The principle of operation of the multivibrator circuit on field-effect transistors. MOSFET: simple designs. FET generators

circuit diagram powerful transistor multivibrator with control, built on transistors KT972, KT973. Many radio amateurs began their creative journey by assembling simple direct-amplification radios, simple audio power amplifiers, and assembling simple multivibrators consisting of a pair of transistors, two or four resistors, and two capacitors.

The traditional symmetrical multivibrator has a number of disadvantages, including a relatively high output impedance, long pulse edges, limited supply voltage, and low efficiency when operating on a low-resistance load.

circuit diagram

On fig. 1. shows a diagram of a controlled symmetrical two-phase multivibrator operating at audio frequencies, the load to which is connected via a bridge circuit the load would be included in one of the arms of the multivibrator.

In addition, a “real” voltage is applied to the load alternating current, which significantly improves the working conditions of the dynamic head connected as a load - there is no effect of indentation or protrusion of the diffuser (depending on the polarity of the speaker). There are also no clicks when turning the multivibrator on or off.

Rice. 1. Schematic diagram of a powerful multivibrator based on transistors KT972, KT973.

A symmetrical two-phase multivibrator consists of two push-pull arms, the voltage on which alternately changes from low level to high. Assume that when the power is turned on, the composite transistor VT2 opens first.

Then the voltage at the terminals of the collectors of transistors VT1, VT2 will become close to zero (VT1 is open, VT2 is closed) composite p-p-p transistor VT5, which will open. A voltage of about 8 V will be applied to the load at a multivibrator supply voltage of 9 V. With the recharge of capacitors C2, C4, the multivibrator will switch - VT1, VT6 will open, VT2, VT5 will close.

The same voltage will be applied to the load, but in reverse polarity. The switching frequency of the multivibrator depends on the capacitance of the capacitors C2, C4, and, to a lesser extent, on the set resistance of the tuning resistor R7. With a supply voltage of 9 V, the frequency can be tuned from 1.4 to 1.5 kHz.

When the resistance R7 decreases below the conditional value, the generation of sound frequencies breaks down. It should be noted that after starting the multivibrator can work without resistors R5, R11. The voltage shape at the output of the multivibrator is close to rectangular.

Resistors R6, R8 and diodes VD1, VD2 protect the emitter junctions of transistors VT2, VT6 from breakdown, which is especially important when the multivibrator supply voltage is more than 10V. Resistors R1, R13 are necessary for stable generation; in their absence, the multivibrator may “wheeze”. The VD3 diode protects powerful transistors from supply voltage reversal. If it is absent and if the power supply is sufficient, when the voltage is reversed, the built-in protective diodes of the transistors may be damaged.

To expand functionality of this multivibrator, it has the ability to turn on / off when a voltage of positive polarity is applied to the control input. If the control input is not connected anywhere or the voltage on it is not more than 0.5 V, the VТЗ, VT4 transistors are closed, the multivibrator is working.

When voltage is applied to the control input high level, for example, from the output of TTLSH. CMOS microcircuits, a sensor of electrical or non-electrical quantities, for example, a humidity sensor, transistors VTZ, VT4 open, the multivibrator slows down. In this state, the multivibrator consumes less than 200 µA of current, excluding the current through R2, R3, R9.

Details and installation

The multivibrator can be mounted on a printed circuit board with dimensions of 70 * 50 mm, a sketch of which is shown in fig. 2 Fixed resistors can be used any small. Trimmer resistor RP1-63M, SP4-1 or similar imported. Oxide capacitors K50-29, K50-35 or analogues Capacitors C2, C4 - K73-9, K73-17, K73-24 or any small film capacitors.

Rice. 2. Printed circuit board for a powerful multivibrator circuit on transistors.

Diodes KD522A can be replaced by KD503. KD521. D223 with any letter index or imported 1N914, 1N4148. Instead of diodes KD226A and KD243A, any of the series KD226, KD257, KD258, 1 N5401 ... 1 N5407 is suitable.

Composite transistors KT972A can be replaced by any of this series or from the KT8131 series, and instead of KT973 by any of the KT973, KT8130 series. If necessary, powerful transistors are installed on small heat sinks. In the absence of such transistors, they can be replaced by analogues of two transistors connected according to the Darlington circuit, Fig. 3. Instead of low-power p-p-p transistors KT315G will fit any of the KT312, KT315, KT342, KT3102, KT645, SS9014 and similar series.

