedback loop is -180 o when the phase shift of each stage is -60 o . This happens when ω = 2πƒ = 1.732 / RC(tan 60 o = 1.732). Then, to achieve the required phase shift in the RC oscillator circuit, multiple RC phase shifting networks must be used, such as the circuit below.

Basic Circuit of RC Oscillator

Base rc generator, also known as phase shift generator, generates a sinusoidal output signal using regenerative feedback obtained from a resistor-capacitor combination. This regenerative feedback from the RC network is due to the capacitor's ability to store electrical charge (similar to an LC tank circuit).

This resistor-capacitor feedback network can be connected as shown above to create an initial phase shift (phase change network) or interchanged to create a delayed phase shift (phase lag network), the result remains the same as a sine wave. which occur only at a frequency at which the total phase shift is 360 o.

By changing one or more resistors or capacitors in a phase shift network, the frequency can be changed, and this is typically done by keeping the resistors the same and using a 3-digit variable capacitor.

If all resistors R and capacitors C in the phase shift network are equal in value, then the frequency of oscillations created by the RC oscillator is determined as:

Where:
ƒ r - output frequency in hertz
R - resistance in ohms
C - capacitance in Farads
N is the number of RC stages, (N = 3)

Since the resistor-capacitor combination in RC oscillator circuits also acts as an attenuator, creating total attenuation -1 / 29th (Vo / Vi = β) in all three stages, the amplifier voltage gain must be high enough to overcome these RC losses. Therefore, in our three-stage RC network above, the amplifier gain must also be equal to or greater than 29.

The effect of the amplifier load on the feedback network affects the oscillation frequency and can cause the generator frequency to be 25% higher than the design frequency. The feedback network must then be driven from a high impedance output source and driven to a low impedance load such as a common emitter transistor amplifier, but it is better to use an op amp as it satisfies these conditions perfectly.

RC Oscillator Operational Amplifier

When used as RC oscillators, RC oscillators with operational amplifier are more common than their bipolar transistor counterparts. The oscillator circuit consists of a negative gain operational amplifier and a three-section RC network that generates a 180 o phase shift. The phase shift network is connected from the op amp's output back to its "inverting" input, as shown below.

Since the feedback is connected to the inverting input, the op-amp is therefore connected in its "inverting amplifier" configuration, which produces the required 180 o phase shift, while the RC network produces another 180 o phase shift at the required frequency (180 o + 180 o O).

Although it is possible to cascade just two single-pole RC stages to achieve the required phase shift of 180 o (90 o + 90 o), the stability of the oscillator at low frequencies is usually poor.

One of the most important features RC generator is its frequency stability, which lies in its ability to provide a constant frequency sinusoidal output signal under varying load conditions. By cascading three or even four RC stages (4 x 45 o) the stability of the generator can be significantly improved.

Commonly used RC generators with four stages because commercially available op amps come in four-layer integrated circuits, so designing a four-stage oscillator with a 45 o phase shift relative to each other is relatively easy.

RC generators are stable and provide a well-formed sinusoidal output with a frequency proportional to 1/RC, and hence a wider frequency range is possible when using a variable capacitor. However, RC oscillators are limited to frequency applications due to bandwidth limitations to obtain the desired phase shift at high frequencies.

In the next lesson about Oscillators we will look at a different type RC generator, called bridge oscillators Wien, which uses resistors and capacitors as a circuit to create a low frequency sine wave signal.

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 oscillatory 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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Department of Internal and Personnel Policy of the Belgorod Region

regional state autonomous

professional educational institution

"Belgorod Polytechnic College"

MDK 01.02 Technology of installation and adjustment of electronic equipment of the electronic part of CNC machines

Subject: “Schemes of an RC generator with an “L”-shaped filter and an “L”-shaped bridge, the purpose of the circuit elements. The principle of operation, design and purpose of a trigger operating in key and counting modes. »

Completed:

Student of group No. 24ASU

Shekhovskoy Dmitry

Checked:

Rotaru T.A.

