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 (POF): 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 possible to divide containers C1 and C2 into sections, then it will be possible to switch ranges, and double variable resistor R1R2 smoothly adjust the frequency in ranges.
An almost practical circuit of an RC oscillator with a Wien bridge is shown in the figure below:
Here: switch SA1 can be used to switch the range, and dual resistor R1 can be used to adjust the frequency. Amplifier DA2 serves to match the generator with the load.
The use of RC type generators with oscillatory circuits of inductance and capacitance, discussed above, becomes more complicated as the frequency of the generated oscillations decreases, since it is difficult to ensure the necessary quality of the circuit and to adjust the frequency of the generator if it operates in a wide frequency range: its dimensions increase. In this regard, rheostatic-capacitive generators of sinusoidal oscillations ( rc generators), which operate stably in a wide range of frequencies (from fractions of a hertz to several thousand kilohertz), are simple in design and small in size.
On rice. 170, a the diagram is shown RC generator, which is a two-stage rheostat-capacitive amplifier with positive and negative feedback. The first ensures the fulfillment of the conditions for self-excitation of the circuit, and the second increases the stability of its operation.
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When the circuit on the grid of lamp L 1 is turned on, due to fluctuation, an alternating voltage arises, which is amplified by lamps L 1 and L 2. So, if the potential of the control grid of lamp L 1 has become higher and has a positive sign, then it is easy to verify that at the output of the circuit, at resistance R c2, the potential will also become higher. A circuit is connected in parallel with resistance R c2 feedback, consisting of two RC units. It is quite obvious that the potential of the point also becomes higher, i.e., due to feedback, the control grid of lamp L 1 receives a voltage in phase with the initial fluctuation oscillations. Rice. 170. RC type generators: a - two-stage rheostatic-capacitive circuit; b - circuit with a phase-shifting chain; c - vector diagram. |
The frequency of the generated oscillations determined by the RC chain can be determined from the following considerations. Amplifier output voltage (at resistance R c2)
U out =U c1 K
where U c 1 is the signal at the input of lamp L 1; K is the gain of the amplifier (we neglect the influence of capacitance C c2). Feedback voltage arising on the control grid of lamp L 1
where Z av is the circuit resistance between points a-c; Z bv - circuit resistance between points b-v.
Generation is possible only under the condition that the phases of the voltage vectors U c1 and U o.c coincide, which will occur if the resistances Z av and Z bv create the same phase shift between the voltages in these sections and the currents. If this condition is met
Z ab =Z ab -Z bv =Z ab e iφ - Z bv e iφ
Considering that
cot φ ab =RωC
cot φ av =1/RωC
then, equating the right-hand sides of the last equalities, we get
from where you can determine the frequency of the generated oscillations
The feedback coefficient β, which must be provided for self-excitation of the circuit, is determined from the relation
Consequently, a third of the output voltage must be supplied to the amplifier input, i.e., to ensure amplitude balance, the amplifier must have a gain factor K = 3.
To reduce the nonlinear distortion that occurs with such strong feedback, automatically controlled negative feedback is introduced into the circuit; its circuit is formed by a thermistor T and resistance R k1. As the output voltage increases, the thermistor current increases, its resistance, and therefore the voltage across it, decreases, and the negative feedback voltage formed at the resistance R k1 increases. Adjustable negative feedback increases the voltage constancy on the control grid of lamp L 1. The circuit also has unregulated negative current feedback: the control grid of lamp L 2 receives feedback voltage from resistance R k2.
Wide practical use have also RC generators with a phase-shifting chain. To rotate the phase of the output voltage (voltage at the anode) by 180°, these circuits use phase shifters, which use RC circuits instead of a lamp, as was the case in the previous circuit. On rice. 170, b A diagram of such an RC generator with a four-link phase-shifting chain is shown. Each link turns the phase by an angle φ = 180/n, where n is the number of links. In the scheme under consideration, the angle φ = 180/4 = 45°.
The self-excitation process is illustrated by a vector diagram ( rice. 170, in). The alternating anode current Ia, which appears in the circuit due to fluctuations, creates an alternating voltage Ua at the anode, which is in antiphase with the current. This voltage is applied to the first link of the phase-shifting chain R 1 C 1, the current in which leads the voltage U R1C1 by 45° and creates a voltage U R1 at the resistance R 1, which is in phase with the current. Voltage U R1 is input in relation to the circuit R 2 C 2.
Thus, by gradually rotating the phase of the anode voltage across resistance R 4 (on the lamp grid), a signal voltage is formed that is in antiphase with the anode voltage, i.e., the phase balance condition is satisfied. In addition, for stable generation it is also necessary that the circuit gain K at the generation frequency be equal to or greater than the attenuation coefficient d of the phase-shifting chain.
The frequency of generated oscillations is determined by the formula
(296)
with the amplifier gain K = 18.4.
Single lamp RC generator It is small in size and simple in design, but has a number of disadvantages:
- a) a slight increase in feedback or gain leads to a sharp distortion of the shape of the generated oscillations;
- b) RC circuits bypass the anode load, and therefore it is often difficult to obtain the necessary gain for self-excitation;
- c) the attenuation of the phase-shifting chain depends on frequency, therefore, when designing a generator designed to operate in a fairly wide frequency range, nonlinear adjustable negative feedback and automatic gain control must be introduced into the circuit.
The use of generators with oscillating circuits to generate low frequency oscillations (below 10 kHz) is difficult due to the significantly increasing ratings of inductors and capacitors, which entails an increase in the size and cost of the generator.
Therefore, at present, RC generators are widely used to generate low and infra-low frequencies, in which RC filters are used instead of an oscillating circuit.
RC generators, operating in a relatively wide frequency range from fractions of a hertz to several megahertz, provide sufficient stability of oscillations and have small dimensions and weight.
The use of field-effect transistors in RC generator circuits distinguishes them favorably from bipolar transistors the possibility of using high-resistance resistors in the positive feedback circuit, which in turn allows the use of capacitors with lower ratings and greater stability.
The simplest RC generators are shown in Fig. 1. As is known, the excitation conditions of the generator require that the feedback circuit changes by 180° (for a single-stage generator) the phase of the signal coming from the drain load to the gate circuit.
In the generator circuit shown in Fig. 1a, this is achieved by implementing a feedback circuit of several simple RC links connected in series. In addition, the attenuation of the signal during the passage of the feedback circuit must be compensated by the amplification of the cascade.
For circuits with elements R and C of equal value, the phase balance condition at the generated frequency f 0 is satisfied under the following relations:
for three-link f 0 =0.065/RC;
for four-bar f 0 =0.133/RC
Rice. 1. Circuits of the simplest RC generators.
a - with a phasing RC chain; b - with a source follower; c - with a T-shaped RC bridge.