Rice. 3. Schematic diagram of the equivalent replacement of transistors KT972, KT973.

The load of this multivibrator can be a dynamic head, a telephone capsule, a piezoceramic sound emitter, a pulse step-up / step-down transformer.

When using a driver with an 8 ohm winding impedance, be aware that with a supply voltage of 9 V, 8 watts of AC power will be supplied to the load. Therefore, a two ... four-watt dynamic head can be damaged after 1 ... 2 minutes of operation.

Establishment

The operating frequency of the multivibrator is significantly affected by the load capacitance and supply voltage. For example, when the supply voltage changes from 5 to 15 V, the frequency changes from 2850 to 1200 Hz when working on a multivibrator for a load in the form of a telephone capsule with a winding resistance of 56 ohms. In the region of low supply voltages, the change in the operating frequency is more significant

By selecting the resistances of resistors R5, R11, R6, R8, you can set the shape of the pulses to be almost strictly rectangular when the multivibrator operates with a specific connected load at a given supply voltage.

This multivibrator can be used in various signaling devices, sound warning devices, when, with a small available voltage of the power source, it is required to obtain significant power at the sound emitter. In addition, it is convenient to use it in low-voltage to high-voltage converters, including those operating at a low frequency of the lighting network of 50 Hz.

Butov A. L. RK-2010-04.

In this article we will talk about the multivibrator, how it works, how to connect the load to the multivibrator and the calculation of a transistor symmetrical multivibrator.

multivibrator is a simple generator rectangular pulses, which works in autogenerator mode. It only needs battery power or other power source to operate. Consider the simplest symmetrical transistor multivibrator. Its scheme is shown in the figure. The multivibrator can be complicated depending on the required functions to be performed, but all the elements shown in the figure are mandatory, without them the multivibrator will not work.

The operation of a symmetrical multivibrator is based on the charge-discharge processes of capacitors, which together with resistors form RC chains.

I wrote about how RC chains work earlier in my article Capacitor, which you can read on my website. On the Internet, if you find material about a symmetrical multivibrator, then it is presented briefly and not intelligibly. This circumstance does not allow novice radio amateurs to understand anything, but only helps experienced electronic engineers to remember something. At the request of one of the visitors to my site, I decided to eliminate this gap.

How does a multivibrator work?

At the initial moment of power supply, the capacitors C1 and C2 are discharged, so their current resistance is small. The low resistance of the capacitors leads to the fact that there is a "fast" opening of the transistors, caused by the flow of current:

- VT2 along the way (shown in red): "+ power supply> resistor R1> low resistance of discharged C1> base-emitter junction VT2> - power supply";

- VT1 along the way (shown in blue): "+ power supply> resistor R4> low resistance of discharged C2> base-emitter junction VT1> - power supply".

This is the "unsteady" mode of operation of the multivibrator. It lasts for a very short time, determined only by the speed of the transistors. And two absolutely identical transistors do not exist. Which transistor opens faster, that one will remain open - the "winner". Suppose that in our diagram it turned out to be VT2. Then, through the low resistance of the discharged capacitor C2 and the low resistance of the collector-emitter junction VT2, the base of the transistor VT1 will be closed to the emitter VT1. As a result, the transistor VT1 will be forced to close - "become defeated."

Since the transistor VT1 is closed, there is a “fast” charge of the capacitor C1 along the path: “+ power source> resistor R1> low resistance of the discharged C1> base-emitter junction VT2> - power source”. This charge occurs almost up to the voltage of the power supply.

At the same time, the capacitor C2 is charged with a current of reverse polarity along the path: “+ power source> resistor R3> low resistance of the discharged C2> collector-emitter junction VT2> - power source”. The duration of the charge is determined by the values ​​of R3 and C2. They determine the time at which VT1 is in the closed state.

