DIY circuit diagrams of functional generators. Abstract:Sawtooth voltage generator. Generator Load Requirements

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

Visual illustration of division frequency range

Classification

Transistor generators have several classifications:

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

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

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

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

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

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

According to the operating principle of generators:

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

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

Generator circuitry

RC and LC sinusoidal generators

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

Transistor oscillator circuit (Colpitts oscillator)

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

Three-point inductively coupled generator circuit (Hartley generator)

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

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

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

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

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

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

Pierce generator circuit without the use of inductance

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

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

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

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

RC generator circuit with phase-shifting chain

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

RC generator circuit with Wien bridge

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

Functional current generators

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

Multivibrator circuit - rectangular pulse generator

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

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

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

Triangular pulse generator circuit

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

Blocking generator

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

Generators based on field-effect transistors

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

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

Noise generators

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

Video

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

RAMP VOLTAGE GENERATOR- generator of linearly varying voltage (current), an electronic device that generates periodic voltage (current) fluctuations in a sawtooth shape. Basic The purpose of gpn is to control the time sweep of the beam in devices using cathode ray tubes. G.p.n. They are also used in devices for comparing voltages, time delays and pulse expansion. To obtain a sawtooth voltage, the process of charging (discharging) a capacitor in a circuit with a large time constant is used. The simplest G. p.n. (Fig. 1, a) consists of RC integrating circuit and a transistor that performs key functions, controlled periodically. impulses. In the absence of pulses, the transistor is saturated (open) and has a low resistance of the collector - emitter, capacitor section WITH discharged (Fig. 1, b). When a switching pulse is applied, the transistor is turned off and the capacitor is charged from a power source with voltage - E k- direct (working) stroke. Output voltage G.p.n., removed from the capacitor WITH, changes by law. WITH At the end of the switching pulse, the transistor is unlocked and the capacitor

quickly discharges (reverse) through low resistance emitter - collector. Basic characteristics of G.p.n.: amplitude of sawtooth voltage, coefficient. nonlinearity and coefficient using power supply voltage. When in this scheme Duration of forward stroke T

p and the frequency of the sawtooth voltage are determined by the duration and frequency of the switching pulses. The disadvantage of the simplest G. p.n. is small k E at low . In G. p.n. with positive By voltage feedback, the output sawtooth voltage is supplied to the charging circuit as a compensating emf. In this case, the charging current is almost constant, which provides values ​​of 1 and = 0.0140.02. G.p.n. used for scanning in cathode ray tubes with electric magnets. beam deflection. To obtain a linear deflection, a linear change in the current in the deflection coils is necessary. For a simplified equivalent coil circuit (Fig. 2, a), the current linearity condition is satisfied when a trapezoidal voltage is applied to the coil terminals. This trapezoidal stress (Fig. 2, b) can be obtained from the State University of Science. when connected to the charging circuit it will supplement. resistance R d (shown in Fig. 1,

A

dotted line). The deflection coils consume large currents, so the trapezoidal voltage generator is supplemented with a power amplifier. Here is a selection of materials: The use of transistor analogues of a dinistor in relaxation generators is typical, since strictly defined parameters of the dinistor are required for the calculation and accurate operation of this generator. Some of these parameters for industrial dinistors either have a large technological spread or are not standardized at all. And make an analogue with strictly

given parameters

is not difficult.

The sawtooth signal shown above is shown. The recovery time is always less than the sweep time. A sawtooth signal is produced when the return time becomes zero. The sweep speed of sawtooth waves depends on the capacitor used in the circuit. The sweep speed is controlled by a resistor placed in the circuit.

The charging and discharging of the capacitor generates the signal shown in the figure below. The transistor provides a low resistance through which the capacitor becomes a discharge. Instantaneous voltage and supply voltage are measured in volts, time is measured in the latter, resistance is measured in ohms, and a capacitor is measured in farads.

Ramp voltage generator circuit The relaxation generator looks like this: (A1)- relaxation generator based on a diode thyristor (dinistor),

The term "sawtooth" refers to the waveform and can therefore have any rise or fall time as long as the waveform maintains the basic saw blade shape. Pilot generator. is a circuit that generates a saw blade signal either from an external input or from self-oscillations, as in a relaxation oscillator. A circuit designed to produce a sawtooth function will have a very slow linear ramp that rises from a steady state to a peak. When the peak voltage of the ramp is reached, the voltage will return to the initial level very quickly.

Resistor R5 selected small (20 - 30 Ohms). It is designed to limit the current through the dinistor or transistors at the moment they open. In the calculations, we will neglect the influence of this resistor and assume that the voltage across it practically does not drop, and the capacitor through it is discharged instantly.

The dinistor parameters used in the calculations are described in the article volt-ampere characteristics dinistor.

