The generator is a self-oscillating system that generates electric current pulses, in which the transistor plays the role of a switching element. Initially, since the 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 took place 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 the division of the frequency range
Classification
Transistor generators have several classifications:
- by the frequency range of the output signal;
- by type of output signal;
- according to the principle of action.
The frequency range is a subjective value, but for standardization the following division of the frequency range is accepted:
- 30 Hz to 300 kHz - low frequency (LF);
- from 300 kHz to 3 MHz - middle frequency (MF);
- 3 MHz to 300 MHz - high frequency (HF);
- above 300 MHz - ultra high frequency (SHF).
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, a high-frequency or low-frequency 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 form. A special case is a rectangular pulse generator;
- noise generators - generators of a wide frequency spectrum, in which, in a given frequency range, the signal spectrum is uniform from the lower to the upper part of the frequency response.
According to the principle of operation of generators:
- RC generators;
- LC generators;
- Blocking generators - short pulse shaper.
Due to fundamental limitations, RC oscillators are usually used in the low and audio ranges, and LC oscillators in the HF frequency range.
Generator circuitry
RC and LC sine wave generators
The generator on a transistor is most simply implemented in a capacitive three-point circuit - the Kolpitz generator (Fig. below).
Transistor oscillator circuit (Colpitz generator)
In the Kolpitz circuit, elements (C1), (C2), (L) are frequency-setting. The remaining elements are a standard transistor piping to provide the necessary DC operation. The same simple circuitry has a generator assembled according to the inductive three-point circuit - the Hartley generator (Fig. below).
Diagram of a three-point generator with inductive coupling (Hartley generator)
In this circuit, the oscillator frequency is determined by a parallel circuit, which includes elements (C), (La), (Lb). Capacitor (C) is needed to form a positive feedback on the alternating current.
The practical implementation of such a generator is more difficult, since it requires an inductor with a tap.
Both self-oscillation generators are mainly used in the MF and HF ranges as carrier frequency generators, in frequency-setting local oscillator circuits, and so on. Radio regenerators are also based on oscillators. 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 in setting the frequency value of the RF generator. Billionths of a percent is 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 values of inductance. To be more precise, in the dimensions of the required inductor.
The Pierce oscillator circuit is a modification of the Kolpitz circuit, implemented without the use of inductance (Fig. below).
Pierce generator circuit without the use of inductance
In Pierce's circuit, the inductance is replaced by a quartz resonator, which made it possible to get rid of the laborious and bulky inductor and, at the same time, limited the upper oscillation range.
Capacitor (C3) does not pass the DC component of the base bias of the transistor 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 values of these elements.
RC current generators use the principle of phase shift in an RC circuit. The most commonly used circuit with a phase-shifting chain (Fig. below).
Schematic of an RC oscillator with a 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 deg). The phase-shifting circuit introduces a strong attenuation of the signal, so such a circuit has increased requirements for the gain of the transistor. The Wien bridge circuit is less demanding on the parameters of the transistor (Fig. below).
Diagram of an RC generator with a Wien bridge
The Wien double T-bridge consists of elements (C1), (C2), (R3) and (R1), (R2), (C3) and is a narrow-band notch filter tuned to the generation frequency. For all other frequencies, the transistor is covered by a deep negative connection.
Functional current generators
Function generators are designed to generate a sequence of pulses of a certain shape (a form describes 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 such a sequence is called a “meander”), triangular and sawtooth pulses. The simplest rectangular pulse generator - a multivibrator, is served as the first circuit for beginner radio amateurs to assemble with their own hands (Fig. below).
Scheme of a multivibrator - a generator of rectangular pulses
A feature of the multivibrator is that almost any transistor can be used in it. The duration of the pulses and pauses between them is determined by the values of the capacitors and resistors in the base circuits of the transistors (Rb1), Cb1) and (Rb2), (Cb2).
The frequency of current self-oscillation can vary from units of hertz to tens of kilohertz. It is impossible to implement HF self-oscillations on a multivibrator.