When the capacitor C2 is charged to a voltage approximately equal to a voltage of 0.7-1.0 volts, its resistance will increase and the transistor VT1 will open with the voltage applied along the path: “+ power supply> resistor R3> base-emitter junction VT1> - power source". In this case, the voltage of the charged capacitor C1, through the open collector-emitter junction VT1, will be applied to the emitter-base junction of the transistor VT2 with reverse polarity. As a result, VT2 will close, and the current that previously passed through the open collector-emitter junction VT2 will run through the circuit: “+ power supply> resistor R4> low resistance C2> base-emitter junction VT1> - power source”. This circuit will quickly recharge the capacitor C2. From this moment, the "steady" mode of autogeneration begins.

The operation of a symmetrical multivibrator in the "steady" generation mode

The first half-cycle of operation (oscillation) of the multivibrator begins.

With the transistor VT1 open and VT2 closed, as I just wrote, capacitor C2 is quickly recharged (from a voltage of 0.7 ... 1.0 volts of one polarity to the power supply voltage of the opposite polarity) along the circuit: “+ power supply> resistor R4 > low resistance C2 > base-emitter junction VT1 > - power supply. In addition, the capacitor C1 is slowly recharged (from the voltage of the power supply of one polarity to a voltage of 0.7 ... 1.0 volts of the opposite polarity) along the circuit: “+ power supply> resistor R2> right plate C1> left plate C1> collector- emitter junction of the transistor VT1> - power supply".

When, as a result of overcharging C1, the voltage at the base of VT2 reaches a value of +0.6 volts relative to the emitter of VT2, the transistor will open. Therefore, the voltage of the charged capacitor C2, through the open collector-emitter junction VT2, will be applied to the emitter-base junction of the transistor VT1 with reverse polarity. VT1 will close.

The second half-cycle of operation (oscillation) of the multivibrator begins.

When the transistor VT2 is open and VT1 is closed, the capacitor C1 is quickly recharged (from a voltage of 0.7 ... 1.0 volts of one polarity to the power supply voltage of the opposite polarity) along the circuit: “+ power supply> resistor R1> low resistance C1> base- emitter junction VT2 > - power supply". In addition, there is a slow recharge of the capacitor C2 (from the voltage of the power supply of one polarity, to a voltage of 0.7 ... 1.0 volts of the opposite polarity) along the circuit: “right plate C2> collector-emitter junction of the transistor VT2> - power supply> + source power > resistor R3 > left plate C2. When the voltage at the base of VT1 reaches +0.6 volts relative to the emitter of VT1, the transistor will open. Therefore, the voltage of the charged capacitor C1, through the open collector-emitter junction VT1, will be applied to the emitter-base junction of the transistor VT2 with reverse polarity. VT2 will close. On this, the second half-cycle of the multivibrator oscillation ends, and the first half-cycle begins again.

The process is repeated until the multivibrator is disconnected from the power source.

Ways to connect the load to a symmetrical multivibrator

Rectangular pulses are taken from two points of a symmetrical multivibrator- collectors of transistors. When there is a “high” potential on one collector, then there is a “low” potential on the other collector (it is absent), and vice versa - when there is a “low” potential on one output, then “high” on the other. This is clearly shown in the timeline below.

The multivibrator load must be connected in parallel with one of the collector resistors, but in no case in parallel with the collector-emitter transistor junction. You can not shunt the transistor with a load. If this condition is not met, then at least the duration of the pulses will change, and as a maximum, the multivibrator will not work. The figure below shows how to connect the load correctly, and how not to do it.

In order for the load not to affect the multivibrator itself, it must have sufficient input impedance. For this, buffer transistor stages are usually used.

The example shows connecting a low-resistance dynamic head to a multivibrator. An additional resistor increases the input resistance of the buffer stage, and thereby eliminates the influence of the buffer stage on the multivibrator transistor. Its value must be at least 10 times the value of the collector resistor. Connecting two transistors in a "composite transistor" scheme greatly increases the output current. In this case, it is correct to connect the base-emitter circuit of the buffer stage in parallel with the collector resistor of the multivibrator, and not in parallel with the collector-emitter junction of the multivibrator transistor.

For connecting a high-impedance dynamic head to a multivibrator buffer stage is not needed. The head is connected instead of one of the collector resistors. The only condition that must be met is that the current flowing through the dynamic head must not exceed the maximum collector current of the transistor.