Belgorod, 2018

INTRODUCTION 3

RC generators.. 4

Triggers.. 9

RS trigger. eleven

D-triggers.. 13

JK trigger. 14

T-trigger. 15

Test questions: 16

List of Internet sources: 18

INTRODUCTION

RC generators are used to produce harmonic oscillations of low and infra-low frequencies (up to fractions of hertz). In such generators it is possible to obtain a frequency of up to 10 MHz. It should be noted that at such low frequencies LC oscillators would be bulky and the quality factor would be lower than necessary. At the same time, RC generators in the low frequency range have smaller dimensions, weight and cost than LC generators.

The following are used as active elements:

– bipolar transistors,

– field-effect transistors,

– Integrated op-amp.

RC generators include an amplification element (amplifier) ​​and a feedback link (FE).

RC generators

The following types of OS links are distinguished:

− L-shaped OS links (Fig. 1),

− Wien Bridge (Fig. 2),

− double T-shaped bridge (Fig. 3).

In Figures 1.1, 1.2, 1.3, the symbol “U 1” indicates the input voltage, and the symbol “U 2” indicates the output voltage.

Fig.1.1. L-shaped OS links

Fig.1.2. Bridge of Wine Fig.1.3. Double T-bridge

RC generators with L-shaped RC OS link

Fig.1.4. Schematic diagram of an RC generator with an L-shaped RC OS link

As is known, in a single-stage amplifier without feedback, U IN and U OUT are shifted in phase relative to each other by 180º. If U OUT of this amplifier is applied to its input, then 100% OOS will be obtained.

To maintain phase balance (for introducing PIC), U OUT, before applying it to the amplifier input, must be shifted in phase by 180º. Such a shift can be accomplished using three identical RC links (Fig. 4), each of which changes the phase by 60º.

According to calculations, phase balance occurs at frequency, and amplitude balance occurs at gain K≥29.

L-shaped RC circuits can be made with a number of links greater than 3 (usually 4) - this can increase the generation frequency.

In addition, the generation frequency can be increased by changing the locations of resistors and capacitors. To change the generation frequency, it is necessary to simultaneously change all resistances R or all capacitances C.

L-shaped RC oscillators typically operate at a fixed frequency or over a narrow frequency range.

One link of an L-shaped RC filter allows for a phase shift of the output voltage relative to the input voltage in the limiting case up to p/2, and when constructing harmonic oscillation generators, as a rule, three L-shaped filters connected in series are used.

This ensures the possibility of a phase shift of the signal in the feedback circuit equal to p (p/3 in each filter link). And to ensure phase balance, signal amplifiers are used whose output signal is antiphase to the input, i.e. – inverting amplifiers. In this case, a phase shift of p is provided in the amplifier and p in the feedback channel, which makes it possible to obtain a total phase shift of the signal equal to 2p and ensure the required phase balance.

In this case, to build a generator, you can use any signal amplifier circuits that provide the required gain K to balance the amplitudes.

The Wien bridge (Fig. 1.5) is connected between the op-amp output and its non-inverting input, thereby achieving PIC. In such a self-oscillator, the amplifier should have K≈3, but in the amplifier K>>3. This can lead to large distortions. To avoid this, an environmental protection system is introduced, which significantly increases the stability of the oscillator.

Fig.1.5. Schematic diagram of an RC generator with a Wien bridge on an op-amp

Resistors R 3 , R 4 , R 5 connect the output to the non-inverting input of the op-amp. Resistors R 4 and R 5 determine the required gain, and thermistor R 3 stabilizes the amplitude and reduces output voltage distortion.

In the circuit diagram of an RC oscillator with an asymmetrical double T-shaped bridge (Fig. 1.6), the output voltage is designated “U”; emitter thermal stabilization chain - “RC”; voltage divider - “Rg 1”, “Rg 2”.

