Rc generator tutorial. Generators of sinusoidal and non-sinusoidal oscillations. RC and LC sinusoidal generators

Department of Internal and personnel policy Belgorod region

regional state autonomous

professional educational institution

"Belgorod Polytechnic College"

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

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

Completed:

Student of group No. 24ASU

Shekhovskoy Dmitry

Checked:

Rotaru T.A.

Belgorod, 2018

INTRODUCTION. 3

RC generators.. 4

Triggers.. 9

RS trigger. 11

D-triggers.. 13

JK trigger. 14

T-trigger. 15

Test questions: 16

List of Internet sources: 18

INTRODUCTION

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

The following are used as active elements:

– bipolar transistors,

– field effect transistors,

– Integrated op-amp.

RC generators include an amplifying element (amplifier) ​​and a link feedback(OS).

RC generators

The following types of OS links are distinguished:

− L-shaped OS links (Fig. 1),

− Wien Bridge (Fig. 2),

− double T-shaped bridge (Fig. 3).

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

Fig.1.1. L-shaped OS links

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

RC generators with L-shaped RC OS link

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Rice. 1.6. Schematic diagram of an RC oscillator

with asymmetrical double T-bridge

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

; ; .

The oscillation frequency is determined by the expression.

Triggers

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

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

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

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

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

Based on their functional purpose, triggers are classified:

RS triggers

D-triggers

· T-triggers

JK trigger

RS trigger

RS trigger

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

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

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

The operating principle is shown in the timing diagram:

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

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

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

Fig.2.3. Synchronized RS trigger

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

D-triggers

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

Rice. 3.1. D flip-flop circuit

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

Fig.3.2. D-trigger. A) schematic illustration b) time diagram of work

JK trigger

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

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

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

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

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

T-trigger

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

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

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

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

Security questions:

What are RC generators used for?

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

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

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

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

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

Structural generator circuit is shown in the figure below:

IE is a source of energy,

UE - amplifier,

POS - positive feedback circuit,

OOS - negative feedback circuit,

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

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

The principle of operation of a self-generator.

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

In this case, the following must be observed:

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

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

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

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

Circuits of LC harmonic generators.

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

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

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

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

Rice. 1a. Autogenerator operation. Transient processes.

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

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

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

Circuits of RC harmonic oscillation generators.

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

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

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

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

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

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

R.C.self-oscillator with matching stage and phase-shifting circuit

The main advantage of RC self-oscillators is the ability to generate stable low-frequency oscillations (up to 20 kHz). The disadvantage of such generators is that they are not economical compared to LC self-oscillators, since RC self-oscillators operate in a soft self-excitation mode.

In RC self-oscillators, RC filters are used to build a selective circuit. In the self-oscillator under consideration, a positive feedback circuit is built sequential connection several RC filters.

Let's consider the processes occurring in the RC filter shown in Figure 16, a. For clarity, we will explain the explanation using a vector diagram (Figure 16, b). When voltage Uin is applied to the input, current i flows in the circuit. This current creates a voltage drop across the capacitor U C and resistor U R. Voltage U R is also the output voltage Uout. Voltage Uout is in phase with current i, and voltage U C is shifted relative to Uout by 90°. The voltage at the input of the circuit is equal to the geometric sum of the vectors Uout and U C and corresponds to the vector Uin. Vectors Uin and Uout are shifted in phase relative to each other by an angle j.

Figure 16 - Schematic diagram of an RC filter and a vector diagram explaining the processes occurring in it.

Angle j can be increased by decreasing the capacitance of the capacitor. As can be seen from diagram j<90°. Поэтому для выполнения баланса фаз необходимо последовательное включение нескольких фильтров. При этом главным условием является равенство сдвига фаз каждым из фильтров, в противном случае каждый из фильтров будет иметь свою резонансную частоту, отличную от других фильтров и колебания будут отсутствовать. На практике используют последовательное включение трех фазосдвигающих звеньев, каждое из которых дает сдвиг фазы 60°, или четырех звеньев, каждое из которых дает сдвиг фазы 45°. На рисунке 17 приведены две возможные трехзвенные фазосдвигающие цепи. Временные диаграммы напряжений на выходе каждого звена этих цепей приведены на рисунке 18.

Figure 17 - Schematic diagrams of three-link phase-shifting circuits

The frequency of generated oscillations when using these circuits is determined by the expressions:

for the diagram shown in Figure 17, a

fg=0.065/R.C. (27)

Figure 18 - Time diagrams of voltages at the output of phase-shifting circuit links

for the diagram shown in Figure 17, b

fg=0.39/R.C. (28)

where R = R 1 = R 2 = R 3 and C = C 1 = C 2 = C 3

Thus, the filters in the generator under consideration perform several functions at once: they determine the frequency of the generated oscillations, determine the shape of the oscillations, and participate in the implementation of phase balance.

The circuit diagram of an RC self-oscillator with a matching stage and a phase-shifting circuit is shown in Figure 19.

