Cognitive experiments with transistors. What is a transistor and how does it work? What can be done from one transistor

We learned how a transistor works, in general terms, we examined manufacturing technologies germanium and silicon transistors and figured out how they are marked.

Today we will conduct several experiments and make sure that the bipolar transistor really consists of two diodes connected back to back, and that the transistor is signal amplifier.

We need a low-power p-n-p germanium transistor from the MP39 - MP42 series, an incandescent lamp rated for a voltage of 2.5 Volts and a 4 - 5 Volt power source. In general, for beginner radio amateurs, I recommend assembling a small adjustable one with which you will power your designs.

1. The transistor consists of two diodes.

To verify this, let's assemble a small circuit: the base of the transistor VT1 connect to the minus of the power source, and the output of the collector with one of the outputs of the incandescent lamp EL. Now, if the second terminal of the lamp is connected to the plus of the power source, the lamp will light up.

The light bulb lit up because we applied to the collector junction of the transistor direct- forward voltage, which opened the collector junction and flowed through it direct current collector Ik. The magnitude of this current depends on the resistance filament lamps and internal resistance power source.

And now let's consider the same circuit, but we will depict the transistor in the form of a semiconductor plate.

Major charge carriers in the base electrons, overcoming the p-n junction, fall into the hole region collector and become irrelevant. Having become minor, the base electrons are absorbed by the majority carriers in the hole region of the collector holes. In the same way, holes from the collector region, falling into the electronic region of the base, become minor and are absorbed by the majority charge carriers in the base. electrons.

The base pin connected to the negative pole of the power supply will act practically unlimited quantity electrons, replenishing the decay of electrons from the base region. And the collector contact, connected to the positive pole of the power source through the filament of the lamp, is capable of to accept the same number of electrons, due to which the concentration of holes in the region will be restored bases.

Thus, the conductivity of the p-n junction will become large and the current resistance will be small, which means that the collector current will flow through the collector junction Ik. And than more this current will be brighter the lamp will be on.

The light bulb will also burn if it is included in the emitter junction circuit. The figure below shows exactly this version of the scheme.

And now we will slightly change the circuit and the base of the transistor VT1 connect to plus power source. In this case, the lamp will not burn, since we included the p-n junction of the transistor in reverse direction. And this means that p-n resistance transition has become great and through it flows only a very small reverse current collector Ikbo incapable of incandescent lamp filament EL. In most cases, this current does not exceed a few microamperes.

And in order to finally verify this, we again consider a circuit with a transistor depicted as a semiconductor plate.

Electrons located in the region bases, will move to plus power source, moving away from the p-n junction. holes in the area collector, will also move away from the p-n junction, moving to negative power supply pole. As a result, the boundary of the regions is, as it were, will expand, which results in the formation of a zone depleted of holes and electrons, which will provide great resistance to the current.

But, since in each of the areas of the base and collector there are minor charge carriers, then small exchange electrons and holes between the regions will still occur. Therefore, a current many times smaller than the direct current will flow through the collector junction, and this current will not be enough to light the filament of the lamp.

2. Transistor operation in switching mode.

Let's make another experiment showing one of the transistor operation modes.
Between the collector and emitter of the transistor, we turn on a power source connected in series and the same incandescent lamp. We connect the plus of the power source to the emitter, and the minus through the filament of the lamp to the collector. The lamp does not light. Why?

Everything is very simple: if you apply a supply voltage between the emitter and the collector, then for any polarity one of the transitions will be in the forward direction, and the other in the opposite direction and will interfere with the passage of current. This is not difficult to see if you look at the following figure.

The figure shows that the emitter base-emitter junction is included in direct direction and is open and ready to accept an unlimited number of electrons. The collector base-collector junction, on the contrary, is included in reverse direction and prevents the passage of electrons to the base.

Hence it follows that the majority charge carriers in the emitter region holes, repelled by the plus of the power source, rush to the base region and there they mutually absorb (recombine) with the main charge carriers in the base electrons. At the moment of saturation, when there are no free charge carriers left on either side, their movement will stop, which means that the current stops flowing. Why? Because from the side of the collector there will be no make-up electrons.

It turns out that the main charge carriers in the collector holes attracted by the negative pole of the power source, and some of them are mutually absorbed electrons coming from the minus side of the power supply. And at the moment of saturation, when there is no left on both sides free charge carriers, holes, due to their predominance in the collector region, will block the further passage of electrons to the base.