Rice. 1.6. Schematic diagram of an RC oscillator

with asymmetrical double T-bridge

In this oscillator circuit K≈11. In such a self-oscillator, the double T-shaped bridge is switched on as an OOS circuit. The phase shift between U IN and U OUT is established when the condition is met

; ; .

The oscillation frequency is determined by the expression.

Triggers

A trigger (from the English “trigger”) is a digital device that can have only two (0 or 1) stable states. In this case, the transition from one state to another is carried out as quickly as possible; in practice, the time of transition processes is usually neglected. Triggers are the main element for building various storage devices. They can be used to store information, but their memory capacity is extremely small - a flip-flop can store bits, individual codes or signals.

Based on how information is written to the trigger, they are divided into:

· asynchronous - information is recorded continuously and depends on the information signals that are supplied to the trigger input

· synchronous - information is recorded only in the presence of an additional signal - synchronizing, in fact - opening the operation of the trigger

In digital circuitry, the following designations are used for trigger inputs:
S – separate input that sets the trigger to a single state (one at Q (direct output))
R - separate input that sets the trigger to the zero state (zero at Q (direct output))
C – synchronization input
D – information input (information is supplied to this input for further entering into the trigger)
T - counting input

Based on their functional purpose, triggers are classified:

RS triggers

D-triggers

· T-triggers

JK trigger

RS trigger

RS trigger

The simplest type of triggers, on the basis of which other types are subsequently created. It can be built either on logical elements 2OR-NOT (direct inputs) or 2AND-NOT (inverse inputs)

Rice. 2.1. RS trigger, construction diagram and designation. A – on OR-NOT elements. B – on AND-NOT elements

On their own, due to their very low noise immunity, RS triggers are practically not used in digital technology. An exception is the elimination of the influence of contact rattling that occurs when switching mechanical switches. In this case, you will need a toggle switch (button) with three outputs, with one of the outputs connected alternately to the other two. To obtain an RS flip-flop, a D flip-flop is used, whose inputs D and C are shorted to zero.

The operating principle is shown in the timing diagram:

Fig.2.2. Scheme for eliminating the influence of contact rattling

The first negative signal received at the –R input puts the trigger into the “0” state, and the first negative signal at the –S input throws the trigger into the one state. All other signals that are caused by contact bounce will no longer be able to influence the trigger in any way. With this switch connection diagram, its upper position will correspond to one at the output of the trigger, and the lower position will correspond to zero.

The RS trigger is asynchronous, but there are cases when there is a need to record (save) recorded information. To do this, use a synchronous (synchronized) RS trigger, which in this case consists of two parts: a regular RS trigger and a control circuit.

Fig.2.3. Synchronized RS trigger

With this scheme, as long as the input C = 0, the value of the pulses arriving at X1 and X2 does not matter, the RS trigger is in the “storage” mode. When C=1, the trigger is activated and goes into recording mode.

D-triggers

The delay flip-flop, which is used to create shift registers and holding registers, is an integral part of any microprocessor.

Rice. 3.1. D flip-flop circuit

It has two inputs - information and synchronization. In the C=0 state, the trigger is stable and the output signal does not depend on the signals arriving at the information input. When C = 1, the information at the direct output will exactly repeat the information supplied to input D. The timing diagram shows the operating principle of the D flip-flop

Fig.3.2. D-trigger. a) schematic diagram b) timing diagram of work

JK trigger

According to the principle of operation, the JK flip-flop almost completely corresponds to the RS flip-flop, but at the same time it was possible to avoid the uncertainty caused by the simultaneous arrival of two “units” at the input.