Triangular (sawtooth) pulse generators are usually built on the basis of rectangular pulse generators (master oscillator) by adding a corrective 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 the capacitor C.
Blocking generator
The purpose of blocking generators is to generate powerful current pulses with steep fronts and low duty cycle. The duration of the pauses between pulses is much longer than the duration of the pulses themselves. Blocking oscillators are used in pulse shapers, comparators, but the main area of application is the line-scan master oscillator in cathode ray tube-based information display devices. Blocking generators are also successfully used in power conversion devices.
FET generators
A feature of field-effect transistors is a very high input resistance, the order of which is commensurate with the resistance of electronic tubes. The circuit solutions listed above are universal, they are simply adapted to the use of various types of active elements. Colpitz, Hartley and other generators made on a field-effect transistor differ only in the ratings of the elements.
Frequency-setting circuits have the same ratios. To generate high-frequency oscillations, a simple generator made on a field-effect transistor according to an inductive three-point circuit is somewhat preferable. The fact is that the field-effect transistor, having a high input resistance, practically does not have a shunting effect on the inductance, and, therefore, the high-frequency generator will work 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 within a given range is the same. Noise generators are used in measuring equipment to assess the frequency characteristics of the tested path. Audio band noise generators are often supplemented with a frequency response equalizer to adapt to subjective loudness to human hearing. Such noise is called "gray".
Video
Until now, there are several areas in which the use of transistors is difficult. These are powerful microwave range generators in radar, and where it is required to receive especially powerful high-frequency pulses. So far, powerful microwave transistors have not been developed. In all other areas, the vast majority of generators are made exclusively on transistors. There are several reasons for this. First, the dimensions. Secondly, power consumption. Thirdly, reliability. On top of that, transistors, due to the peculiarities of their structure, are very easy to miniaturize.
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. At present, generators of the type are widely used for this purpose. rc, in which selective RC filters are used instead of an oscillating circuit. Type Generators RC can generate very stable sinusoidal oscillations in a relatively wide frequency range from fractions of a hertz to hundreds of kilohertz. In addition, they are small in size and weight. The most complete advantages of type generators RC appear in the low frequency region.
Structural diagram of the generator of sinusoidal oscillations of the type RC shown in fig. 1.5.
Rice. 1.5
The amplifier is built according to the usual resistive circuit. For self-excitation of the amplifier, i.e., for the transformation of 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 in magnitude and coincides with it in phase, in other words, to cover the amplifier with a positive feedback of sufficient depth . When the amplifier output is directly connected to its input, self-excitation occurs, however, the shape of the generated oscillations will differ sharply from the sinusoidal one, since the self-excitation conditions will be simultaneously fulfilled for oscillations of many frequencies. To obtain sinusoidal oscillations, it is necessary that these conditions are satisfied only at one specific frequency and are sharply violated at all other frequencies.
Rice. 1.6
This task is solved using phase shift chain, which has several links RC and serves to rotate the phase of the output voltage of the amplifier by 180 °. Phase change depends on the number of links P and equal
Due to the fact that one link RC changes the phase by an angle< 90°, минимальное число звеньев фазовращающей цепочки P -- 3. In practical generator circuits, three-link phase-shifting chains are usually used.
On fig. 1.6 shows two variants of such chains, called, respectively, "R-parallel" and "C-parallel". The frequency of the generated sinusoidal oscillations for these circuits, provided R1 = R 2 = R 3 = R And C t = C 2 = C3 = C is calculated by the following formulas: for the circuit in fig. 1.6, a:
for the circuit in fig. 4.6, b:
To ensure the balance of amplitudes, the gain of the amplifier must be equal to the attenuation introduced by the phase-shifting circuit, through which the voltage from the output enters the input of the amplifier, or exceed it.
Calculations show that for the given circuits, the attenuation
Therefore, circuits using three-link phase-shifting chains having the same links can generate sinusoidal oscillations with a frequency f 0 only if the gain of the amplifier is greater than 29.
In a phase-shifting circuit with identical links, each subsequent link has a shunting effect on the previous one. To reduce the shunting action 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 greater than 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.