Thus, a zone depleted of holes and electrons is formed between the collector and the base, which will provide great resistance to the current.

Of course, due to the magnetic field and thermal effects, a meager current will still flow, but the strength of this current is so small that it is not capable of heating the filament of the lamp.

Now add to the diagram wire jumper and we will close the base with the emitter to it. The light bulb included in the collector circuit of the transistor will again not light up. Why?

Because when the base and emitter are closed with a jumper, the collector junction becomes just a diode, to which reverse voltage. The transistor is in the closed state and only a small reverse collector current flows through it. Ikbo.

And now we will change the circuit a little more and add a resistor Rb resistance 200 - 300 Ohm, and another voltage source GB in the form of a finger battery.
Connect the battery minus through a resistor Rb with a transistor base, and plus batteries with an emitter. The lamp is on.

The lamp lit up because we connected the battery between the base and the emitter, and thereby applied to the emitter junction direct release voltage. The emitter junction opened and went through it straight current, which opened collector junction of the transistor. The transistor opened and along the circuit emitter-base-collector drip collector current Ik, many times greater circuit current emitter base. And thanks to this current, the light bulb lit up.

If we change the polarity of the battery and apply a plus to the base, then the emitter junction will close, and the collector junction will close with it. The reverse collector current will flow through the transistor Ikbo and the lamp will turn off.

Resistor Rb limits the current in the base circuit. If the current is not limited and all 1.5 volts are applied to the base, then too much current will flow through the emitter junction, as a result of which thermal breakdown transition and the transistor will fail. As a rule, for germanium transistors, the trigger voltage is not more than 0,2 volt, and for silicon no more 0,7 volt.

And again we will analyze the same circuit, but we will present the transistor in the form of a semiconductor plate.

When a trigger voltage is applied to the base of the transistor, the emitter transition and free holes from the emitter begin to mutually absorb with electrons bases, creating a small forward base current Ib.

But not all holes introduced from the emitter into the base recombine with its electrons. Typically, the base area is done thin, and in the manufacture of transistors of the p-n-p structure, the concentration of holes in emitter and collector make many times greater than the concentration of electrons in base, therefore, only a small part of the holes is absorbed by the base electrons.

The bulk of the emitter holes passes through the base and falls under the action of a higher negative voltage acting in the collector, and already together with the holes of the collector moves to its negative contact, where it is mutually absorbed by the input electrons by the negative pole of the power source GB.

As a result, the resistance of the collector circuit emitter-base-collector decreases and direct collector current flows in it Ik many times the base current Ib chains emitter base.

How more more holes is introduced from the emitter into the base, the more significant current in the collector circuit. And vice versa than less unlocking voltage on the base, the less current in the collector circuit.

If, at the time of transistor operation, a milliammeter is included in the base and collector circuits, then with the transistor closed, there would be practically no currents in these circuits.

With the transistor open, the base current Ib would be 2-3 mA, and the collector current Ik would be around 60 - 80 mA. All this suggests that the transistor can be current amplifier.

In these experiments, the transistor was in one of two states: open or closed. Switching the transistor from one state to another occurred under the action of the trigger voltage on the base Ub. This type of transistor is called switching mode or key. This mode of operation of the transistor is used in instruments and automation devices.

We will finish this, and in the next part we will analyze the operation of a transistor using the example of a simple audio frequency amplifier assembled on a single transistor.
Good luck!

Literature:

1. Borisov V.G. - Young radio amateur. 1985
2. E. Iceberg - Transistor? .. It's very simple! 1964

How are transistors of different types made?.. How are semiconductors cleaned and given a single-crystal structure?.. What methods allow you to introduce impurities of positive and negative types into a semiconductor?.. the form of the base in transistors for amplifying the RF poses dilemmas? .. All these questions are considered here by Professor Radiol.

I listened with interest to your talk about transistors and I note with satisfaction that Luboznaikin explained to you all the basic concepts related to these active components, which in a few years have successfully replaced vacuum tubes in most types of electronic equipment.

You understood well, Neznaikin, that weak alternating currents applied between the base and the emitter determine the base current, which in turn causes the collector current. We can say that the gain of a transistor is determined by the ratio of the change in collector current to the change in base current that caused it.

Semiconductor Cleaning

I think you would like to know what types of transistors exist and how they are made. Therefore, I will try to describe to you the main characteristics of transistors and the technology for their manufacture.