For a three-bar RC feedback circuit, the required cascade gain must be greater than 29, and in a four-bar RC circuit at least 18.4.
To increase the stability of the generator (due to the shunting effect of the load resistor Rc by the feedback circuit), an additional stage is often introduced - a source follower (Fig. 1, b), which has a high input resistance.
Generator circuit with a double T-shaped RC filter (Fig. 1, c), the elements of which are selected as follows: C1=C2=C; C3=C/0.207; R1=R2=R; R3=0.207R - operates provided that the cascade gain is at least 11. In this case, the oscillation frequency
The considered simplest RC generators on PTs have not found widespread use due to their inherent shortcomings.
The first drawback is the need to obtain a large cascade gain, which for a generator with a three-link feedback circuit must be at least 29. The practical implementation of such a gain is difficult due to the small value of the DC slope. If we take into account that negative feedback is introduced to improve the shape of the generated oscillations, then the gain of the cascade should be even greater.
The second drawback is the impossibility of tuning in a wide frequency range of generators made according to a circuit with RC circuits and a T-shaped bridge in the feedback circuit.
GENERATORS TUNABLE IN A WIDE FREQUENCY RANGE
The most widely used among RC generators is a circuit with a phase RC bridge (Win bridge generator), circuit diagram which is shown in Fig. 2. The advantages of such a circuit include low attenuation and zero phase shift in the feedback circuit at the generation frequency.
Thus, when turning on the RC phase bridge, to satisfy the phase balance condition, it is necessary that the generator amplifier provide a 360° phase shift.
The generation frequency with equality R1=R2=R and C1=C2=C is determined by the expression
f 0 =1/2RCπ (1)
At this frequency, the attenuation of the phase RC bridge is minimal and equal to 3. (Attenuation β - the amount of attenuation that the phase RC bridge introduces into the passing signal depending on the detuning Δf - is determined by the expression β = (9 + (2Δf) 2 / f 0 ) 1/2) It follows that the minimum gain at which the amplitude balance condition is satisfied must be at least 3. Due to the low value of the required gain, it becomes possible to introduce deep negative feedback, which leads to a decrease in the level of nonlinear distortions when operating in a wide range frequency range.
In the diagram of Fig. 2, and negative feedback is carried out due to the resistor in the source circuit of transistor T1 and the introduction of chain R5C3. A TVD-4 low-inertia thermistor was used as resistor R5, resistors R1, R2 were of the PTMN type, and capacitors C1 and C2 were of the KSO-G type. At the ratings indicated in the diagram, the generation frequency f 0 = 1500 Hz. When the temperature changed in the range from 10 to 50 ° C, relative frequency instability was obtained
Δf/f=0.05% at 10° C.
The phase RC bridge contains only two elements of the same name; therefore, it can be tuned in a wide range of frequencies by changing the value of only two elements R1, R2 or C1, C2), which makes the restructuring of generators with such bridges structurally convenient.
In Fig. Figure 2b shows a diagram of a tunable low-frequency generator with a phase RC bridge. The frequency of the generated oscillations is smoothly adjusted using a dual potentiometer R2, R3. The generator amplifier is two-stage with direct coupling. To stabilize the amplitude of oscillations of the generator and its operating mode, deep negative feedback has been introduced for both direct and alternating current (chain R8, R6, R5). To cover the entire audio range, a switch should be introduced that would simultaneously change the capacitances of capacitors RC and C2 in both shoulders of the bridge.
Rice. 2. Schematic diagrams of generators with a phase RC bridge.
a - with a two-stage amplifier and capacitive coupling; b - with a two-stage amplifier and direct coupling.
Rice. 3. Generator tunable over a wide range
a - schematic diagram; b - block diagram.
A more complex circuit of an RC generator using field-effect transistors, which makes it possible to adjust the frequency in the ten-day range, is shown in Fig. 3. For the parameters indicated in the diagram, the generator frequency lies in the range of 500 kHz - 5 MHz; however, by changing the capacitances of the capacitors, frequencies in other ranges can be obtained.
Two phase shifters, a bass reflex, an amplifier and an attenuator are connected in such a way that they form a feedback loop. The circuit will oscillate at a frequency such that the total phase shift is 360°. At this frequency, each of the two identical phase shifters provides a 90° phase shift.
The voltage-controlled phase shifter consists of a capacitor C1 and a transistor T2.
Transistors T3, T4 and capacitor C3 form a second phase shifter, which operates similarly to the first. Thanks to high resistance phase shifters eliminate the need for buffer stages. The gates of transistors T2 and T4 are AC grounded and therefore can be connected. Transistor T5 is designed to amplify the signal.
Transistor T7 and resistor R6 form a voltage-controlled attenuator, with transistor T7 used as a controlled resistor.
The amplitude detector consists of an amplifier based on transistor T6, a diode detector D1 and a filter R5C5. As the amplitude of the input signal increases, the gate voltage of transistor T7 becomes more negative, thereby increasing the dynamic resistance of the transistor and decreasing the gain in the feedback loop.
STABILIZATION OF THE AMPLITUDE OF OSCILLATIONS
The property of a field-effect transistor to change the channel resistance depending on the control voltage applied to the gate has found wide application in generators for automatically stabilizing the output signal level.
In Fig. 4, a shows a diagram of an RC sinusoidal oscillation generator with adjustable negative feedback. A two-stage amplifier based on field-effect transistors T1 and T3 is covered by positive feedback through elements R1-R4, C1, C3. Negative feedback is carried out through a divider consisting of resistor R6 and the controlled resistance of the channel of field-effect transistor T2. The establishment of a stationary amplitude occurs due to the influence of Uout (through detector D1 and its elements R7, C5) on the depth of negative feedback and on the power supply mode of transistor T1. The inertia of the AGC is determined mainly by the capacitance of capacitor C5 and the resistance of resistor R7. This automatically controlled negative feedback makes it possible to increase the stability of the generator characteristics compared to a conventional circuit when the supply voltage and ambient temperature change. When the power supply changed from 18 to 10 V, the amplitude of the output signal decreased by 8%.
Rice. 4. Generators with stabilization of the amplitude of generated oscillations.
a - RC generator with adjustable feedback; b - LC generator with an attenuator on the DC.
The level of the output signal of the generator is automatically stabilized somewhat differently, the circuit diagram of which is shown in Fig. 4, b. The drain-source voltage of field-effect transistor T1 is regulated by a variable resistor R3 installed in the gate circuit of the second transistor T2. Part of the output voltage through transformer L1, L2 is supplied to rectifier D1 and filter R3C7. Depending on the position of potentiometer R3, the operating point of the field-effect transistor changes, the resistance of its channel and, accordingly, the amplitude of the signal at the output of the generator changes. Potentiometer R3 sets the required amplitude of the output voltage, which is then automatically maintained at a given level.