If you want to connect ordinary LEDs to the multivibrator- to make a flasher, then buffer cascades are not required for this. They can be connected in series with collector resistors. This is due to the fact that the current of the LED is small, and the voltage drop across it during operation is not more than one volt. Therefore, they do not have any effect on the operation of the multivibrator. True, this does not apply to super-bright LEDs, in which the operating current is higher and the voltage drop can be from 3.5 to 10 volts. But in this case, there is a way out - to increase the supply voltage and use transistors with big power providing sufficient collector current.

Please note that oxide (electrolytic) capacitors are connected with pluses to the collectors of transistors. This is due to the fact that on the bases of bipolar transistors, the voltage does not rise above 0.7 volts relative to the emitter, and in our case, emitters are a minus of power. But on the collectors of transistors, the voltage changes almost from zero to the voltage of the power source. Oxide capacitors are not able to perform their function when they are connected with reverse polarity. Naturally, if you use transistors of a different structure (not N-P-N, a P-N-P structures), then in addition to changing the polarity of the power source, it is necessary to turn the LEDs with cathodes "up the circuit", and the capacitors - pluses to the bases of the transistors.

Let's figure it out now what parameters of the multivibrator elements set the output currents and generation frequency of the multivibrator?

What are the collector resistor values? I have seen in some incompetent Internet articles that the values ​​​​of the collector resistors are insignificant, but they affect the frequency of the multivibrator. All this is complete nonsense! With the correct calculation of the multivibrator, the deviation of the values ​​\u200b\u200bof these resistors by more than five times from the calculated one will not change the frequency of the multivibrator. The main thing is that their resistance should be less than the base resistors, because the collector resistors provide a fast charge of the capacitors. But on the other hand, the values ​​​​of the collector resistors are the main ones for calculating the power consumption from the power source, the value of which should not exceed the power of the transistors. If you figure it out, then correct connection they do not even directly affect the output power of the multivibrator. But the duration between switching (multivibrator frequency) is determined by the "slow" recharge of the capacitors. The recharge time is determined by the values ​​of RC chains - basic resistors and capacitors (R2C1 and R3C2).

The multivibrator, although it is called symmetrical, refers only to the circuitry of its construction, and it can produce both symmetrical and non-symmetrical output pulses. The duration of the pulse (high level) on the VT1 collector is determined by the values ​​of R3 and C2, and the duration of the pulse (high level) on the VT2 collector is determined by the values ​​of R2 and C1.

The duration of the recharge of capacitors is determined by a simple formula, where Tau is the pulse duration in seconds, R is the resistance of the resistor in ohms, FROM is the capacitance of the capacitor in Farads:

Thus, if you have not already forgotten what was written in this article a couple of paragraphs earlier:

If equal R2=R3 and C1=C2, at the outputs of the multivibrator there will be a “meander” - rectangular pulses with a duration equal to the pauses between the pulses, which you see in the figure.

The total period of oscillation of the multivibrator is T is equal to the sum of the pulse and pause durations:

Oscillation frequency F(Hz) related to period T(sec) through the ratio:

As a rule, if there are any calculations of radio circuits on the Internet, they are scarce. That's why we will calculate the elements of a symmetrical multivibrator using an example .

Like any transistor cascades, the calculation must be carried out from the end - the output. And at the output we have a buffer stage, then there are collector resistors. Collector resistors R1 and R4 perform the function of loading transistors. Collector resistors have no effect on the generation frequency. They are calculated based on the parameters of the selected transistors. Thus, we first calculate the collector resistors, then the base resistors, then the capacitors, and then the buffer stage.

The order and example of calculating a transistor symmetrical multivibrator

Initial data:

Supply voltage Ui.p. = 12 V.

Required multivibrator frequency F = 0.2 Hz (T = 5 seconds), and the pulse duration is equal to 1 (one) second.

An incandescent car light bulb is used as a load. 12 volts, 15 watts.

As you guessed, we will calculate the flasher, which will flash once every five seconds, and the duration of the glow will be 1 second.

Choosing transistors for the multivibrator. For example, we have the most common transistors in Soviet times KT315G.

For them: Pmax=150 mW; Imax=150 mA; h21>50.

Transistors for the buffer stage are selected based on the load current.