Transistors are made from germanium or silicon, and at the beginning of the production cycle you need to have a very pure semiconductor with an impeccable crystal structure.

To eliminate impurities, a heating method called zone melting is used. The semiconductor rod is placed in a quartz crucible and heated until the narrow zone of the rod melts. This molten zone is then slowly moved from one end of the semiconductor rod to the other. What's going on here? Impurities tend to remain in the molten part. By moving this zone from one end of the rod to the other, we collect impurities at one end and thoroughly clean the rest of the rod from them. After that, the end of the rod, in which impurities have accumulated, is cut off, and in the well-cleaned part there remains no more than one atom of impurities per hundred million atoms of the semiconductor.

high frequency heating

Perhaps you want to know how it is possible to heat a semiconductor in a narrow zone, in which the temperature is reached during the purification of germanium and during the purification of silicon? In this case, electronics are called to help. The area to be melted, together with the crucible, is placed in a coil through which a strong high-frequency current flows. This current induces currents in the mass of the semiconductor, which greatly heat it up. The coil is slowly moved along the crucible, which causes a corresponding movement of the molten zone (Fig. 132).

Heating by a magnetic field induced by high-frequency currents and, in turn, generating currents in the mass of a semiconductor, is fundamentally different from heating with a flame.

Heating with a flame increases the temperature of the surface of the body, and already from the surface, due to thermal conductivity, calories penetrate deep into the body. With high-frequency heating, the heat immediately covers the entire mass of the heated body.

I will add that this method can also be used to heat dielectrics, but then an electric (rather than magnetic) field is created in the heated body. To do this, a heated body is placed between the plates of a capacitor, to which an RF voltage is applied. This method is used in medicine, where it is called high-frequency diathermy.

Rice. 132. Purification of a semiconductor by zone melting.

Rice. 133. The arrangement of the three elements that form the transistor.

Obtaining a single crystal

Let us return, however, to semiconductors. Now that they are well cleaned, they need to be given an impeccable crystalline structure. The fact is that usually a semiconductor consists of a large number randomly arranged crystals. Such an accumulation of crystals must be turned into one single crystal with an exceptionally uniform crystal structure throughout the mass.

To do this, the entire semiconductor must be melted again; this operation is also performed using RF currents flowing through the coil. A tiny crystal is introduced into the melt, which serves as a seed for the perfect crystallization of the entire mass, and the required amount of impurities of the n or p type, depending on the type of future transistors.

After cooling, a single crystal with a mass of several kilograms is obtained. Then it will have to be cut into a large number of small pieces, each of which will subsequently be turned into a transistor. With the exception of blanks for high power transistors, these pieces are about 2 mm long and wide, and a few tenths of a millimeter thick.

fusion

Here we have blanks for the base. How to make transistors from them? You can easily guess that for this, on both sides of the base, you need to have impurities of a type opposite to that which the base contains.

There are several ways to accomplish this task. If the base is made of type p germanium, then tiny tablets of indium, which is an n type impurity, can be applied on both sides of it. Let us heat all this up to the temperature at which indium begins to melt; germanium manium, as I already told you, turns into a liquid only when heated to 940 ° C.

Indium atoms interspersed with germanium; this penetration is facilitated by thermal motion.

Thus, an emitter is formed on one side of the base, and a collector on the other (Fig. 133). The latter must have a larger volume than the emitter, since the currents dissipate more power on it. It goes without saying that a lead wire must be soldered to each of these three electrodes.

Diffusion and electrolysis

The emitter and collector formation method I have just described is used in the manufacture of Fusion Transistors. But the emitter and collector can also be created by diffusion. To do this, the semiconductor is heated to a temperature close to the melting point, and placed in a neutral gas atmosphere containing impurity vapor, designed to form the emitter and collector. Impurity atoms easily penetrate into the semiconductor. Depending on the dosage of impurity vapors and the duration of the operation, the penetration depth may be greater or lesser. This determines the thickness of the base.

The diffusion method is very well suited for the production of high-power transistors, as it allows the introduction of impurities over large areas - in this way it is possible to form an emitter and collector of the required size, sufficient to pass relatively large currents.

The diffusion method is similar to the electrolytic method, in which the semiconductor is exposed to jets of a liquid containing an impurity of the opposite type.