Operation of a unipolar transistor circuit

The falling time is much shorter than the rising time, but is not instantaneous, although it looks the same compared to the rising time. Fall time is also referred to as flyback when the signal is used as a sweep generator. The circuit functions as an oscillator and switches off the charging and discharging of the capacitor. Of course, you can also make the frequency variable by adding a trimmer as the current setting. The top side of the trimmer remains connected to the supply voltage. While the other end of the trimmer remains unconnected as in the configuration.

[Minimum output voltage, V] =

[Maximum output voltage, V] =

Calculation of the resistance of resistor R4

For resistor R4, two relationships must be met:

[Resistance R4, kOhm] > 1.1 * ([Supply voltage, V] - [Dinistor turn-off voltage, V]) / [Holding current, mA]

This is necessary so that the dinistor or its analogue is securely locked when the capacitor is discharged.

This charging time is the increasing ramp of the sawtooth shaft, as well as the sweep time in specific applications. The ramp time depends on the resistor and capacitor values. Fall time is the time required for the capacitor to discharge through the transistor. The vacuum tube circuit on the right is another example of a circuit that outputs a sawtooth waveform. This circuit was used as a sweep generator in an oscilloscope or other display. The ramp or sweep portion of the output is used to move the electron beam from left to right across the display, while the retrace or flyback portion returns the beam to its starting point.

[Resistance R4, kOhm] Supply voltage, V] - [ Dinistor unlocking voltage, V]) / (1.1 * [Release current, mA])

This is necessary so that the capacitor can be charged to the voltage required to unlock the dinistor or its equivalent.

The coefficient of 1.1 was chosen conditionally out of the desire to obtain a 10% margin.

If these two conditions conflict with each other, then this means that the circuit supply voltage for this thyristor is selected too low.

This circuit is used as an example to show the vacuum tube used as a sawtooth generator and the second method of changing the sweep time. A switch is used to change the sweep time, just as a variable resistor is used in the circuit above it.

This is a measure of time based on the amount of voltage change. Another important consideration is the use of the linear part of the rise time of the capacitors. Only the first time the constant is a linear ramp or some linear one. As the capacitor is able to charge further, the charging time slows down more and more. Of course, the saw ramp is linear in its rise time. The same applies to the capacitor discharge time. The longer the discharge time, the smaller the linear discharge will be.

Calculation of the relaxation oscillator frequency

The frequency of the generator can be approximately estimated from the following considerations. The oscillation period is equal to the sum of the capacitor charging time to the dinistor unlocking voltage and the discharge time. We agreed to assume that the capacitor discharges instantly. So we need to estimate the charging time.

Could you show me how to make a variable frequency sawtooth oscillator? A sawtooth wave is characterized by a positive linear voltage reversal accompanied by a sharp drop to zero. One way to generate a sawtooth surface is to slowly charge a capacitor through a DC source, and then quickly discharge the capacitor, shorting it.

By repeating this process, a sawtooth wave is created. But DC supplies can be tricky, especially if you want to customize it. Instead of a constant current source, a fixed resistor is often used to limit the cap's charging current. However, the voltage across the charging capacitor using a fixed resistor is not linear. But, choosing a section of the curve that is more or less linear, as shown in red dotted lines, we can create a pseudopilos. The 555 timer is an astable oscillator that uses charging and discharging a capacitor.

Second option: R1- 1 kOhm, R2, R3- 200 Ohm, R4- trimmer 3 kOhm (set to 2.5 kOhm), Supply voltage- 12 V. Transistors- KT502, KT503.

Generator Load Requirements

The above relaxation generators can operate with a load that has a high input resistance so that the output current does not affect the charging and discharging process of the capacitor.

Not perfect, but good enough for most electronics. The waveform is then buffered and conditioned. The frequency bank changes the frequency, and the waveform control adjusts the wave so that the top and bottom of the waveform are not clipped.

A more linear ramp wave can be generated using a digital counter with weighted outputs. Look at the sawtooth generator in Figure 3. Does it look like number 3? These currents are summed at the non-inverting op-amp and output node as a voltage.

[Load resistance, kOhm] >> [Resistor R4 resistance, kOhm]

In this case, at the output of the circuit there is a constant voltage level equal to
,
,
At a moment in time

. (7)

the transistor turns off and the capacitor begins to charge. The processes occurring in the circuit are described by the following equations From (6) we obtain Let us introduce the notation

. (8)

, then the resulting equation can be rewritten in the form
It's patchy
, therefore, (8) can be written as

.

Then the output voltage will change according to the law

(9)

Here
has the same meaning as before.

Since the voltage at the system output after the operating stroke time must be equal to the value
, Where
is the amplitude of the sawtooth voltage, then, solving (9) with respect to time, we obtain

. (10)

Similarly for the discharge circuit, taking into account that
And
.

2. Calculation of the scheme.



For the circuit to operate correctly, the gain at the inverting input must be greater than unity.
Let

, choose resistor R2 with a nominal value of 20 kOhm, then R1 = 10 kOhm.

Let's calculate the gain for the non-inverting input.