In order not to depict the circuit twice, I have already signed the values ​​​​of the elements on the diagram. Their calculation is given later in the Decision.

Solution:

1. First of all, it is necessary to understand that the operation of a transistor at high currents in the key mode is the safest for the transistor itself than operation in the amplifying mode. Therefore, there is no need to calculate the power for the transition state at the moments of the passage of an alternating signal, through the operating point "B" of the static mode of the transistor - the transition from the open state to the closed state and vice versa. For pulse circuits built on bipolar transistors, usually calculate the power for transistors in the open state.

First, we determine the maximum power dissipation of the transistors, which should be a value that is 20 percent less (a factor of 0.8) than the maximum power of the transistor indicated in the reference book. But why should we drive the multivibrator into a rigid frame of high currents? yes and from increased power the energy consumption from the power supply will be large, and the benefit will be small. Therefore, having determined the maximum power dissipation of transistors, we will reduce it by 3 times. A further reduction in dissipated power is undesirable because the operation of a multivibrator on bipolar transistors in the low current mode is an “unstable” phenomenon. If the power supply is used not only for the multivibrator, or it is not quite stable, the frequency of the multivibrator will also “float”.

Determine the maximum power dissipation: Pras.max = 0.8 * Pmax = 0.8 * 150mW = 120mW

We determine the rated power dissipation: Pras.nom. = 120 / 3 = 40mW

2. Determine the collector current in the open state: Ik0 = Pras.nom. / Ui.p. = 40mW / 12V = 3.3mA

Let's take it as the maximum collector current.

3. Find the value of the resistance and power of the collector load: Rk.total = Ui.p. / Ik0 = 12V / 3.3mA = 3.6 kOhm

We select resistors as close as possible to 3.6 kOhm in the existing nominal range. In the nominal series of resistors there is a nominal value of 3.6 kOhm, therefore, we first consider the value of the collector resistors R1 and R4 of the multivibrator: Rk \u003d R1 \u003d R4 \u003d 3.6 kOhm.

The power of the collector resistors R1 and R4 is equal to the rated power dissipation of the transistors Pras.nom. = 40 mW. We use resistors with a power exceeding the specified Pras.nom. - MLT-0.125 type.

4. Let's proceed to the calculation of the basic resistors R2 and R3. Their value is found based on the gain of the transistors h21. At the same time, for reliable operation of the multivibrator, the resistance value must be within: 5 times the resistance of the collector resistors, and less than the product Rk * h21. In our case Rmin \u003d 3.6 * 5 \u003d 18 kOhm, and Rmax \u003d 3.6 * 50 \u003d 180 kOhm

Thus, the resistance values ​​Rb (R2 and R3) can be in the range of 18...180 kOhm. We pre-select the average value = 100 kOhm. But it is not final, since we need to provide the required frequency of the multivibrator, and as I wrote earlier, the frequency of the multivibrator directly depends on the base resistors R2 and R3, as well as on the capacitance of the capacitors.

5. Calculate the capacitances of capacitors C1 and C2 and, if necessary, recalculate the values ​​of R2 and R3.

The values ​​of the capacitance of the capacitor C1 and the resistance of the resistor R2 determine the duration of the output pulse on the collector VT2. It is during the action of this pulse that our light bulb should light up. And in the condition, the pulse duration was set to 1 second.

determine the capacitance of the capacitor: C1 \u003d 1 sec / 100kOhm \u003d 10 uF

A capacitor with a capacity of 10 microfarads is available in the nominal range, so it suits us.

The values ​​of the capacitance of the capacitor C2 and the resistance of the resistor R3 determine the duration of the output pulse on the collector VT1. It is during the action of this pulse that a "pause" operates on the VT2 collector and our light should not light up. And in the condition, a full period of 5 seconds was set with a pulse duration of 1 second. Therefore, the duration of the pause is 5 seconds - 1 second = 4 seconds.

By transforming the recharge duration formula, we determine the capacitance of the capacitor: C2 \u003d 4sec / 100kOhm \u003d 40 uF

A 40 uF capacitor is not in the nominal series, so it does not suit us, and we will take a 47 uF capacitor as close as possible to it. But as you understand, the “pause” time will also change. To prevent this from happening, we recalculate the resistance of the resistor R3 based on the duration of the pause and the capacitance of the capacitor C2: R3 = 4sec / 47uF = 85kΩ

According to the nominal series, the nearest value of the resistance of the resistor is 82 kOhm.