As you can see, for the production of transistors, substances are used in the solid state - fusion, in the liquid state - electrolysis and in the gaseous state - diffusion.

A transistor created by one of the described methods is placed in a sealed and opaque case so that light does not cause a photoelectric effect in the semiconductor. A vacuum is created in the housing or filled with a neutral gas, such as nitrogen, to prevent oxidation of germanium or silicon by atmospheric oxygen. Packages for power transistors are made in such a way that they can dissipate heat and thereby prevent excessive heating of semiconductors. Such a case is a heat sink radiator, it has a large size.

High frequencies pose problems

The high-frequency transistor is subject to requirements regarding the thickness of the base.

If its thickness is very small, then a relatively high capacitance is formed between the emitter and the collector. Then the RF currents, instead of passing through two transitions, pass directly from the emitter to the collector, which are a kind of capacitor plates.

Should the thickness of the base be increased to reduce this unwanted capacitance? You, Neznaykin, are undoubtedly going to propose this solution. Let's see how rational it is.

By increasing the distance separating the emitter and collector, you will force the electrons to make a longer path between the two junctions. However, in a semiconductor, the speed of movement of electrons and holes is rather low: about . Assume that the thickness of the base is OD mm. It takes 2.5 microseconds for the electrons to cover this more than short distance.

This is equal to the duration of one half-cycle of current with a frequency corresponding to a wave length. As you can see, with such a base thickness, only currents corresponding to long waves can be amplified.

That is why in RF transistors the base thickness must be made much smaller. With a base thickness of 0.001 mm, it is possible to amplify waves up to , and to receive decimeter waves, on which, in particular, television broadcasts are conducted, the base must be even thinner.

As you can see, here we are faced with two conflicting requirements: so that the capacitance of the emitter - collector is not too large, you need to increase the thickness of the base, and in order for the electrons to pass through the base fast enough, it needs to be made as thin as possible.

Problem Solutions

How to get out of this dilemma? It is very simple to reduce the capacitance not by reducing the distance between the two plates, which are the emitter and collector here, but by reducing their areas at the junctions as much as possible.

Rice. 134. Electrolytic treatment with liquid streams.

Rice. 135. A transistor in which there is a zone of semiconductor with its own conductivity between the base and the collector, which improves the gain at high frequencies.

For this purpose, impurities are introduced in such a way that the emitter and collector have the shape of cones, the tops of which are turned towards the base. This result is achieved, in particular, by treating both sides of the semiconductor plate with jets of liquid, which, under the influence of voltage, causes electrolysis and thereby gradually pulls out atoms, creating real craters in the semiconductor. When the bottoms of these recesses are close enough to each other, the direction of the voltage is changed, and a sufficient amount of impurities is added to the liquid, which are introduced into the recesses that form the emitter and collector by electrolysis (Fig. 134).

There is a category of RF transistors in which the base layer facing the emitter contains an increased amount of impurities, which increases the speed of the electrons and thereby allows higher frequencies to be amplified. Such transistors are called drift; they allow you to amplify decimeter waves.

One can go further in this direction by placing between the base and the collector what is called a self-conducting zone (Fig. 135). It is a layer of very pure germanium or silicon and therefore has a mediocre conductivity. This zone separates the very thin base from the collector, which reduces the capacitance between emitter and collector and allows very high frequencies to be amplified.

Mesa Transistors

Another method is used to manufacture transistors capable of operating at frequencies of several thousand megahertz, due to which they are, in particular, used in the input circuits of televisions.

For the manufacture of such transistors, a type p germanium plate is taken, which will serve as a collector. A strip of gold is firmly soldered to the underside of the plate - a future conclusion. The top side of the plate is exposed to antimony vapors. This n-type impurity, which is denser near the surface, forms the base. Then, on the same side of the plate, a p-type impurity (usually aluminum) is introduced by diffusion, which forms the emitter. This diffusion is carried out through a lattice, as a result of which aluminum is deposited on the surface in narrow stripes (Fig. 136, a).

After these operations are completed, tiny droplets of wax are applied to the surface, each of which covers with one side a section of a p-type semiconductor - the future emitter, and with its other part - a section of type n - the future base (Fig. 136, b).

Rice. Fig. 136. Successive steps in the manufacture of a mestransistor: a - diffusion through a p-type impurity lattice; b - deposition of wax droplets on the surfaces forming the emitter and base; c - acid treatment and separation of the plate into separate transistors.

