RC generators. Generators of sinusoidal and non-sinusoidal oscillations Autogenerator rc type

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. 11

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, three L-shaped filters connected in series are usually 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 constructing 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

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 trigger output, 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 state C=0, the trigger is stable and the output signal does not depend on the signals arriving at the information input. When C = 1 at the direct output, the information 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

Security 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)

We looked at one of the types of generators using an oscillatory circuit. Such generators are mainly used only at high frequencies, but for the share of generation at lower frequencies, the use of an LC generator can be difficult. Why? Let's remember the formula: the frequency of the KC generator is calculated by the formula

That is: in order to reduce the generation frequency, it is necessary to increase the capacitance of the master capacitor and the inductance of the inductor, and this, of course, will entail an increase in size.
Therefore, to generate relatively low frequencies, they use RC generators
the principle of operation of which we will consider.

Circuit of the simplest RC generator(it is also called a circuit with a three-phase phasing chain), shown in the figure:

The diagram shows that this is just an amplifier. Moreover, it is covered by positive feedback (POS): its input is connected to the output and therefore it is constantly in self-excitation. And the frequency of the RC oscillator is controlled by the so-called phase-shifting chain, which consists of elements C1R1, C2R2, C3R3.
Using one chain of a resistor and a capacitor, 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. 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 using a phase-shifting chain, there is another, more common option. The generator is also built on a transistor amplifier, but instead of a phase-shifting chain, the so-called Wien-Robinson bridge is used (the last name Vin is spelled with one “H”!!). This is what it looks like:

The left side of the circuit is a passive RC bandpass filter, at point A the output voltage is removed.
The right side is like 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 gain of the divider is equal to the gain of the 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:

R3=2R4

But there’s just one problem: all this can only be considered under ideal conditions. In reality, everything is not so simple: the slightest deviation from the condition R3 = 2R4 will lead either to a breakdown in generation or to saturation of the amplifier. To make it more clear, let's connect a Wien bridge to an op-amp:

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. Or you can also replace resistor R4 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 R1 and R2 are replaced with a variable variable (but only a dual one), then the generation frequency can be adjusted within certain limits.
It is also possible to divide the capacitors C1 and C2 into sections, then it will be possible to switch ranges, and use a dual variable resistor R1R2 to smoothly regulate the frequency in the ranges.

An almost practical circuit of an RC oscillator with a Wien bridge is shown in the figure below:

Here: switch SA1 can switch the range, and dual resistor R1 can adjust the frequency. Amplifier DA2 serves to match the generator with the load.

Generators with an oscillatory circuit are indispensable as sources of sinusoidal high-frequency oscillations. They are inconvenient for generating oscillations with frequencies less than 15...20 kHz, since the oscillatory circuit is too bulky.

Another disadvantage of low-frequency LC oscillators is the difficulty of tuning them in the frequency range. All this has led to the widespread use of RC generators at the above frequencies, in which frequency electric RC filters are used instead of an oscillating circuit. Generators of this type can generate fairly stable sinusoidal oscillations over a relatively wide frequency range from fractions of a hertz to hundreds of kilohertz. They are small in size and weight, and these advantages of RC generators are most fully manifested in the low frequency region.

4.2 Block diagram of the rc generator

This diagram is shown in Fig. No. 7.

Fig. No. 7. Block diagram of an RC oscillator.

The circuit contains an amplifier 1, loaded with a resistor and receiving power from a constant voltage source 3. To self-excite the amplifier, i.e. to obtain continuous oscillations, it is necessary to apply to its input a part of the output voltage that is greater than the input voltage (or equal to it) and is in phase with it. In other words, the amplifier must be covered by positive feedback, and the four-terminal feedback network 2 must have a sufficient transmission coefficient. This problem is solved in the case when the four-terminal network 2 contains a phase-shifting circuit consisting of resistors and capacitors, the phase shift between the input and output voltages is 180 0.

4.3 Operating principle of the phase-shifting circuit

The diagram of which is shown in Fig. No. 8a, illustrated using the vector diagram in Fig. No. 8b.

Fig.8. Phase-shifting circuits: a - schematic diagram; b- vector diagram; c, d - three-link chains

Let voltage U1 be applied to the input of this RC circuit. It causes a current I in the circuit, creating a voltage drop across the capacitor

(where ω is the frequency of voltage U1) and across the resistor U R =IR, which is also the output voltage U2. In this case, the phase shift angle between current I and voltage Uс is equal to 90 0, and between current I and voltage U R – zero. The voltage vector U1 is equal to the geometric sum of the vectors U C and U R and makes an angle φ with the vector U2. The smaller the capacitance of the capacitor C, the closer the angle φ to 90 0.

4.4 Conditions for self-excitation of an rc oscillator

The largest angle φ that can be obtained by changing the values of the elements of the RC circuit is close to 90 0. In practice, circuit elements R and C are selected as follows. So that the angle φ=60 0. Consequently, to obtain the phase shift angle φ=180 0, necessary to fulfill the phase balance condition. It is required to connect three RC links in series.

In Fig. No. 8 c, d shows two variants of three-link phase-shifting circuits. The phase shift between the output and input voltage by an angle of 180 0 with R1=R2=R3=R and C1=C2=C3=C is provided at frequencies: f 01 ≈ (in the circuit in Fig. No. 8c) and f 02 ≈ (in the circuit in Fig. No. 8d), where R is expressed in ohms, C is in farads, and f 0 is in hertz. The values of f 01 and f 02 are simultaneously the frequency of self-oscillations.

To ensure amplitude balance, the gain of the amplifier K ac should not be less than the transmission coefficient of the feedback circuit K os. =. Calculations show that for the given schemes K o.s =. Thus, self-oscillations in RC generators containing three-link phase-shifting circuits with identical links are possible only if the following conditions are met:

f aut = f 01 (or f aut = f 02); K us ≥29.

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 in the subsequent absence of external influences on the oscillatory system (the 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 with electrical signals from an oscillation source. 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 oscillation 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 necessary operating mode for direct current 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 inductance coefficient M, an alternating current will arise in the base circuit, 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. Condition for phase balance The result is that each of the RC links rotates the signal phase 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.

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? It's 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 in 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 chain, which is also called a circuit with reactive elements of the same sign. It is shown in Fig. 1.

Rice. 1 - RC 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:

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 a Wien 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 the conders C1-C8, the double rezik R1 and the 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.