So, we got the values ​​​​of the elements of the multivibrator:

R1 = 3.6 kΩ, R2 = 100 kΩ, R3 = 82 kΩ, R4 = 3.6 kΩ, C1 = 10 uF, C2 = 47 uF.

6. Calculate the value of the resistor R5 of the buffer stage.

The resistance of the additional limiting resistor R5 to eliminate the influence on the multivibrator is selected at least 2 times the resistance of the collector resistor R4 (and in some cases more). Its resistance, together with the resistance of the emitter-base junctions VT3 and VT4, in this case will not affect the parameters of the multivibrator.

R5 = R4 * 2 = 3.6 * 2 = 7.2 kΩ

According to the nominal series, the nearest resistor is 7.5 kOhm.

With the value of the resistor R5 = 7.5 kOhm, the buffer stage control current will be equal to:

I ex. \u003d (Ui.p. - Ube) / R5 \u003d (12v - 1.2v) / 7.5 kOhm \u003d 1.44 mA

In addition, as I wrote earlier, the value of the collector load of the multivibrator transistors does not affect its frequency, so if you do not have such a resistor, then you can replace it with another "close" value (5 ... 9 kOhm). It is better if this is in the direction of decreasing, so that there is no drop in the control current at the buffer stage. But keep in mind that the additional resistor is an additional load on the VT2 transistor of the multivibrator, so the current flowing through this resistor adds up to the current of the collector resistor R4 and is a load for the VT2 transistor: Itotal \u003d Ik + Iupr. = 3.3mA + 1.44mA = 4.74mA

The total load on the collector of the transistor VT2 is within normal limits. If it exceeds the maximum collector current specified in the reference book and multiplied by a factor of 0.8, increase the resistance R4 until the load current is sufficiently reduced, or use a more powerful transistor.

7. We need to provide current to the light bulb In \u003d Rn / Ui.p. = 15W / 12V = 1.25 A

But the buffer stage control current is 1.44mA. The multivibrator current must be increased by a value equal to the ratio:

In / I ex. = 1.25A / 0.00144A = 870 times.

How to do it? For a significant increase in output current use transistor cascades built according to the "composite transistor" scheme. The first transistor is usually low-power (we will use KT361G), it has the highest gain, and the second must provide sufficient load current (let's take the no less common KT814B). Then their gains h21 are multiplied. So, for the transistor KT361G h21> 50, and for the transistor KT814B h21=40. And the overall transfer coefficient of these transistors, connected according to the "composite transistor" scheme: h21 = 50 * 40 = 2000. This figure is more than 870, so these transistors are enough to drive a light bulb.

Well, that's all!

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

Visual illustration of division frequency range

Classification

Transistor generators have several classifications:

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

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

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

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

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

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

According to the principle of operation of generators:

• RC generators;
• LC generators;
• Blocking generators - short pulse shaper.

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

Generator circuitry

RC and LC sine wave generators

The generator on a transistor is most simply implemented in a capacitive three-point circuit - the Kolpitz generator (Fig. below).

Transistor oscillator circuit (Colpitz generator)

In the Kolpitz circuit, elements (C1), (C2), (L) are frequency-setting. The remaining elements are a standard transistor piping to provide the required operating mode for direct current. The same simple circuitry has a generator assembled according to the inductive three-point circuit - the Hartley generator (Fig. below).

Diagram of a three-point generator with inductive coupling (Hartley generator)

In this circuit, the oscillator frequency is determined by a parallel circuit, which includes elements (C), (La), (Lb). Capacitor (C) is needed to form a positive feedback on the alternating current.

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

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

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

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

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

Pierce generator circuit without the use of inductance

In Pierce's circuit, the inductance is replaced by a quartz resonator, which made it possible to get rid of the laborious and bulky inductor and, at the same time, limited the upper oscillation range.