Rice. 4.1. Graphic representation of a JK flip-flop

Fig.4.2. JK flip-flop at the input with 3I logic

In this case, the JK flip-flop switches to counting flip-flop mode. In practice, this leads to the fact that when “single” signals are simultaneously received at the input, the trigger changes its state to the opposite. Below is the truth table for JK flip-flop:

JK triggers are very versatile devices, and their versatility is twofold. On the one hand, these triggers are successfully used for digital devices, so to speak, in their pure form: in digital counters, registers, frequency dividers, etc. On the other hand, it is very easy to get any desired type of trigger from a JK trigger by connecting certain pins. Below is an example of obtaining a D-trigger from the original JK-trigger using an additional inverter

T-trigger

Another name is counting flip-flops, on the basis of which binary counters and frequency dividers are created. This type of trigger has only one input. The principle of its operation is that when a pulse arrives at the input of the trigger, its state changes to the opposite; when a second pulse arrives, it returns to its original state.

Rice. 5.1. Timing diagram of frequency divider based on T-flip-flop

From it it becomes clear why the T-trigger is called a frequency divider. The trigger switches at the moment when the leading edge of the clock pulse arrives at the input. As a result, the frequency with which the pulses at the output of the trigger follow is 2 times less than the original one - the frequency of the clock pulses arriving at the input. If the installation of one counting trigger allows the pulse frequency to be divided into two, then two series-connected triggers will, accordingly, reduce this frequency by 4 times.
Below is an example of obtaining a T flip-flop from a JK flip-flop:

Rice. 5.2. T-trigger based on JK-trigger

Control questions:

What are RC generators used for?

RC generators are used to produce harmonic oscillations of low and infra-low frequencies (up to fractions of hertz)

The use of generators with oscillatory circuits (such as LC) to generate oscillations with frequencies less than 15-20 kHz is difficult and inconvenient due to the bulkiness of the circuits. Currently, generators such as R.C. in which selective RC filters are used instead of an oscillating circuit. Generators type R.C. can generate very stable sinusoidal oscillations in a relatively wide frequency range from fractions of a hertz to hundreds of kilohertz. In addition, they have small dimensions and weight. The most complete advantages of type generators R.C. appear in the low frequency region.

Block diagram of a sinusoidal oscillation generator type R.C. shown in Fig. 1.5.

Rice. 1.5

The amplifier is built according to a conventional resistive circuit. To self-excite the amplifier, i.e., to transform the initially occurring oscillations into undamped ones, it is necessary to apply to the input of the amplifier a part of the output voltage that exceeds the input voltage or is equal to it in magnitude and coincides with it in phase, in other words, to cover the amplifier with positive feedback of sufficient depth . When the output of the amplifier is directly connected to its input, self-excitation occurs, but the shape of the generated oscillations will differ sharply from sinusoidal, since the conditions of self-excitation will be simultaneously satisfied for oscillations of many frequencies. To obtain sinusoidal oscillations, it is necessary that these conditions are met only at one specific frequency and are sharply violated at all other frequencies.

Rice. 1.6

This problem is solved using phase shifting chain, which has several links R.C. and serves to rotate the phase of the amplifier's output voltage by 180°. The phase change depends on the number of links P and equal

Due to the fact that one link R.C. changes phase by angle< 90°, минимальное число звеньев фазовращающей цепочки P -- 3. In practical generator circuits, three-link phase-shifting chains are usually used.

In Fig. Figure 1.6 shows two variants of such chains, called “R-parallel” and “C-parallel”, respectively. The frequency of generated sinusoidal oscillations for these circuits under the condition R1 = R 2 = R 3 = R And C t = C 2 = C3 = C is calculated using the following formulas: for the circuit in Fig. 1.6, a:

for the diagram in Fig. 4.6, b:

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 schemes the attenuation

Consequently, circuits using three-link phase-shifting chains having identical links can generate sinusoidal oscillations with a frequency f 0 only if the amplifier gain exceeds 29.

In a phase-shifting chain with identical links, each subsequent link has a shunting effect on the previous one. To reduce the shunting effect of the links and reduce attenuation in the phase-shifting feedback circuit, so-called progressive chains. In this case, the resistance of the resistor of each subsequent link is selected in tn times the resistance of the previous link, and the capacitance of subsequent links decreases by the same amount:

Usually the value T does not exceed 4--5.