On fig. 1.7 shows one of the possible schemes of an autogenerator of the type RC with a phase shifter.
From the point of view of ensuring the phase balance condition, such a generator could be built on a single transistor (T2) with a common emitter. However, in this case, the feedback circuit shunts the resistor R K amplifying 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 the transistor T1.
The operation of the oscillator begins at the moment the power source is turned on. The resulting collector current pulse contains a wide and continuous frequency spectrum, 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 satisfied, 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 oscillators of the type RC built a little differently. Let's consider this question in more detail.
If the amplifier rotates the phase of the input signal by 2? (for example, an amplifier with an even number of cascades), then when it is covered with a sufficient depth of positive feedback, it can generate electrical oscillations without including a special phase-shifting chain. 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 chain can be included in the feedback circuit, the scheme of which is shown in Fig. 1.8.
Rice. 1.7
Let us define the properties of this circuit, 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 a quasi-resonant frequency w 0, the voltage transfer coefficient must be equal to a real number. This is possible only if the resistances expressed by the corresponding mathematical notation in the numerator and denominator of the last formula will have the same character. This condition is provided 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, then at the quasi-resonant frequency it is equal to
Substituting into this formula the value
Assuming R1 = R 2 = R And C 1 = С 2 = С, we find the final values f 0
The attenuation introduced by the considered selective chain at the quasi-resonant frequency is equal to
This means that the minimum gain factor at which the amplitude balance condition is satisfied must also be equal to 3. It is obvious that this requirement is quite easy to fulfill. A real transistor amplifier having two stages (smallest even number) allows for a voltage gain much greater than TO O = 3. Therefore, along with positive feedback, it is advisable 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. A schematic diagram of such a generator is shown in fig. 1.9.
Frequency tunable transistor RC oscillator circuit
The thermistor in the emitter circuit of transistor T1 is designed to stabilize the amplitude of the output voltage when the 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 simplicity, parameter stability, and versatility. The flexibility and versatility of the op amp make it possible to create, with a minimum number of external components, simple, but at the same time convenient for setting up and adjusting generators of almost all types with satisfactory parameters.
The principle of operation of such generators is based on the use of phase-shifting or resonant elements in the OS circuits: a Wien bridge, a double T-shaped bridge, shifting RC circuits.
There are other ways to generate sinusoidal oscillations, such as filtering triangular pulses or extracting the first harmonic component of rectangular pulses.
We considered one of the varieties of generators using an oscillatory circuit. Such generators are mainly used only at high frequencies, but the use of an LC generator can be difficult to generate at lower frequencies. 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, RC generators
the principle of operation of which we will consider.
Diagram of the simplest RC generator(it is also called a three-phase phasing circuit), is 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 generator is controlled by the so-called phase-shifting chain, which consists of elements C1R1, C2R2, C3R3.
With the help of one chain of a resistor and a capacitor, a phase shift of no more than 90º can be obtained. In reality, the shift is close to 60º. Therefore, to obtain a phase shift of 180º, three chains have to be set. From the output of the last RC circuit, the signal is fed to the base of the transistor.
Operation starts the moment the power supply is turned on. The collector current pulse arising in this case contains a wide and continuous frequency spectrum, in which the required generation frequency will necessarily be. 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 circuit, 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 Vin-Robinson bridge is used (Vin's surname is spelled with one "H" !!). This is how it looks like:
The left side of the circuit is a passive band-pass RC 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 divider gain is equal to the bandpass filter gain, then at the resonant frequency the voltage between points A and B will be zero, and the PFC at the resonant frequency jumps from -90º to +90º. In general, the following condition must be met:
R3=2R4
But there is only one problem: all this can be considered only for ideal conditions. In reality, everything is not so simple: the slightest deviation from the condition R3 = 2R4 will either lead to a breakdown in generation or to saturation of the amplifier. To make it clearer, let's connect a Wien bridge to an op-amp:
In general, this scheme cannot be used in this way, since in any case there will be a spread in the parameters of the bridge. Therefore, instead of the resistor R4, some kind of non-linear or controlled resistance is introduced.