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

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

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

Schematic of an RC oscillator with a phase-shifting chain

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

Diagram of an RC generator with a Wien bridge

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

Functional current generators

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

Scheme of a multivibrator - a generator of rectangular pulses

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

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

Triangular (sawtooth) pulse generators are usually built on the basis of rectangular pulse generators (master oscillator) by adding a corrective chain (Fig. below).

Triangular pulse generator circuit

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

Blocking generator

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

FET generators

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

Frequency-setting circuits have the same ratios. To generate high-frequency oscillations, a simple generator made on a field-effect transistor according to an inductive three-point circuit is somewhat preferable. The fact is that the field-effect transistor, having a high input resistance, practically does not have a shunting effect on the inductance, and, therefore, the high-frequency generator will work more stable.

Noise generators

A feature of noise generators is the uniformity of the frequency response in a certain range, that is, the amplitude of oscillations of all frequencies within a given range is the same. Noise generators are used in measuring equipment to assess the frequency characteristics of the tested path. Audio band noise generators are often supplemented with a frequency response equalizer to adapt to subjective loudness to human hearing. Such noise is called "gray".

Video

Until now, there are several areas in which the use of transistors is difficult. These are powerful microwave range generators in radar, and where it is required to receive especially powerful high-frequency pulses. So far, powerful microwave transistors have not been developed. In all other areas, the vast majority of generators are made exclusively on transistors. There are several reasons for this. First, the dimensions. Secondly, power consumption. Thirdly, reliability. On top of that, transistors, due to the peculiarities of their structure, are very easy to miniaturize.

INTRODUCTION

Electronic Computer Engineering- a relatively young scientific and technical direction, but it has the most revolutionary effect on all areas of science and technology, on all aspects of society. The constant development of the computer element base is characteristic. The elemental base is developing very quickly; new types appear logic circuits, existing ones are modified. There are many different electronic devices: logic elements, registers, adders, decoders, multiplexers, counters, frequency dividers, triggers, generators, etc.

Generators convert the energy of the power source into the energy of periodic or quasi-periodic electrical oscillations. The main purpose of generators in electronics is the formation of pulses initial installation and synchronization, control signals of various shapes and durations.

The whole variety of generators can be divided into the following types:

Rectangular pulse generators;

Linear voltage generators (LIN);

Step voltage generators;

Sinusoidal generators

Typical square wave shapes are shown in Figure 1

Rectangular pulse generators with energy-accumulating elements in the feedback loop are called multivibrators.

Multivibrators are divided into two groups:

Self-oscillating multivibrators;

Waiting multivibrators or single vibrators.

The main difference between these multivibrators is that self-oscillating multivibrators form a pulse sequence when the supply voltage is applied to the circuit, since they have two feedback circuits with energy storage devices, and standby multivibrators form a single pulse with given parameters on external launch, since one feedback loop has no energy storage. A single vibrator is a cross between a multivibrator and a trigger.

There are soft and hard modes of excitation of multivibrators. In soft mode, any voltage changes in the feedback circuit at the time of power-up lead to the occurrence of generation mode; in hard mode, generation occurs when the voltage in the feedback circuit reaches a certain threshold.

Multivibrators are divided into restartable and non-restartable. In the first case, when a trigger pulse is applied, the generation of output signals starts anew with initial state. Restarts allow you to unlimitedly increase the duration of the output pulse, regardless of the parameters of the multivibrator circuit. Non-restartable multivibrators do not respond to external trigger pulses

Description of the multivibrator circuit on field effect transistors

The high input resistance of field-effect transistors (FETs) makes it possible to design multivibrators for very low frequencies repetition of pulses at small capacitances of time-setting capacitors. Due to this, the shape of the output pulses is less distorted, and the duty cycle is greater than that of multivibrators based on bipolar transistors.

For self-oscillating multivibrators, FETs with a control p-n junction, since during the charging of the capacitors, the voltage in the gate-source section is applied in the forward direction and therefore the resistance of this section is small and the charge time of the capacitors becomes small.

Scheme of multivibrators from PT with managing p-n junction and a p-type channel is shown in Fig.2. In this multivibrator, a small negative voltage is applied through the resistors to the gate relative to the source, which increases the stability of the oscillation period and the duration of the output pulses. ).