In Fig. 1.7 shows one of the possible circuits of a self-oscillator of the type R.C. with a phase-shifting chain.

From the point of view of ensuring phase balance conditions, such a generator could be built on a single transistor (T2) with a common emitter. However, in this case the feedback circuit bypasses the resistor R K amplifier transistor and reduces its gain, and the low input resistance of the transistor sharply increases the attenuation in the feedback circuit. Therefore, it is advisable to separate the output of the phase-shifting circuit and the input of the amplifier using an emitter follower assembled on transistor T1.

The operation of the self-generator begins the moment the power source is turned on. The resulting collector current pulse contains a wide and continuous spectrum of frequencies, which necessarily includes the required generation frequency. Due to the fulfillment of the self-excitation conditions, the oscillations of this frequency become undamped, while the oscillations of all other frequencies, for which the phase balance condition is not met, quickly decay.

Autogenerators with phase-shifting circuits are usually used to generate sinusoidal oscillations of a fixed frequency. This is due to the difficulty of frequency tuning over a wide range. Range autogenerators type R.C. are built a little differently. Let's consider this issue in more detail.

If the amplifier rotates the phase of the input signal by 2? (for example, an amplifier with an even number of stages), then when covered by positive feedback of sufficient depth, it can generate electrical oscillations without turning on a special phase-shifting circuit. To isolate the required frequency of sinusoidal oscillations from the entire spectrum of frequencies generated by such a circuit, it is necessary to ensure that the self-excitation conditions are met for only one frequency. For this purpose, a series-parallel selective circuit can be included in the feedback circuit, the diagram of which is shown in Fig. 1.8.

Rice. 1.7

Let's determine the properties of this chain, considering it as a voltage divider.

There is an obvious relationship between the output and input voltages

The voltage transfer coefficient of this circuit

At the quasi-resonant frequency w 0, the voltage transfer coefficient must be equal to a real number. This is only possible if the resistances expressed by the corresponding mathematical notation in the numerator and denominator of the last formula are of the same nature. This condition is met only if the real part of the denominator is equal to zero, i.e.

Hence the quasi-resonance frequency

As for the voltage transfer coefficient, at the quasi-resonant frequency it is equal to

Substituting the value into this formula

Considering R1 = R 2 = R And C 1 = C 2 = C, let’s find the final values ​​of f 0

The attenuation introduced by the selective circuit under consideration at the quasi-resonant frequency is equal to

This means that the minimum gain at which the amplitude balance condition is satisfied must also be equal to 3. Obviously, this requirement is quite easy to satisfy. A real transistor amplifier, having two stages (the smallest even number), allows you to obtain a voltage gain much higher than TO O = 3. Therefore, it is advisable, along with positive feedback, to introduce negative feedback into the amplifier, which, while reducing the gain, at the same time significantly reduces possible nonlinear distortions of the generated oscillations. The schematic diagram of such a generator is shown in Fig. 1.9.

Circuit diagram of a transistor RC oscillator with frequency tuning

The thermistor in the emitter circuit of transistor T1 is designed to stabilize the amplitude of the output voltage when temperature changes. Frequency adjustment is carried out using a paired potentiometer R1R2.

Currently, discrete elements (transistors) are rarely used to build generators. Most often, various types of integrated circuits are used for these purposes. Circuits built on op-amps, multipliers, comparators and timers are distinguished by their simplicity, stable parameters, and versatility. The flexibility and versatility of the op-amp allows you to create generators of almost all types with satisfactory parameters with a minimum number of external components, but at the same time easy to configure and adjust.

The operating principle of such generators is based on the use of phase-shifting or resonant elements in OS circuits: Wien bridge, double T-shaped bridge, shifting RC circuits.

There are other ways to generate sinusoidal oscillations, for example by filtering triangular pulses or extracting the first harmonic component of rectangular pulses.