For example, a non-linear resistor: controlled resistance using transistors. Or you can also replace the resistor R4 with a micropower incandescent lamp, the dynamic resistance of which increases with increasing current amplitude. The filament has a sufficiently large thermal inertia, and at frequencies of several hundred hertz it practically does not affect the operation of the circuit within one period.
Wien bridge oscillators have one good property: if R1 and R2 are replaced by variables (but only doubled), then it will be possible to regulate the generation frequency within certain limits.
It is possible to divide the capacitances C1 and C2 into sections, then it will be possible to switch the ranges, and smoothly adjust the frequency in the ranges with a double variable resistor R1R2.
An almost practical circuit of an RC oscillator with a Wien bridge in the figure below:
Here: with switch SA1 you can switch the range, and with a double resistor R1 you can adjust the frequency. Amplifier DA2 is used to match the generator with the load.
Harmonic Oscillator called a device that creates an alternating sinusoidal voltage in the absence of input signals. Generator circuits always use positive feedback.
The fluctuations are called free(or own), if they are performed at the expense of the initially perfect energy with 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 value changes over time according to the sine (cosine) law.
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 generators of harmonic oscillations, an important problem is the automatic stabilization of the output voltage amplitude. If the circuit does not provide for 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. Schemes for automatic amplitude stabilization are quite complex.
Structural generator circuit shown in the figure below:
IE - source of energy,
UE - amplifier,
POS - positive feedback circuit,
OOS - negative feedback circuit,
FK - oscillation shaper (LC-circuit or phasing RC-circuit).
By way to get vibrations generators are divided into two groups: generators with external excitation and generators with self-excitation. A generator with external excitation is a power amplifier, to the input of which electrical signals are supplied from an oscillation source. Self-excited generators contain vibration generators; such generators are often called oscillators .
The principle of operation of the autogenerator.
It is based on the automatic replenishment of the energy that the oscillation shaper expends.
In doing so, the following must be observed:
-amplitude balance rule- the product of the gain and the feedback factor should be equal to 1.
-phase balance rule- it means that oscillations occur at a well-defined frequency, at which phase coincidence occurs.
If both conditions are met, the 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 varieties of sine wave generator circuits. Generators for frequencies from several tens of kilohertz and higher contain LC circuits , and generators for low frequencies, as a rule, RC filters .
Schemes of LC generators of harmonic oscillations.
In generators with LC contours inductive coils and high quality capacitors are used. Auto-oscillator - oscillation shaper - is one or more amplifying stages with positive frequency-dependent feedback circuits; feedback circuits contain oscillatory circuits. There are various options for switching on the oscillatory circuit relative to the RE electrodes: only at the input, only at the output, or simultaneously in several sections of the circuit. According to the methods of connecting LC elements with the electrodes of amplifying elements, a transformer connection and the so-called three-point connection - inductive or capacitive - are distinguished. Transformer-coupled oscillator is shown in fig. 1.
Rice. 1. Autogenerator-shaper of sinusoidal oscillations with transformer connection.
The oscillatory circuit, consisting of the Lk coil and the capacitor C, is the collector load of the transistor V1. The inductive connection between the output and the input of the amplifier is provided by the Lb coil connected to the base of the transistor. Elements R1, R2, Re, Se are designed to provide the necessary mode of operation for direct current and its thermal stabilization.
Thanks to the capacitor C1, which has a low resistance at the generation frequency, a circuit is created for the variable current component between the base and emitter of the transistor. The dots indicate the beginning of the windings Lb and Lk, since it is necessary to observe 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 the OE circuit, the phases of the input and output signals are mutually shifted by 180 ° C. Therefore, the ends of the Lb coil must be connected so that the input and output oscillations are in phase. Amplitude balance condition consists in the fact that the losses in the circuit and the load are continuously replenished by the power source.
Rice. 1a. The work of the generator. Transition processes.