Timing diagrams of the operation of an asymmetric multivibrator are shown in Fig.3. In basic terms, the principle of operation of this multivibrator is the same as that of a tube multivibrator. It differs from a BT multivibrator in that in temporarily stable equilibrium states, the discharge of capacitors occurs practically only through resistors and not to zero voltage, but to a value at which the gate voltage becomes equal to the cutoff voltage (usually 1-6 V)

To generate rectangular pulses with a frequency above, you can use circuits operating on the same principle as the circuit in Fig. 18.32. As shown in fig. 18.40, the simplest differential amplifier is used as a comparator in such circuits.

Positive feedback in the Schmitt trigger circuit is provided by directly connecting the amplifier output to its -input, i.e., the resistance of the resistor in the voltage divider is chosen to be zero. According to formula (18.16), in such a scheme an infinitely large period of oscillations should have turned out, but this is not entirely true. When deriving this equation, it was assumed that the amplifier used as a comparator has an infinitely large gain, i.e. that the switching process of the circuit occurs when the input voltage difference is equal to zero. In this case, the switching threshold of the circuit will be equal to the output voltage, and the voltage across the capacitor C will reach this value only after a very long time.

Rice. 18.40 Multivibrator based on differential amplifier.

The differential amplifier circuit, on the basis of which the generator is made in fig. 18.40 has a rather low gain. For this reason, the circuit will switch even before the difference between the input signals of the amplifier becomes zero. If, for example, such a scheme is implemented, as shown in Fig. 18.41, based on a linear amplifier made using ESL technology (for example, based on an integrated circuit, the difference in input signals at which the circuit switches will be about When the amplitude of the output voltage is about typical for circuits made on the basis of ESL technology, the pulse period generated signal is equal to

The considered circuit makes it possible to generate a pulsed voltage with a frequency of up to

A similar generator can also be made on the basis of TTL circuits. For these purposes, a ready-made Schmitt trigger chip (for example, 7414 or 74132) is suitable, since it already has an internal positive feedback. The corresponding inclusion of such a microcircuit is shown in Fig. 18.42. Since the input current of the TTL element must flow through the Schmitt trigger resistor, its resistance should not exceed 470 ohms. This is necessary for confident switching of the circuit at the lower threshold. The minimum value of this resistance is determined by the output load capacity of the logic element and is about 100 ohms. The Schmitt trigger thresholds are 0.8 and 1.6 V. For an output signal amplitude of about 3 V, typical for a TTL-type IC, the pulse frequency of the generated signal is

The maximum achievable frequency value is about 10 MHz.

The highest generation frequencies are achieved using special multivibrator circuits with emitter couplings (for example, microcircuits or a circuit diagram of such a multivibrator is shown in Fig. 18.43. In addition, these integrated microcircuits are equipped with additional terminal stages based on TTL or ESL circuits.

Consider the principle of operation of the circuit. Let's assume that the amplitude variable voltages at all points of the circuit does not exceed the value When the transistor is closed, the voltage on its collector is almost equal to the supply voltage. The emitter voltage of the transistor is the emitter current

Rice. 18.41. Multivibrator based on a linear amplifier made using ESL technology.

Rice. 18.42. Multivibrator based on the Schmitt trigger, made using TTL technology. Frequency

Rice. 18.43. Multivibrator with emitter couplings.

transistor is equal to In order for a signal of the desired amplitude to be emitted on the resistor, its resistance should be Then in the considered state of the circuit, the voltage at the emitter of the transistor will be equal to . During the time when the transistor is closed, the current of the source left in the circuit flows through the capacitor C. As a result, the voltage at the emitter of the transistor decreases at a rate

Transistor T opens when the voltage at its emitter drops to a value At the same time, the voltage at the base of the transistor decreases by 0.5 V and the transistor closes, and the voltage at its collector increases to a value Due to the presence of an emitter follower on the transistor, with increasing voltage at the collector of the transistor increases so is the base voltage of the transistor. As a result, the voltage at the emitter of the transistor jumps up to a value. This jump in voltage through the capacitor C is transmitted to the emitter of the transistor so that the voltage at this point jumps from to

During the time that the transistor is closed, the current flowing through the capacitor C causes the voltage at the emitter of the transistor to decrease at a rate

The transistor remains closed until the potential of its emitter decreases from value to value For a transistor, this time is