As can be seen from the above examples, the use of field-effect transistors in circuits for automatic stabilization of the output voltage of generators makes it possible to significantly simplify such circuits and reduce the required control power of the regulated element.
FM GENERATORS
In automation and telemechanics, and measurement technology, there is a need for broadband frequency modulation at a low carrier frequency. For example, in frequency division radio telemetry, each channel is assigned its own subcarrier frequency. Subcarrier frequency generators are low frequency oscillators whose frequencies are modulated by signals from sensors. The use of LC generators in such systems is undesirable due to the cumbersome implementation in the low-frequency range. Therefore, an RC generator is used as a master frequency-modulated subcarrier frequency generator.
The frequency of the RC generator, as mentioned above, is determined by the parameters of the phasing RC chain, changing which in a certain way, the frequency modulation of the generator oscillations is carried out. To obtain a linear modulation characteristic, it is necessary that the ratios 1/R or 1/C of the phasing chain change simultaneously according to a linear law.
Rice. 5. FM generator on PT, and - circuit diagram; b - modulation characteristic.
Semiconductor diodes and transistors are used as voltage-tunable capacitances, using the dependence of the capacitance p-n junction from reverse voltage. A significant disadvantage of this method is the large nonlinearity of the modulation characteristic of the FM generator due to the nonlinear change in capacitance depending on the applied voltage.
Semiconductor diodes and bipolar transistors can also be used as variable resistances. However, this method of obtaining FM has the following disadvantages: nonlinearity of the modulation characteristic at large frequency deviations; large amplitude modulation; poor isolation of the modulating signal source and the self-oscillator; significant power consumed by the control circuit.
The method of implementing FM using field-effect transistors does not have the listed disadvantages. The use of PTs as variable resistances in the phasing circuit of an RC generator makes it possible to realize their important advantage - the linear dependence of the channel conductivity on the control voltage and the high input impedance of the frequency modulator.
In Fig. Figure 5 shows a schematic diagram of an FM generator with a phase RC bridge and its modulation characteristics for PTs (T(G2) of the KP103Zh and KP103M types, used as variable resistors.
Resistors R1 and R2 are included to reduce the deviation depth to the required level; In addition, using resistors with negative TCR, it is possible to reduce the influence of temperature changes in the resistance of the DC channel on the stability of the generator frequency. Using the bias source Ecm, the required resistance value of the DC channels is set with the control (modulating) signal UBX=0.
MULTIVIBRATORS
Relaxation low-frequency generators have a large time constant. In multivibrators made on bipolar transistors, to obtain a large time constant, electrolytic capacitors with a large capacity and low stability are used. The high input resistance of field-effect transistors makes it possible to obtain the necessary time constant in relaxation circuits without the use of capacitors with large capacitance. Therefore, in cases where it is necessary to implement time constants of approximately several seconds or minutes, it is advisable to use field effect transistors.
In the diagram shown in Fig. 6, a, two field-effect transistors are connected according to the source follower circuit, and two bipolar transistors are switches. The principle of operation of the circuit is similar to the principle of operation of a conventional multivibrator, and the combination of a bipolar and field-effect transistor should be considered as some kind of active element. This introduces high input impedance into the FETs while simultaneously providing high total gain. Bipolar transistors do not enter a saturation state, since the voltage from their collectors powers the drains of the field-effect transistors. As a result of this connection, the multivibrator is stably self-excited; Since the operating points of the transistors are shifted to the linear region, any change in the input current causes a change in the collector voltage. This circuit also works well at high frequencies.
Rice. 6. Circuits of multivibrators on PT.
a - with unsaturated bipolar transistors; b - with saturated bipolar transistors.
The duration of the multivibrator's stay in each state is determined by the discharge of capacitor C1 or C2 through the gate circuit resistor. When the voltage reaches a value equal to the cutoff voltage of the FET, the change in source current causes the circuit to switch to a different state. If the capacitance of each capacitor C1 and C2 is 4 μF, then by changing R1 and R2 upward, you can increase the duration of the multivibrator period from 8 ms to 6 min. If the capacitance of each capacitor is chosen equal to 100 pF, then the frequency can be changed from 100 Hz to 3 MHz
The multivibrator is designed somewhat differently, the circuit of which is shown in Fig. 6, b. Let's consider the principle of operation of this scheme. Let's assume that transistor T1 goes into saturation state, then a positive potential appears at the gate of T4 and transistors T4 and T2 close. A voltage surge at the collector T2 leads to a reliable opening of transistors T1 and T3. The bias current flowing to the gate of T3 through resistor R2 maintains it in this state. Capacitor C1, discharging through a resistor, reduces the bias voltage at gate T4. When the voltage Uzi of transistor T4 decreases to the cutoff voltage, transistors T4 and T2 begin to conduct and quickly open, while T1 and T3 close. The multivibrator pulse duration is determined by the formula
(2)
where Ec is the voltage of the power source.
With the nominal values of the parts indicated in the diagram in Fig. 8b, a pulse duration of approximately 25 s was obtained.
RAMP VOLTAGE GENERATORS
By using a constant current source on a field effect transistor in a sawtooth voltage generator, it is possible to obtain a saw whose linearity and slope are almost independent of random changes in the control voltage. In addition, field-effect transistors make it possible to implement scan generator circuits with linearity and duration values that are difficult to achieve when using bipolar transistors.
The sawtooth voltage generator shown in Fig. 7, consists of a direct current source on a field-effect transistor T1, a variable capacitor C1 and a unijunction transistor T2. Using potentiometer R2, the value of the direct drain current of field-effect transistor T1 is set, corresponding to the thermostable point of the PT. The negative feedback created by resistors R1 and R2 with high resistance included in the source circuit ensures a stable drain current despite changes in the supply voltage. This current linearly charges the variable capacitor C1 to the trigger voltage of the unijunction transistor T2. The charging time is a function of the capacitance of capacitor C1.
Rice. 7. Scheme of a sawtooth voltage generator.
By changing the capacitance of capacitor C1, you can adjust the repetition frequency of the generator output signal in the range from 500 Hz to 50 kHz. The storage capacitor is quickly discharged through a conductive switch on transistor T2. The sawtooth voltage from capacitor C1 is supplied to the output through the emitter follower on transistor T3. The amplitude of the output signal is determined by the position of the potentiometer R4 and can be adjusted from 0 to 8 V. Over the entire frequency range, the nonlinearity of the sawtooth voltage in this circuit does not exceed 1%.
QUARTZ GENERATORS
One of the most important parameters of generators is the stability of the frequency of generated oscillations. Strict requirements for frequency stability and reproducibility in modern radio devices can be met using quartz oscillators.
Rice. 8. Crystal oscillator circuit.
Tube quartz oscillators are unacceptable in most practical cases due to such disadvantages as high power consumption, large dimensions and weight. In addition, the lamp itself is a source of heat, which makes it difficult to thermostat the generator.