Antogenerator operation(Fig. 1a) starts when the Ek source is turned on. The initial current pulse excites oscillations in the LkC 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 connection between the coils Lb and Lk with a mutual inductance coefficient M, an alternating current will appear 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). Amplified oscillations are transmitted from the circuit back to the base circuit, and the amplitude of oscillations gradually increases, reaching a predetermined value.
Rice. 2. Shapers of sinusoidal oscillations based on an oscillatory circuit assembled according to a three-point inductive (a) and capacitive (b) circuit.
Autogenerator assembled according to three-point pattern, is shown in fig. 2, a. The oscillatory circuit, consisting of a sectioned coil Lk and a capacitor Sk, is the load of the transistor V1. Coil Lk is divided into two parts: one output 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 inclusion ensures the implementation of the phase balance and is characterized by great simplicity and reliability. The operation mode of the transistor in direct current and its thermal stabilization are carried out by the same elements as in the transformer generator circuit (see Fig. 1). The capacitive three-point circuit (Fig. 2b) contains two capacitors in the capacitive branch of the oscillatory circuit, the midpoint between which is connected to the emitter of the transistor V1. The oscillatory circuit is connected in series between the energy source and the RE. The voltages on the capacitors have the opposite polarity with respect to the common point, which ensures the fulfillment of the phase balance condition.
Schemes of RC-generators of harmonic oscillations.
RC oscillators are used to generate infra-low and low frequency oscillations (from fractions of a hertz to several tens of kilohertz); RC oscillators can generate oscillations of higher frequencies, however, low-frequency oscillations are more stable.
Rice. 3. Self-oscillators of sinusoidal oscillations with a target of L-shaped RC-links (a) and a 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 multilink 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 amplifying 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 It lies in the fact 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.
Auto oscillator with RC filter bridge type shown in fig. 3b. The two arms of the bridge - links R1C1 and R2C2 - are connected to the non-inverting input of amplifier 2 (the number inside the triangle means the number of stages). These links form the POS chain. Another diagonal is connected to the inverting input of the same amplifier, made up of non-linear elements R3 and r, which creates the OOS chain. In this circuit, the bridge has a selective property and the phase balance condition is provided at one frequency (at which the output signal of the bridge is in phase with the input signal). Frequency adjustment in this autogenerator is simple and convenient, and it 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 bridge capacitors.
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-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 ( RC-circuit) with frequency selectivity.
The exclusion of inductors from the circuit makes it possible to significantly reduce the dimensions and weight of the generator, especially at low frequencies, since the dimensions of the inductors sharply increase with decreasing frequency. An important advantage RC-generators compared to LC-generators is the possibility of their manufacture by integrated technology. However RC-generators have low frequency stability of generated oscillations due to low quality factor RC-circuits, as well as a poor shape of oscillations due to poor filtering of higher harmonics in the spectrum of the output oscillation.
RC-generators can operate in a wide frequency range (from fractions of a hertz to tens of megahertz), however, they have found application in communication equipment and measuring technology mainly at low frequencies.
Basics of the theory RC generators were developed by Soviet scientists V. P. Aseev, K. F. Teodorchik, E. O. Saakov, V. G. Kriksunov and others.
RC-oscillator usually includes a broadband amplifier, made on a tube, transistor or integrated circuit and RC-feedback circuit, which has selective properties and determines the frequency of oscillations. The amplifier compensates for energy losses in the passive elements and ensures that the self-excitation amplitude condition is met. The feedback circuit ensures that the phase condition of self-excitation is met only at one frequency. Type of feedback loop RC 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 RC 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 such a limitation, the form of oscillations does not change, they remain harmonic even in the stationary regime.
Consider both types RC-autogenerators.
Auto-oscillator with a phase shift of 180 in the feedback circuit.
Such an oscillator is also called an oscillator with a three-link chain. RC.
In schemes RC-generators with a phase shift in the feedback circuit by 180 use amplifiers that invert the phase of the input voltage. Such an amplifier can, for example, be an op-amp 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 fulfilled, the feedback circuit must provide a phase shift OS = 180.