Rice. 137. Stages of manufacturing a transistor using planar technology: a - an insulating layer of silicon dioxide is applied to the epitaxial layer; b - a “window” is created in the insulating layer, through which an impurity of type p is introduced by diffusion; c - after applying a new insulating layer, a "window" smaller than the first one is created in it, and an impurity of type n is introduced through it; d - for access to the base and emitter zones, holes are opened, filled with metal, to which the leads are then soldered; e - the substrate is fixed on a metal plate, which serves as the output of the collector.

Then the entire plate is treated with acid, which pits all areas of the emitters and bases, except for those protected by wax. Now it remains only to cut the plate into as many transistors as there are emitters and bases, which form small peculiar hills with a flat top on the collector (Fig. 136, c). Transistors with this structure became known as mesa, because in South America this word is called a mountain with a flat top.

epitaxial layer

Let us now descend from this mountain to the plain. By this, I mean the planar technology for manufacturing transistors, which has become very widespread, since it allows you to prepare thousands of transistors on a single crystal in one technological cycle. These transistors also make it possible to amplify high frequencies and obtain significant power.

Most often, such transistors are formed on the epitaxial layer of a semiconductor. What is it?

The collector must have a small specific electrical resistance to easily pass current. Therefore, it is desirable to make it from a semiconductor with a high content of impurities. The base and emitter, on the contrary, should have significantly less impurities.

To create the necessary difference, the impurity-rich semiconductor is coated with a thin epitaxial layer. To do this, a semiconductor, such as silicon, is heated in an atmosphere of hydrogen to a temperature of about one hundred degrees below its melting point. The temperature is then slightly lowered and the semiconductor is simultaneously introduced into the silicon tetrachloride. The latter decomposes, and an epitaxial layer is deposited on the surface of the semiconductor, consisting of silicon atoms arranged in the ideal order of the crystal lattice. The thickness of this layer is a hundredth of a millimeter, and its high purity determines the high electrical resistivity.

Manufacturing of transistors using planar technology

Imagine that we have a silicon wafer coated with an epitaxial layer. First, let's apply an insulating layer of silicon dioxide to the epitaxial layer (Fig. 137). Then, acting with the appropriate chemical composition, we will open a hole in the insulating layer, through which we will introduce an impurity of the p type, for example, boron, into the epitaxial layer by diffusion; this area with impurities will serve as the base of the future transistor.

Again we cover the entire plate with an insulating layer of silicon dioxide and by repeated chemical etching we open a small hole in the center. Through this hole, we introduce an impurity of type n, for example, phosphorus, by diffusion. Thus, an emitter is created.

Once again, cover the entire plate with an insulating layer of silicon dioxide and then open two holes in this layer: one above the emitter, and the other, located in the very center, above the base. Through these holes, by sputtering aluminum or gold, we will create the conclusions of the emitter and base. As for the output of the collector, its manufacture is not difficult - it is enough to strengthen the conductive plate on the underside of the collector.

You, Neznaykin, will undoubtedly notice that the transition edges of a transistor made in this way do not have contact with the surrounding atmosphere; they are protected by a layer of silicon dioxide, which completely eliminates the possibility of damage to the transistor. Silicon dioxide is better known as quartz.

If you want to increase the power of a planar transistor, in principle, you should increase the area of ​​the emitter-base junction; To do this, it is also possible to increase the area of ​​contact between these two zones, making the emitter not in the form of a small circle, but in the form of a star or a closed broken line.

Use of photosensitive films

Having learned from my explanations about the large number of operations required to produce a transistor using planar technology, you, Neznaikin, undoubtedly think that its cost should be very high. So I hasten to reassure you.

At one time, several tens or even hundreds of transistors are made. In production, photolithographic methods are used, which are even more widely used in the manufacture of integrated circuits which we will discuss another time.

Remember that to open the tiny holes (“windows”), the entire surface is first covered with a photosensitive film, which, when exposed to light, becomes hard and resistant to the solvent used in the next step. Thus, the exposed areas of the surface are protected by a kind of varnish, into which the hardened film has turned.

As I hope, you have guessed that light images of areas of the epitaxial layer are projected onto the film, which should not be chemically treated. Usually, light projection is carried out through lenses that allow the projected image to be reduced, which contributes to microminiaturization ...

I could tell you about other transistors, such as field-effect transistors. But I don't want to bore you. You can turn off the tape recorder.