Due to the low input resistance of bipolar transistors, the quartz resonator in self-oscillators is connected only between the base and collector.
Field-effect transistors, which do not have the above-mentioned disadvantages of electron tubes and bipolar transistors, are now quite often used in quartz oscillator circuits.
A.G. Milekhin
Literature:
- Gozling V. Application of field-effect transistors. M., "Energy", 1970.
- Barsukov F.I. Generators and selective amplifiers of low frequency. M., "Energy", 1964.
- Gonorovsky I. S. Radio engineering circuits and signals. M., “Soviet Radio”, 1971.
- Van der Geer. Tuning an RC oscillator in the ten-day range using field-effect transistors. - “Electronics”, No. 4, 1969.
- Krisilov Yu. D. Automatic adjustment and stabilization of amplification of transistor circuits. M., “Soviet Radio”, 1972.
- Prosser L. Stable oscillators using field-effect transistors. - “Electronics”, 1966, No. 20.
- Hanus, Martinez. Stable low-frequency multivibrator with two PTs. - “Electronics”, 1967, No. 1.
- Iled L. Using a field-effect transistor to obtain a stable sawtooth voltage. - “Electronics”, 1966, No. 16.
- Express information "PEA and VT", 1973, No. 47.
- King L. Stable quartz oscillator based on a field-effect transistor. - “Electronics”, 1973, No. 13.
- Ignatov A.N. Application of field-effect transistors of the KP103 type in communication equipment. - In the book: Trends in the development of low-power active radio components. Novosibirsk, "Science", 1971.
1.1 Purpose and types of generators.
An electronic signal generator is a device through which the energy of third-party power sources is converted into electrical oscillations of the required shape, frequency and power. Electronic generators included integral part in many electronic devices and systems. For example, generators of harmonic or other waveforms are used in universal measuring instruments, oscilloscopes, microprocessor systems, in various technological installations, etc. In televisions, horizontal and vertical scanning generators are used to form a luminous screen.
The classification of generators is carried out according to a number of characteristics: the shape of the oscillations, their frequency, output power, purpose, type of active element used, type of frequency-selective feedback circuit, etc. Based on their purpose, generators are divided into technological, measuring, medical, and communication. Based on the shape of the oscillations, they are divided into generators of harmonic and non-harmonic (pulse) signals.
Based on the output power of the generator, they are divided into low-power (less than 1 W), medium-power (below 100 W) and high-power (over 100 W). By frequency, generators can be divided into the following groups: infra-low-frequency (less than 10 Hz), low-frequency (from 10 Hz to 100 kHz), high-frequency (from 100 kHz to 100 MHz) and ultra-high-frequency (above 100 MHz).
According to the active elements used, generators are divided into tube, transistor, operational amplifiers, tunnel diodes, or dinistors, and according to the type of frequency-selective feedback circuits - into LC-, RC- and ^L-type generators. In addition, feedback in generators can be external or internal.
1.2 Sine wave generators
This group of generators is designed to produce sinusoidal oscillations of the required frequency. Their operation is based on the principle of self-excitation of an amplifier covered by positive feedback (Figure 1). The gain and transmission coefficient of the feedback link are assumed to be complex, i.e. their dependence on frequency is taken into account. In this case, the input signal for the amplifier in the circuit of Fig. 1.1 is part of its output voltage transmitted by the feedback link
Figure 1. Generator block diagram
To excite oscillations in the system Figure 1, two conditions must be met:
1.3 Generator self-excitation modes
Soft mode.
If the operating point is located in the section of the iK(uBE) characteristic with the greatest steepness, then the self-excitation mode is called soft.
Let us follow the changes in the amplitude of the first harmonic current depending on the value of the feedback coefficient of the CBS. A change in the CBS leads to a change in the angle of inclination a of the direct feedback (Fig. 2)
Figure 2. Soft self-excitation mode
When KOS = KOS1 the state of rest is stable and the generator is not excited, the amplitude of oscillations is zero (Fig. 2 b). The value of KOS = KOS2 = KKR is the boundary (critical) value between the stability and instability of the state of rest. When KOS = KOS3 > KKR, the state of rest is unstable, the generator will be excited, and the value of Im1 will be established corresponding to point A. With an increase in KOS, the value of the first harmonic of the output current will gradually increase and at KOS = KOS4 it will be established at point B. With a decrease in KOS, the amplitude of oscillations will decrease along the same curve and the oscillations will break down at the feedback coefficient KOS = KOS2
As conclusions, the following features of the soft self-excitation mode can be noted:
excitation does not require a large value of the feedback coefficient of the CBS;
excitation and disruption of oscillations occur at the same value of the feedback coefficient KKR;
it is possible to smoothly adjust the amplitude of stationary oscillations by changing the value of the feedback coefficient of the CBS;
As a disadvantage, it should be noted the large value of the constant component of the collector current, which leads to a low efficiency value.
Hard mode.
If the operating point is located in the characteristic section iK = f (uBE) with a low slope S
Figure 3. Hard self-excitation mode
The self-oscillator will be excited when the feedback coefficient exceeds the value of KOS3 = KOSKR. A further increase in CBS leads to a slight increase in the amplitude of the first harmonic of the output (collector) current Im1 along the V-G-D path. Reducing the KOS to KOS1 does not lead to a breakdown of the oscillations, since points B and B are stable, and point A is stable on the right. The oscillations break down at point A, i.e. at CBS
Thus, we can note the following features of the generator operation in a hard self-excitation mode:
self-excitation requires a large value of the feedback coefficient of the CBS;
excitation and disruption of oscillations occur stepwise when different meanings feedback coefficient CBS;
the amplitude of stationary oscillations cannot change within large limits;
the DC component of the collector current is less than in soft mode, therefore, the efficiency is significantly higher.
Comparing the positive and negative aspects of the considered self-excitation modes, we come to a general conclusion: reliable self-excitation of the generator is ensured by the soft mode, and economical operation, high efficiency and a more stable amplitude of oscillations are provided by the hard mode.
The desire to combine these advantages led to the idea of using automatic bias, when the generator is excited in a soft mode of self-excitation, and its operation occurs in a hard mode. The essence of automatic offset is discussed below.
Automatic offset.
The essence of the mode is that to ensure excitation of the self-oscillator in soft mode, the initial position of the operating point is selected on the linear section of the flow characteristic with maximum steepness. The equivalent resistance of the circuit is selected such that the self-excitation conditions are met. In the process of increasing the oscillation amplitude, the direct current mode automatically changes and in a stationary state the operating mode with cutoff of the output current (collector current) is established, i.e. the self-oscillator operates in a hard self-excitation mode in the section of the flow characteristic with a low slope (Fig. 4).