To justify the structure of the feedback circuit, we reproduce the phase-frequency characteristics of the simplest RC-links (Fig. 3.4).
Rice. 3 option RC-link and its PFC
Rice. 4 Option RC-link and its PFC
It can be seen from the graphs that one of the simplest RC-link introduces a phase shift not exceeding 90. Therefore, a phase shift of 180 can be achieved by cascading three elementary RC- links (Fig. 5).
Rice. 5 Schemes and PFC of three-link RC-chains
Elements RC-circuits are calculated so as to obtain a phase shift of 180 at the generation frequency. One of the generator options with a three-link circuit RC shown in figure 6
Rice. 6 Three-link generator RC
The generator consists of a resistive transistor amplifier and a feedback circuit. A single-stage amplifier with a common emitter performs a phase shift between the voltage at the collector and the base K \u003d 180. Therefore, in order to balance the phases, the feedback circuit must provide OS \u003d 180 at the frequency of the generated oscillations.
We will analyze the feedback circuit, for which we will compose a system of equations using the method of loop currents.
Solving the resulting system with respect to the feedback coefficient, we obtain the expression
It follows from the expression that the phase shift 180 is obtained when it is a real and negative value, i.e.
therefore, generation is possible at a frequency
At this frequency, the module of the feedback coefficient
This means that in order 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 oscillations of a harmonic form, a thermistor is included in the emitter circuit R T with a positive temperature coefficient of resistance. With an increase in the amplitude of oscillations, the resistance R T increases and the depth of negative feedback in the AC amplifier increases, respectively, the gain decreases. When the stationary regime of oscillations sets in ( TO= 1), the amplifier remains linear and there is no distortion of the collector current waveform.
Self-oscillator with zero phase shift in the feedback circuit.
A characteristic feature of the schemes RC-generators 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 us consider some possible options for feedback circuits that provide a zero phase shift (Fig. 7).
Rice. 7 Variants of OS circuits providing zero phase shift
They consist of two parts, one of which is RC-link with a positive phase shift, and the second - with a negative phase shift. As a result of adding the PFC at a certain frequency (generation frequency), you can get a phase shift equal to zero.
In practice, most often, as a selective circuit with a zero phase shift, a phase-balancing bridge is used, or in another way, the Wien bridge (Fig. 7 c), the use of which is shown in the diagram RC-oscillator with zero phase shift, made on an operational amplifier (Fig. 8).
Rice. 8 RC-generator with zero phase shift in the OS circuit
In this circuit, the voltage from the output of the amplifier is fed to its non-inverting input through the feedback circuit formed by the elements of the Wien bridge R 1 C 1 and R 2 C 2. Resistive chain RR T forms another feedback - negative, which is designed to limit the increase in the amplitude of oscillations and preserve their harmonic shape. 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 loop gain
must be a real and positive value, and this is possible when 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 equal 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 capacitances of capacitors WITH, which are part of the Wien bridge, and the amplitude of the oscillations is controlled by the resistance R.
Main advantage RC-generators before LC-oscillators is that the former are easier to implement for low frequencies. For example, if in a generator circuit with a zero phase shift in the feedback circuit (Fig. 8) R 1 = R 2 = 1 MΩ, C 1 = C 2 = 1 uF, then the generated frequency
.
To get the same frequency in LC generator would require an inductance L= 10 16 H at WITH= 1 uF, which is difficult to implement.
IN RC-generators can, by simultaneously changing the values of capacitances WITH 1 and WITH 2 , get a wider frequency tuning range than is the case in LC-generators. For LC-generators
while for RC- generators, at WITH 1 = WITH 2
To disadvantages RC-generators should be attributed to the fact that at relatively high frequencies they are more difficult to implement than LC-generators. Indeed, the capacitance value cannot be reduced below the mounting capacitance, and a decrease in the resistance of the resistors leads to a drop in the gain, which makes it difficult to fulfill the amplitude self-excitation condition.
Listed advantages and disadvantages RC-generators led to their use in the low-frequency range with a large frequency overlap ratio.