3. 

   It is required to ensure a nonlinearity coefficient of 0.3%, then the time constant for charging the capacitor must be no less than

4. ,

   Then the output voltage will change according to the law:
   So if you ask
   = 1067

   B, then

   then K = = = 0.014, provided the supply voltage in the transistor circuit is 15 V.

   .

   Taking into account the previously obtained notation, we calculate the resistance ratio of resistances R3 and R4

   Let's set the resistance in the collector circuit of the transistor R3 = 10 kOhm, then we get that R4 = 20 kOhm.

   In turn, c, therefore, the capacitance of the capacitor will be about 224 pF, choose 220 pF.

   . (13)

   Let's move on to calculating the discharge circuit. For the discharge circuit it is true

   .

   Let us substitute the formulas from (11) into (13), resolve with respect to R6, and obtain

   Whence it follows, when substituting numerical values, that R6 = 2 mOhm.

   , (11)

   Where
   ,
   ,
   .

   We obtain an expression for the return time

   If expression (9) is differentiated by time and multiplied by C1, then the voltage nonlinearity coefficient will be determined by the formula t p / ,Where

   =RC

   Based on the research carried out, let’s move on to calculating parameters and selecting circuit elements.
   We will estimate the current flowing at the moment when the transistor opens through resistance R6 based on the following reasoning.

   As a key, you can use a transistor with suitable parameters like KT342B. Resistor R5, which limits the base current, will be about 1 kOhm. Since the maximum collector current is 50 mA, and the current gain is 200, the base saturation current will be equal to 250 μA, therefore the voltage across the resistor will be 0.25 V. Let us take the base-emitter saturation voltage - 1 V. The voltage drop across the resistance R6, at the maximum current flowing through R3 and R4 added to R6 will be 6.08 V. Thus, to reliably unlock the transistor and keep it open, a pulse with an amplitude of 8 V is required.

The principle of operation of the relaxation generator is based on the fact that the capacitor is charged to a certain voltage through a resistor. Upon reaching required voltage the control element opens. The capacitor is discharged through another resistor to a voltage at which the control element closes. So the voltage on the capacitor increases according to an exponential law, then decreases according to an exponential law.

You can read more about how a capacitor is charged and discharged through a resistor by following the link.

A The use of transistor analogues of a dinistor in relaxation generators is typical, since strictly defined parameters of the dinistor are required for the calculation and accurate operation of this generator. Some of these parameters for industrial dinistors either have a large technological spread or are not standardized at all. And making an analogue with strictly specified parameters is not difficult. Ramp voltage generator circuit The charging and discharging of the capacitor generates the signal shown in the figure below. The transistor provides a low resistance through which the capacitor becomes a discharge. Instantaneous voltage and supply voltage are measured in volts, time is measured in the latter, resistance is measured in ohms, and a capacitor is measured in farads. Ramp voltage generator circuit The relaxation generator looks like this: (A1)- in circuit A1 the dinistor is replaced with a transistor analogue. You can calculate the parameters of the transistor analog depending on the transistors used and resistor values. Resistor R5 selected small (20 - 30 Ohms). It is designed to limit the current through the dinistor or transistors at the moment they open. In the calculations, we will neglect the influence of this resistor and assume that the voltage across it practically does not drop, and the capacitor through it is discharged instantly. The dinistor parameters used in the calculations are described in the article Volt-ampere characteristics of the dinistor. [Minimum output voltage, V] = [Maximum output voltage, V] = Calculation of the resistance of resistor R4 For resistor R4, two relationships must be met: [Resistance R4, kOhm] > 1.1 * ([Supply voltage, V] - [Dinistor turn-off voltage, V]) / [Holding current, mA] This is necessary so that the dinistor or its analogue is securely locked when the capacitor is discharged. [Resistance R4, kOhm] Supply voltage, V] - [ Dinistor unlocking voltage, V]) / (1.1 * [Release current, mA]) This is necessary so that the capacitor can be charged to the voltage required to unlock the dinistor or its equivalent. The coefficient of 1.1 was chosen conditionally out of the desire to obtain a 10% margin. If these two conditions conflict with each other, then this means that too much has been chosen. low voltage supply circuit for this thyristor. Calculation of the relaxation oscillator frequency The frequency of the generator can be approximately estimated from the following considerations. The oscillation period is equal to the sum of the capacitor charging time to the dinistor unlocking voltage and the discharge time. We agreed to assume that the capacitor discharges instantly. So we need to estimate the charging time. Second option: R1- 1 kOhm, R2, R3- 200 Ohm, R4- trimmer 3 kOhm (set to 2.5 kOhm), Supply voltage- 12 V. Transistors- KT502, KT503. Generator Load Requirements The above relaxation generators can operate with a load that has a high input resistance so that the output current does not affect the charging and discharging process of the capacitor. [Load resistance, kOhm] >> [Resistor R4 resistance, kOhm]