Figure 4. Principle of automatic biasing of a self-oscillator
The automatic bias voltage is usually obtained due to the base current by including the chain R B C B in the base circuit (Fig. 5).
Figure 5. Automatic bias circuit due to base current
The initial bias voltage is provided by the voltage source E B. As the oscillation amplitude increases, the voltage across the resistor R B increases, created by the constant component of the base current I B0. The resulting bias voltage (E B - I B0 R B) decreases, tending to E B S T.
In practical circuits, the initial bias voltage is provided using a basic divider R B1, R B2 (Fig. 6).
Figure 6: Automatic offset using base divider
In this circuit, the initial bias voltage
E B.START =E K -(I D +I B0)R B2,
where I D =E K /(R B1 +R B2) – divider current.
As the oscillation amplitude increases, the constant component of the base current IB 0 increases and the displacement EB decreases in magnitude, reaching the EBST value in steady state. The capacitor SB prevents a short circuit of the resistor RB1 with direct current.
It should be noted that the introduction of an automatic bias circuit into the generator circuit can lead to the phenomenon of intermittent generation. The reason for its occurrence is the delay of the automatic bias voltage relative to the increase in the oscillation amplitude. With a large time constant t = RBSB (Fig. 8.41), the oscillations quickly increase, and the displacement remains practically unchanged - EB.START. Further, the displacement begins to change and may be less than the critical value at which stationarity conditions are still met, and the oscillations will break down. After the oscillations stop, the capacitance SB will slowly discharge through RB and the bias will again tend to EB.START. As soon as the slope becomes large enough, the generator will be excited again. Further processes will be repeated. Thus, oscillations will periodically arise and break down again.
Intermittent fluctuations are generally considered to be undesirable phenomena. Therefore, it is very important to calculate the elements of the automatic bias circuit in such a way as to exclude the possibility of intermittent generation.
To eliminate intermittent generation in the circuit (Fig. 4), the value of SB is selected from the equality
Autogenerator with transformer feedback
Let's consider a simplified circuit of a transistor self-oscillator of harmonic oscillations with transformer feedback (Fig. 7).
Figure 7. Autogenerator with transformer feedback
Purpose of the circuit elements:
transistor VT p-n-p type, acts as an amplifying nonlinear element;
the oscillatory circuit LKCKGE sets the frequency of oscillations of the generator and ensures their harmonic form, the real conductivity GE characterizes the energy losses in the circuit itself and in the external load associated with the circuit;
coil LB provides positive feedback between the collector (output) and base (input) circuits; it is inductively coupled to the circuit coil LK (mutual induction coefficient M);
power supplies EB and EK provide the necessary constant voltages at the transitions of the transistor to ensure the active mode of its operation;
capacitor CP separates the generator and its DC load;
blocking capacitors SB1 and SB2 shunt power supplies via alternating current, eliminating useless energy losses on their internal resistances.
1.3 Types of generators
Depending on the way in which the condition of phase and amplitude balance is ensured in the generator, generators are distinguished:
LC oscillators using frequency dependent circuit oscillatory circuit. The time setting parameter in them is the period of natural oscillations of the oscillatory circuit;
RC oscillators in which frequency-dependent feedback circuits are a combination of R and C elements (Wien bridge, double T-bridge, shifting RC circuits, etc.). Time the setting parameter here is the time of charging, discharging or recharging the capacitor;
generators with electromechanical resonators (quartz, magnetostrictive), in which the timing parameter is the period of natural oscillations of the resonating element.
1.3.1 RC oscillators
RC generators are based on the use of frequency-selective RC circuits and are implemented according to the block diagram shown in Fig. 1.
There are RC generators with phase-shifting and bridge RC circuits.
1.3.2 Three-link RC circuit diagram
RC oscillators with a phase-shifting circuit are an amplifier with a phase rotation of 180°, in which, to fulfill the phase balance condition, a feedback circuit is connected, which also changes the phase of the output signal by 180° at the generation frequency. Three-bar RC circuits (less often four-bar) are usually used as a phase-shifting feedback circuit. The diagram of such a circuit is shown in Fig. 8.
Figure 8. Diagram of a three-bar RC circuit
The phase-shifting circuit significantly reduces the feedback signal entering the amplifier input. Therefore, for three-link RC circuits, the gain of the amplifier must be at least 29. Then the second condition for the occurrence of oscillations will also be satisfied - the amplitude balance condition.
With the same resistances of resistors R and capacitances of capacitors C, the oscillations of a generator with a phase-shifting circuit are determined by the formula:
To change the oscillation frequency, it is enough to change the resistance or capacitance in the phase-shifting RC circuit.
1.3.3 Bridge of Wine
R 3
Three bridge frequency-selective RC circuits are most widely used by the Wien bridge (Fig. 9.).
R 4
Figure 9. Wien Bridge
The phase balance condition is ensured here at one frequency at which the output signal of the bridge is in phase with the input.
The generation frequency is equal to the bridge tuning frequency and is determined by the relation:
Frequency adjustment in a generator with a Wien bridge is simple and convenient, and is possible over a wide frequency range. It is carried out using a dual variable capacitor or a dual variable resistor included in the circuit instead of constant capacitors C or resistors R.
Since the transmission coefficient of the Wien bridge at the generation frequency is 1/3, the gain of the amplifier should be equal to 3. Then stable generation occurs in the generator with the Wien bridge.
1.3.4 Double T-bridge diagram
In addition, a double T-shaped bridge is also used in RC generators (Fig. 10).
Figure 10. Diagram of a double T-bridge
To stabilize the amplitude of the output signal of an RC generator, various nonlinear elements are used: thermistors, photoresistors, incandescent lamps, diodes, LEDs, zener diodes, field-effect transistors, etc. Strictly regulated feedback is also used.
RC oscillators are characterized by good stability, are easily tuned and allow you to obtain oscillations with very low frequencies (from fractions of a hertz to several kilohertz). Stability of oscillation frequency. RC oscillators depend more on the quality of the R and C elements than on the structure of the frequency-selective circuit and the characteristics of the amplifier. The best performance is achieved by RC generators, in which additional stabilization of the oscillation frequency is carried out using quartz resonators.
1.3.6 Generator circuit with a Wien bridge on an op-amp
Figure 6 shows a circuit with a Wien bridge, one arm of which is formed by a resistive voltage divider, and the other by differentiating and integrating circuits. The transfer coefficient from the output of the phase-setting circuit , , , to the non-inverting input of the op-amp at the resonant frequency is 1/3. To balance the amplitudes, the amplifier's transmission coefficient from the output to the non-inverting input must be equal to three, i.e. the condition = must be met. To achieve phase balance, the time constant of the differentiating circuit must be equal to the time constant of the integrating circuit, i.e. =.