The principle of semiconductor control of electric current was known as early as the beginning of the 20th century. Despite the fact that engineers working in the fields of radio electronics knew how the transistor worked, they continued to design devices based on vacuum tubes. The reason for such distrust of semiconductor triodes was the imperfection of the first point transistors. The family of germanium transistors did not differ in the stability of their characteristics and was highly dependent on temperature conditions.

Serious competition electronic tubes made monolithic silicon transistors only at the end of the 50s. Since that time, the electronics industry began to develop rapidly, and compact semiconductor triodes actively replaced energy-intensive lamps from circuits. electronic appliances. With the advent integrated circuits, where the number of transistors can reach billions, semiconductor electronics has won a decisive victory in the fight for the miniaturization of devices.

What is a transistor?

In the modern sense, a transistor is called a semiconductor radio element designed to change the parameters of an electric current and control it. A conventional semiconductor triode has three outputs: a base to which control signals are applied, an emitter and a collector. There are also high power composite transistors.

The size scale of semiconductor devices is striking - from a few nanometers (unpackaged elements used in microcircuits) to centimeters in diameter of powerful transistors designed for power plants and industrial equipment. Reverse voltages industrial triodes can reach up to 1000 V.

Device

Structurally, the triode consists of semiconductor layers enclosed in a housing. Semiconductors are materials based on silicon, germanium, gallium arsenide and others. chemical elements. Today, research is being carried out that prepares some types of polymers, and even carbon nanotubes, for the role of semiconductor materials. Apparently in the near future we will learn about the new properties of graphene field-effect transistors.

Previously, semiconductor crystals were located in metal cases in the form of hats with three legs. This design was typical for point transistors.

Today, the designs of most flat, including silicon, semiconductor devices are made on the basis of a single crystal doped in certain parts. They are pressed into plastic, glass-metal or ceramic-metal housings. Some of them have protruding metal plates for heat dissipation, which are mounted on radiators.

electrodes modern transistors located in one row. This arrangement of legs is convenient for automatic board assembly. The terminals are not marked on the housings. The type of electrode is determined by reference books or by measurements.

For transistors, semiconductor crystals with different structures are used, pnp type or n-p-n. They differ in the polarity of the voltage on the electrodes.

Schematically, the structure of a transistor can be represented as two semiconductor diodes separated by an additional layer. (See figure 1). It is the presence of this layer that makes it possible to control the conductivity of the semiconductor triode.

Rice. 1. The structure of transistors

Figure 1 schematically shows the structure of bipolar triodes. There is another class of field-effect transistors, which will be discussed below.

Basic principle of operation

At rest, no current flows between the collector and emitter of a bipolar triode. The resistance of the emitter junction, which arises as a result of the interaction of the layers, prevents the electric current. To turn on the transistor, it is required to apply a slight voltage to its base.

Figure 2 shows a diagram explaining how a triode works.

Rice. 2. Working principle

By controlling the base currents, you can turn the device on and off. If you apply to the base analog signal, then it will change the amplitude of the output currents. In this case, the output signal will exactly repeat the oscillation frequency at the base electrode. In other words, there will be an amplification of the electrical signal received at the input.

Thus, semiconductor triodes can operate in the mode of electronic keys or in the mode of amplifying input signals.

Device operation in mode electronic key can be understood from figure 3.

Rice. 3. Triode in key mode

Designation on the diagrams

Common notation: "VT" or "Q" followed by a positional index. For example, VT 3. In earlier diagrams, obsolete designations can be found: “T”, “PP” or “PT”. The transistor is depicted as symbolic lines indicating the corresponding electrodes, circled or not. The direction of the current in the emitter is indicated by an arrow.

Figure 4 shows a ULF circuit, in which transistors are labeled in a new way, and Figure 5 shows schematic representations of different types of field-effect transistors.

Rice. 4. An example of a ULF circuit on triodes

Types of transistors

According to the principle of operation and structure, semiconductor triodes are distinguished:

• field;
• bipolar;
• combined.

These transistors perform the same functions, but there are differences in the principle of their operation.

Field

This type of triode is also called unipolar, because of the electrical properties - they have a current of only one polarity. According to the structure and type of control, these devices are divided into 3 types:

1. Transistors with managing p-n transition (Fig. 6).
2. With an insulated gate (there are with a built-in or with an induced channel).
3. MDP, with the structure: metal-dielectric-conductor.