In this generator, the amplifier stage is assembled using transistor VT1. The amplifier load is resistor R3. The three-link phase-shifting chain consists of elements C4 C5 C6 and R4 R5 R6. Is a matching stage used to match the low input resistance of transistor VT1 with the resistance of the phase-shifting circuit? emitter follower. This cascade is assembled on transistor VT2 connected according to a circuit with a common collector. In the absence of this cascade, the low input resistance of VT1 will bypass the feedback circuit and significantly reduce the feedback coefficient, and this

Figure 19 - Schematic diagram of an RC oscillator with a matching stage and a phase-shifting circuit

will lead to non-compliance with the amplitude balance condition. The emitter follower load is resistor R9. The bias voltage is supplied to the transistors by voltage dividers R1 R2 and R7 R8. Elements C1 R10 are a power filter. C2 C3 C7 are isolation capacitors. The feedback coefficient of such a generator is 1/29, therefore, in order to balance the amplitudes, the amplifier gain must be Kus? 29.

RC self-oscillator with phase-balance circuit

In generators with an even number of amplification stages, there is no need to use phase-shifting circuits in the positive feedback circuit. To isolate oscillations of the required frequency in the output voltage of such generators, a four-terminal network with frequency-selective properties (phase-balance circuit) is included in the feedback circuit. The schematic electrical diagram of such a four-terminal network is shown in Figure 20.

To generate oscillations, it is necessary that this four-terminal network does not introduce a phase shift between the input voltage Uin and the output voltage Uout, i.e. jin must be equal to jout. The frequency at which j in = j out is determined by the expression

Figure 20 - Schematic diagram of a frequency-selective quadripole

fg=1/2p ? R 1 C 1 R 2 C 2 (29)

It is convenient to choose R 1 =R 2 =R, C 1 =C 2 =C then expression 26 will take the form

fg=1/2p R.C. (30)

At all other frequencies, a phase shift will occur, which means that at these frequencies the phase balance condition will not be met and there will be no oscillations at these frequencies.

The feedback coefficient in this case will be equal to 1/3, and therefore, in order to balance the amplitudes, the gain of the oscillator amplifier must be at least 3.

The circuit diagram of an RC self-oscillator with a phase-balance circuit is shown in Figure 21.

Figure 21 - Schematic diagram of an RC oscillator with a phase-balance circuit

In this generator, the amplifier is assembled on two amplification stages assembled on transistors VT1 and VT2. The load of these stages is resistors R3 and R5. The bias voltage is supplied to the transistors by a fixed base current through resistors R2 and R4. Elements C1 R1 C2 R2 form a phase-balance circuit in the positive feedback circuit. Elements C4 C5 are isolation capacitors. R6 C3 power filter elements. The amplitude balance condition in this circuit is met by two amplification stages, with the help of which a gain of 3 is easily achieved. Phase balance is achieved by connecting two transistors according to a circuit with a common emitter (the total phase shift in this case is 180°+180°=360°) .

RC oscillator with Wien bridge

The advantage of this generator is the ability to change the frequency of the generated oscillations. The electrical circuit diagram of this generator is shown in Figure 22.

Figure 22 - Schematic diagram of an RC oscillator with a Wien bridge

In this generator, the amplifier also has two amplification stages assembled on transistors VT1 and VT2. The load of these stages is resistors R4 and R9. The bias voltage is supplied to the resistors through voltage dividers R2 R3 and R7 R8.

The output voltage is supplied to the amplifier input through the phase-balance circuit C1 R1 C2 R3, which is one of the arms of the Wien bridge, the second arm is formed by the elements R6 R5. The second branch is connected to the output of the amplifier through a large capacitor C5, due to which the circuit R5 R6 does not create a noticeable phase shift. Along with positive feedback, negative feedback is introduced formed by elements R5 R10 C5 R6. Negative feedback, by reducing the gain, significantly reduces the nonlinear distortions of the generated oscillations. Reducing the gain does not lead to an imbalance of amplitudes since a real two-stage amplifier has a gain much greater than 3. In addition, elements R5 R10 provide temperature stabilization of the operating point of the transistors. Adjustment of the frequency of generated oscillations in the generator under consideration is carried out by simultaneous adjustment of the resistances of resistors R1 R3, however, it can also be carried out by simultaneous adjustment of the capacitances of capacitors C1 C2.

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 are an integral part of 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. According to 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;

possible smooth adjustment amplitudes of stationary oscillations by changing the value of the feedback coefficient of the CBS;

as a disadvantage it should be noted great value 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 at different values ​​of the feedback coefficient of the 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 oscillation amplitude 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 transistor transitions to ensure the active mode of its operation;

capacitor CP separates the generator and its DC load;

blocking capacitors SB1 and SB2 bypass power supplies according to alternating current, excluding 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 generators using an oscillatory circuit as a frequency-dependent 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 fixed 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 feedback in 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 according to 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 either identical transistors or transistors with different types of electrical conductivity in one cascade. Cascades that use transistors with different types of electrical conductivity (p-n-p and n-p-n) are called cascades with additional symmetry.

Based on 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 the 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 the 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 using 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 (implementation of a circuit with single-polar power supply is possible). When using complementary pairs of transistors in this circuit types n-p-n 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 MIS 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 low;

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 - amplitude value voltage across 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.

This device can be used as an RC oscillator with a powerful output stage, generation frequency output stage power, load resistance electronic device on ... generator: - R.C.- external R.C. generator- INTOSC - internal R.C. generator ...