To improve self-excitation, stabilize the oscillation amplitude and reduce nonlinear distortions in the circuit, it is necessary to use an amplifier with an adjustable transmission ratio or include a nonlinear voltage limiter at the op-amp output.
Figure 11. Generator circuit with a Wien bridge on an op-amp
1.4 LC-type generator
Such a generator is built on the basis of an amplifier stage on a transistor, including an oscillatory LC circuit in its collector circuit. To create a PIC, a transformer connection is used between windings W1 (having inductance L) and W2 (Fig. 12).
Figure 12. LC-type generator
1.5 Powerful amplifier stages.
A powerful cascade is understood as an amplification cascade for which the load and the power dissipated in this load are specified. Typically, the power ranges from several to tens - hundreds of watts. Therefore, powerful cascades, which, as a rule, are output, are calculated based on the given values of and. To estimate how much power the pre-amp stage should produce, you have to estimate the power gain of the stage.
The powerful output stage is the main energy consumer. It introduces the bulk of nonlinear distortion and occupies a volume commensurate with the volume of the rest of the amplifier. Therefore, when selecting and designing an output stage, the main attention is paid to the possibility of obtaining the highest efficiency, low nonlinear distortion and overall dimensions.
The output stages are single-ended and push-pull. Active devices in power amplifiers can operate in modes A, B or AB. To create powerful output stages, circuits with OE, OB and OK are used.
In single-ended output stages, active devices operate in mode A. When creating them, three transistor switching circuits are used. To match the load with the output stage, transformers are sometimes used, which provide maximum power gain, but significantly worsen its frequency characteristics.
Transformerless output stages have become increasingly widespread. They allow direct communication with the load, which makes it possible to do without bulky transformers and isolation capacitors; have good frequency and amplitude characteristics; can easily be made using integrated technology. In addition, due to the absence of frequency-dependent elements in the communication circuits between stages, it is possible to introduce deep common negative feedbacks for both alternating and direct currents, which significantly improves the conversion characteristics of the entire device. In this case, ensuring the stability of the amplifying device can be achieved by introducing the simplest corrective circuits.
Transformerless powerful output stages are assembled mainly using push-pull circuits on transistors operating in mode B or AB and connected according to circuits with OK or OE. In these circuits, it is possible to combine in one cascade either identical transistors or transistors with different types of electrical conductivity. Cascades that use transistors with different types of electrical conductivity (p-n-p and n-p-n) are called cascades with additional symmetry.
According to the method of connecting the load, there are two types of circuits: powered from one source and powered from two sources.
1.6 Classification of output power amplifiers
I will consider the classification of amplifiers by operating mode, i.e., by the amount of current flowing through the amplifier transistors in the absence of a signal.
1.6.1 Class A amplifiers
Class A amplifiers operate without signal cutoff in the most linear section of the current-voltage characteristic of the amplifying elements. This ensures a minimum of nonlinear distortions (THD and IMD), both at rated power and at low powers.
For this minimum you have to pay with impressive power consumption, size and weight. On average, the efficiency of a class A amplifier is 15-30%, and the power consumption does not depend on the output power. Power dissipation is maximum at small output signals.
1.6.2 Class B amplifiers
If we change the bias of the emitter junction so that the operating point coincides with the cutoff point, then we get the class B amplification mode. To do this, a more negative voltage must be applied to the base of the n-p-n transistor than in class A mode (for transistors of the type pnp mode class B is ensured by applying a more positive voltage to the base than in class A mode). In either case, for class B mode, the forward bias of the emitter junction is reduced and the transistor is turned off.
If the Class B amplifier stage includes only one transistor, the harmonic distortion of the signal will be significant. This is explained by the fact that the resulting collector current in shape repeats only the positive half-wave of the input signal, and not the entire signal, since for the negative half-wave the transistor remains off. To recreate an output signal that is completely similar in shape to the input signal, you can use two transistors (one for each half-wave of the input signal), combining them in a so-called push-pull circuit.
The voltage amplitude of the output signal is slightly less than the voltage of the power source. Since in class B mode the current flows through the transistor for only half a cycle, it becomes possible to double the collector current (compared to class A mode) with the same average power dissipated at the transistor collector.
The output voltage amplitude of a Class B amplifier is equal to twice the output voltage amplitude of a Class A amplifier. Thus, a push-pull transistor stage in Class B mode allows an output voltage that is twice that of Class A mode.
1.6.3 Class AB amplifiers
As the name suggests, class AB amplifiers are an attempt to combine the advantages of class A and class B amplifiers, i.e. achieve high efficiency and an acceptable level of nonlinear distortion. In order to get rid of the step transition when switching amplifier elements, a cutoff angle of more than 90 degrees is used, i.e. the operating point is selected at the beginning of the linear section of the current-voltage characteristic. Due to this, in the absence of a signal at the input, the amplifying elements are not switched off, and some quiescent current flows through them, sometimes significant. Because of this, the efficiency decreases and a minor problem arises in stabilizing the quiescent current, but nonlinear distortions are significantly reduced.
Class AB is the most economical for ULF, since in this case the amplifier consumes minimal current from the power supply. This is explained by the fact that at the operating point the transistors are locked and the collector current flows only when an input signal arrives. However, Class B amplifiers distort the waveform.
In a real class B amplifier, the transistor remains closed at very low input signal levels (since the transistor has a very small current gain near the cutoff) and opens sharply as the signal increases.
Nonlinear distortion can be reduced if class AB (or something in between B and AB) is used instead of class B mode. To do this, the transistor is turned on somewhat so that a small current flows at the operating point in the collector circuit. Class AB is less economical than class B, as it consumes more current from the power source. Typically, class AB is used only in push-pull circuits.
1.6.4 Class C amplifiers
Class C mode is obtained by biasing the transistor in the opposite direction, well to the left of the cutoff point. Part of the input signal is used to forward bias the emitter junction. As a result, the collector current flows for only part of one half-cycle of the input voltage. The negative half-wave of the input voltage lies in the deep cutoff region of the transistor. Since the collector current flows only during some part of the positive half-cycle, the duration of the collector current pulse is significantly less than the half-cycle of the input signal
Obviously, the shape of the output signal differs from the input signal and it cannot be restored by the methods used in push-pull amplifiers of classes B and AB. For this reason, Class C mode is used only when signal distortion is not a concern. As a rule, class C operating mode is used in high-frequency amplifiers and is not used in ULF.
1.7 Circuit solutions for powerful amplifier stages.
Power amplifiers using transistors of the same conductivity.
When the cascade is powered from two sources, and having a common point, the load is connected between the connection point of the emitter and collector of the transistors, and the common point of the power sources. The operating mode of the transistors is provided by dividers , , and . The transistors are controlled by antiphase input signals and, to obtain which, the previous stage must be phase inverted.