A distinctive feature of an insulated gate is the presence of a dielectric between it and the channel.

Parts are very sensitive to static electricity.

Field triode circuits are shown in Figure 5.

Rice. 5. Field-effect transistors
Rice. 6. Photo of a real field triode

Pay attention to the name of the electrodes: drain, source and gate.

FETs consume very little power. They can last over a year on a small battery or accumulator. Therefore, they are widely used in modern electronic devices such as remote controls, mobile gadgets, etc.

Bipolar

Much has been said about this type of transistor in the subsection “Basic principle of operation”. We only note that the device received the name "Bipolar" because of the ability to pass charges of opposite signs through one channel. Their feature is a low output impedance.

Transistors amplify signals and act as switching devices. A sufficiently powerful load can be included in the collector circuit. Due to the large collector current, the load resistance can be reduced.

We will consider in more detail about the structure and principle of operation below.

Combined

In order to achieve certain electrical parameters from the use of one discrete element transistor designers are inventing hybrid designs. Among them are:

• with resistors embedded and their circuit;
• combinations of two triodes (identical or different structures) in one case;
• lambda diodes - a combination of two field triodes forming a section with negative resistance;
• constructions in which an insulated gate field triode controls a bipolar triode (used to control electric motors).

Combined transistors are, in fact, an elementary microcircuit in one package.

How does a bipolar transistor work? Instructions for dummies

The operation of bipolar transistors is based on the properties of semiconductors and their combinations. To understand the principle of operation of triodes, we will deal with the behavior of semiconductors in electrical circuits.

Semiconductors.

Some crystals, such as silicon, germanium, etc., are dielectrics. But they have one feature - if you add certain impurities, they become conductors with special properties.

Some additives (donors) lead to the appearance of free electrons, while others (acceptors) form “holes”.

If, for example, silicon is doped with phosphorus (donor), then we get a semiconductor with an excess of electrons (n-Si structure). When boron (acceptor) is added, doped silicon will become a hole-conducting semiconductor (p-Si), that is, positively charged ions will predominate in its structure.

Unidirectional conduction.

Let's conduct a thought experiment: let's connect two heterogeneous semiconductors to a power source and bring current to our design. Something unexpected will happen. If you connect the negative wire to an n-type crystal, the circuit will close. However, when we reverse the polarity, there will be no electricity in the circuit. Why is this happening?

As a result of the combination of crystals with different types conductivity, a region with a p-n junction is formed between them. Part of the electrons (charge carriers) from the n-type crystal will flow into a crystal with hole conductivity and recombine holes in the contact zone.

As a result, uncompensated charges arise: in the n-type region - from negative ions, and in the p-type region from positive ones. The potential difference reaches a value of 0.3 to 0.6 V.

The relationship between voltage and impurity concentration can be expressed by the formula:

φ= V T*ln( N n* Np)/n 2 i , where

V T – thermodynamic stress value, N n and Np – the concentration of electrons and holes, respectively, and n i denotes the intrinsic concentration.

When connecting a plus to a p-conductor, and a minus to an n-type semiconductor, electric charges will overcome the barrier, since their movement will be directed against the electric field inside the p-n junction. In this case, the transition is open. But if the poles are reversed, the transition will be closed. Hence the conclusion: the p-n junction forms one-way conduction. This property is used in the design of diodes.

From diode to transistor.

Let's complicate the experiment. Let's add one more layer between two semiconductors with the same structures. For example, between p-type silicon wafers, we insert a conductive layer (n-Si). It is not difficult to guess what will happen in the contact zones. By analogy with the process described above, regions with p-n junctions are formed that block the movement of electric charges between the emitter and collector, regardless of the polarity of the current.

The most interesting thing happens when we apply a slight voltage to the interlayer (base). In our case, we apply a current with a negative sign. As in the case of a diode, an emitter-base circuit is formed, through which current will flow. At the same time, the layer will begin to be saturated with holes, which will lead to hole conduction between the emitter and collector.

Look at Figure 7. It shows that positive ions have filled the entire space of our conditional design and now nothing interferes with the conduction of current. We have obtained a visual model of a p-n-p bipolar transistor.

Rice. 7. The principle of operation of the triode

When the base is de-energized, the transistor very quickly returns to its original state and the collector junction closes.

The device can also operate in amplifying mode.