The principle of operation of the cascade according to the diagram in Figure 13 is to alternately amplify the half-waves of the input signal. If in the first cycle the negative half-wave is amplified by a transistor, while the transistor is closed by the positive half-wave, then in the second cycle the second half-wave of the signal is amplified by a transistor with the transistor closed.
When the cascade is powered from a single source (Fig. 14), the load is connected through an electrolytic separating capacitor of a sufficiently large capacity, but otherwise the circuit is similar to the previous one.
Figure 13. Output stage of a power amplifier using transistors of the same conductivity
The operating principle of the circuit is as follows. In the absence, the capacitor is charged to voltage. It is at this voltage that the capacitor enters the rest mode. During the cycle of operation (open state), a current flows through the load, which recharges the capacitor. During the cycle of operation, the capacitor discharges and current flows through the load. Thus, a bipolar signal is realized at the load.
In the considered circuits, transistors , and have different connections: - according to the OK circuit, and - according to the OE circuit. Since with these two connection schemes the transistors have different voltage amplification factors, without taking additional measures, an asymmetry of the output signal is obtained. Reducing signal asymmetry, in particular, can be achieved by appropriately selecting the gain factors for the two outputs of the previous phase-inverted stage. The asymmetry can also be reduced by using negative feedback covering the output and pre-output stages.
Figure 14. Output stage of a power amplifier based on transistors of the same conductivity with unipolar power supply
Power amplifiers using transistors of different conductivity, connected according to the OK circuit.
Figure 15. Output stage of a power amplifier using transistors of different conductivities
In Fig. Figure 15 shows a circuit diagram of a cascade powered from two sources (it is possible to implement a circuit with a single-polar supply). When using complementary pairs of transistors in this circuit n-p-n types and p-n-p there is no need to supply two antiphase input signals. With a positive half-wave of the signal, the transistor is open and closed; with a negative half-wave, on the contrary, it is open and closed. The rest of the operation of the circuit in Fig. 15 is similar to the operation of the corresponding circuits in Fig. 14 and fig. 13. A distinctive feature of the considered circuits is that the voltage gain of the cascade is always less than 1, and the output signal has less asymmetry, since both transistors are connected in the same circuit with OK.
In order to switch the power amplifier into AB mode to reduce nonlinear distortion, the bases are separated from each other by a pair of diodes, which provide bias for the transistors, at which current flows in them in quiescent mode (Fig. 16).
R 1
R 2
Figure 16. Power amplifier output stage in AB mode
Figure 17 shows a diagram of a transformerless power amplifier with a push-pull output stage based on MIS transistors with induced channels of type n (VT2) and type p (VT3). The substrate is usually connected to the source inside high-power MIS transistors. Field-effect transistors introduce less nonlinear distortion and are not subject to thermal instability. The threshold voltage of the drain-gate characteristic of modern high-power MIS transistors with an induced channel is close to zero. Their disadvantage is increased residual stress and production variation in parameters, however, as technology improves, they decrease.
Figure 17. Power amplifier output stage in AB to DC mode
Choice electrical diagram electronic device and its description
The circuit consists of two stages: the first stage is an RC oscillator on a Wien bridge, the second stage is a class AB power amplifier.
The Wien bridge is connected to the non-inverting input of the op-amp.
Let , then the frequency of the signal will be determined by the formula:
In order for oscillations to be established in a generator with a Wien bridge, the amplifier must have a gain greater than 3. The gain is set by resistors. Therefore, the following condition must be met:
Diodes connected in parallel serve to stabilize the amplitude of the generated signals (i.e., they introduce symmetrical nonlinear feedback).
Advantages of an RC generator with a Wien bridge:
The main disadvantage is that the output voltage reaches the voltage of the supply rails, which causes saturation of the output transistors of the op-amp and creates significant distortion.
The second stage is a push-pull transformerless stage with field-effect MOS transistors of different conductivity types.
MIS - transistor VT1 has n-type conductivity, and transistor VT2 has p-type. If a voltage of positive polarity is applied between the gates and sources of the transistors, then the transistor VT2 will be closed, and the transistor VT1 will be open, and the current will flow through the circuit from the plus of the power source E1 drain-source of the transistor VT1, across the load, to the negative pole of the power source E1. And if a gate-source voltage of negative polarity is applied, then transistor VT1 will be closed, and transistor VT2 will be open, and current will flow through the circuit from the plus of power source E2 through the load, source-drain of transistor VT2, to the negative pole of power source E2. The arrival of a signal with a voltage of either positive or negative polarity at the input leads to either turning off one transistor and unlocking the other, or vice versa. In other words, the transistors operate in antiphase. Transistors VT1 and VT2 are selected so that their parameters and characteristics in the working area are as close as possible.
Advantages:
it is possible to obtain high efficiency; with the correct choice of transistors, nonlinear distortions are small;
the cascade develops a greater maximum output power compared to a single-ended cascade with the same transistor;
due to the absence of transformers, there are no strict restrictions on the frequency range of amplified signals;
In addition, without bulky and heavy transformers, the device is lightweight, small in size and low in cost.
Flaws:
the need for careful selection of transistors and their rapid destruction when the output stage is overloaded, if it does not have a current protection system.
Figure 18. RC oscillator with a powerful output stage
CALCULATION AND SELECTION OF ELEMENTS OF AN ELECTRONIC DEVICE CIRCUIT
3.1 Power amplifier calculation
where is the amplitude value of the voltage at the load resistance;
Amplitude value of the current at the load resistance;
Load power.
The voltage of the power source of one half of the output stage with bipolar power supply is determined based on the amplitude of the output signal, and the voltage value is selected at least n V more, since the residual voltage must be taken into account, and for field-effect transistors it can reach one volt:
The maximum power dissipated by one transistor is determined by: Since the transistors are complementary, it is enough to calculate one arm of the amplifier. . Let
let's assemble an electronic device in MicroCap.
measure the output voltage,
measure the output current,
let's determine the frequency of the signal,
determine the power at the load,
compare with the terms of the technical specifications,
let's conclude.
Oscilloscope connection diagram:
Figure 4.1 RC Generator Test Scheme
CONCLUSION
In progress course work a methodology for developing an electronic device was considered using the example of an RC generator with a Wien bridge and a powerful output stage. The resulting device satisfies all the conditions of the technical specifications.
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R.C.-generator is a generator of harmonic oscillations, in which instead of an oscillatory system containing elements L And WITH, a resistive-capacitive circuit is used ( R.C.-circuit) with frequency selectivity.
The exclusion of inductors from the circuit makes it possible to significantly reduce the size and weight of the generator, especially at low frequencies, since as the frequency decreases, the dimensions of the inductors sharply increase. An important advantage R.C.-generators compared to L.C.- generators is the possibility of their manufacture using integrated technology. However R.C.- generators have low stability of the frequency of generated oscillations due to low quality factor R.C.-circuits, as well as poor oscillation shape due to poor filtering of higher harmonics in the output oscillation spectrum.
R.C.-generators can operate in a wide range of frequencies (from fractions of a hertz to tens of megahertz), but they have found application in communication equipment and measuring technology mainly at low frequencies.
Basic theory R.C.-generators were developed by Soviet scientists V.P. Aseev, K.F. Teodorchik, E.O. Saakov, V.G. Kriksunov and others.
R.C.- the generator usually includes a wideband amplifier made of a tube, transistor or integrated circuit and R.C.- a feedback circuit that has selective properties and determines the oscillation frequency. The amplifier compensates for energy losses in passive elements and ensures that the amplitude conditions for self-excitation are met. The feedback circuit ensures that the phase condition of self-excitation is fulfilled at only one frequency. By type of feedback circuit R.C.-generators are divided into two groups:
with zero phase shift in the feedback circuit;
with a phase shift in the feedback circuit by 180.
To improve the shape of the generated oscillations in R.C. generators use elements that have nonlinearity, which limit the increase in the amplitude of oscillations. The parameters of such an element change depending on the amplitude of the oscillations, and not on their instantaneous values (a thermistor, the resistance of which depends on the degree of heating by the current passing through it). With this limitation, the shape of the oscillations does not change; they remain harmonic even in a stationary mode.
Let's consider both types R.C.-autogenerators.
Self-oscillator with a 180 phase shift in the feedback circuit.
Such a self-generator is also called a self-generator with a three-link chain. R.C..
In the diagrams R.C.-oscillators with a phase shift of 180 use amplifiers in the feedback circuit to invert the phase of the input voltage. Such an amplifier can, for example, be an operational amplifier with an inverting input, a single-stage amplifier, or a multi-stage amplifier with an odd number of inverting stages.
In order for the phase balance equation to be satisfied, the feedback circuit must provide a phase shift OS = 180.
To substantiate the structure of the feedback circuit, we reproduce the phase-frequency characteristics of the simplest R.C.-links (Fig. 3,4).
Rice. Option 3 R.C.-link and its phase response
Rice. 4 Option R.C.-link and its phase response
From the graphs it is clear that one of the simplest R.C.-link introduces a phase shift not exceeding 90. Therefore, a phase shift of 180 can be achieved by cascading connection of three elementary R.C.-links (Fig. 5).
Rice. 5 Circuits and phase response of three-element R.C.-chains
Elements R.C.- the circuits are designed so as to obtain a phase shift of 180 at the generation frequency. One of the options for a generator with a three-link circuit R.C. shown in Figure 6
Rice. 6 Generator with three-link chain R.C.
The generator consists of a resistive transistor amplifier and a feedback circuit. A single-stage amplifier with a common emitter produces a phase shift between the voltage on the collector and the base K = 180. Therefore, to achieve phase balance, the feedback circuit must provide OS = 180 at the frequency of the generated oscillations.
Let us analyze the feedback circuit, for which we will compile a system of equations using the loop current method.
Solving the resulting system with respect to the feedback coefficient, we obtain the expression
From the expression it follows that the phase shift 180 is obtained in the case when it is a real and negative value, i.e.
therefore, generation is possible at a frequency
At this frequency the modulus of the feedback coefficient
This means that to excite self-oscillations, the amplifier coefficient must be greater than 29.
The output voltage of the generator is usually taken from the collector of the transistor. To obtain harmonic oscillations, a thermistor is included in the emitter circuit R T with positive temperature coefficient of resistance. As the oscillation amplitude increases, the resistance R T increases and the depth of negative feedback in the amplifier for alternating current increases, respectively, the gain decreases. When a stationary oscillation mode occurs ( TO= 1), the amplifier remains linear and no distortion of the collector current shape occurs.
Self-oscillator with zero phase shift in the feedback circuit.
A characteristic feature of the circuits R.C.-oscillators with zero phase shift in the feedback circuit is the use of amplifiers in them that do not invert the phase of the input signal. Such an amplifier can, for example, be an operational amplifier with a non-inverting input or a multi-stage amplifier with an even number of inverting stages. Let's consider some possible options for feedback circuits that provide zero phase shift (Fig. 7).
Rice. 7 Options for feedback circuits providing zero phase shift
They consist of two links, one of which represents RС-link with a positive phase shift, and the second – with a negative phase shift. As a result of adding the phase response at a certain frequency (generation frequency), a phase shift equal to zero can be obtained.
In practice, the phase-balance bridge, or in other words the Wien bridge (Fig. 7c), the use of which is shown in the diagram, is most often used as a selective circuit with zero phase shift R.C.-oscillator with zero phase shift, made on an operational amplifier (Fig. 8).
Rice. 8 R.C.-generator with zero phase shift in the OS circuit
In this circuit, the voltage from the output of the amplifier is supplied to its non-inverting input through a feedback circuit formed by the elements of the Wien bridge R 1 C 1 and R 2 C 2. Resistive circuit R.R. T forms another feedback - negative, which is designed to limit the increase in the amplitude of oscillations and maintain their harmonic form. The negative feedback voltage is applied to the inverting input of the operational amplifier. Thermistor R T must have a negative temperature coefficient of resistance.
Feedback circuit gain
must be a real and positive quantity, and this is possible if the equality
From here the frequency of the generated oscillations is determined. If R 1 = R 2 =R, C 1 = C 2 = C, That
The amplitude condition for self-excitation at frequency 0 requires the fulfillment of the inequality
If there is equality R 1 = R 2 = R And C 1 = C 2 = C gain TO > 3.
The oscillation frequency can be changed by changing the resistances R or capacitor capacities WITH, included in the Wien bridge, and the amplitude of oscillations is regulated by resistance R.
Main advantage R.C.- generators in front L.C.-generators is that the former are easier to implement for low frequencies. For example, if in a generator circuit with zero phase shift in the feedback circuit (Fig. 8) R 1 = R 2 = 1 MOhm, C 1 = C 2 = 1 µF, then the generated frequency
.
To get the same frequency in L.C.-generator, inductance would be required L= 10 16 Hn at WITH= 1 µF, which is difficult to implement.
IN R.C.- generators, it is possible by simultaneously changing the values of the capacitors WITH 1 and WITH 2, obtain a wider frequency tuning range than is the case in L.C.-generators. For L.C.-generators
while for R.C.- generators, with WITH 1 = WITH 2
To the disadvantages R.C.-generators should be attributed to the fact that at relatively high frequencies they are more difficult to implement than L.C.-generators. Indeed, the capacitance value cannot be reduced below the installation capacitance, and a decrease in resistor resistance leads to a drop in the gain, which makes it difficult to satisfy the amplitude self-excitation condition.
Listed advantages and disadvantages R.C.-generators led to their use in the low-frequency range with a large frequency overlap coefficient.