The collector current is directly proportional to the base current. : Ito= ß* IB , where ß – current gain, IB – base current.

If you change the value of the control current, then the intensity of the formation of holes on the base will change, which will entail a proportional change in the amplitude of the output voltage, while maintaining the frequency of the signal. This principle is used to amplify signals.

By applying weak pulses to the base, at the output we get the same amplification frequency, but with a much larger amplitude (set by the voltage applied to the collector-emitter circuit).

NPN transistors work in a similar way. Only the polarity of the voltages changes. Devices with an n-p-n structure have direct conduction. Reverse conduction p-n-p transistors type.

It remains to add that a semiconductor crystal reacts in a similar way to the ultraviolet spectrum of light. By turning the photon flux on and off, or by adjusting its intensity, one can control the operation of the triode or change the resistance of a semiconductor resistor.

Bipolar transistor switching circuits

Circuit engineers use the following connection schemes: with a common base, common emitter electrodes and switching on with a common collector (Fig. 8).

Rice. 8. Wiring diagrams for bipolar transistors

For amplifiers with a common base is typical:

• low input impedance, which does not exceed 100 ohms;
• good temperature properties and frequency characteristics of the triode;
• high allowable voltage;
• it takes two different sources for food.

Common emitter circuits have:

• high current and voltage gains;
• low power gain;
• inversion of the output voltage relative to the input.

With this connection, one power supply is sufficient.

The connection scheme according to the "common collector" principle provides:

• high input and low output impedance;
• low voltage gain (< 1).

How does a field effect transistor work? Explanation for dummies

The structure of a field-effect transistor differs from a bipolar one in that the current in it does not cross the p-n junction zones. The charges move along an adjustable area called the gate. Gate bandwidth is regulated by voltage.

Space p-n zones decreases or increases under the action of an electric field (see Fig. 9). Accordingly, the number of free charge carriers changes - from complete destruction to ultimate saturation. As a result of such an impact on the gate, the current at the drain electrodes (contacts that output the processed current) is regulated. The incoming current flows through the source contacts.

Figure 9. FET with p-n junction

Field triodes with a built-in and induced channel work on a similar principle. You saw their schemes in Figure 5.

FET switching circuits

In practice, connection schemes are used by analogy with a bipolar triode:

• with a common source - gives a large amplification of current and power;
• common-gate circuits providing low input impedance and low gain (of limited use);
• common-drain circuits that work in the same way as common-emitter circuits.

Figure 10 shows various wiring diagrams.

Rice. 10. Image of field triode connection diagrams

Almost every circuit is capable of operating at very low input voltages.

Video explaining the principle of operation of the transistor in simple terms

After we began to study bipolar transistors, a lot of messages about them began to come to personal messages. The most common questions are something like this:

If a transistor consists of two diodes, then why not just use two diodes and make a simple transistor out of them?

Why electricity flows from the collector to the emitter (or vice versa) if the transistor consists of two diodes that are connected either by cathodes or anodes? After all, current will flow only through a diode connected in the forward direction, after all, it cannot flow through another one, can it?

But the truth is yours ... Everything is logical ... But something seems to me that somewhere there is a catch ;-). And here is where this “highlight” we will consider in this article ...

The structure of the transistor

So, as you all remember from previous articles, any bipolar transistor, let's say, consists of two diodes. For

the equivalent circuit looks like this:

And for NPN transistor

something like that:

And what to be wise? Let's do a simple experiment!

We all have our favorite Soviet transistor KT815B. It is an NPN silicon conductance transistor:

Assembling a simple schematic OE (O general E mitter) to demonstrate some of its properties. I have shown this experience in previous articles. But as they say, repetition is the mother of learning.

To demonstrate the experience, we need a low-power incandescent light bulb and a couple of power supplies. Putting it all together like this:

where are we Bat1- this is a power supply that we turn on between the base and the emitter, and Bat2- the power supply, which we turn on between the collector and the emitter, and in addition, another light bulb clings in series.

It all looks like this:

Since the light bulb normally shines at a voltage of 5 V, I also set 5 V on the Bat 2.

On Bat 1, we gradually increase the voltage ... and at a voltage of 0.6 V

we have a light bulb. Therefore, our transistor "opened"

But since a transistor is made up of diodes, why don't we take two diodes and "make" a transistor out of them? No sooner said than done. We assemble the equivalent circuit of the KT815B transistor from two diodes of the 1N4007 brand.

In the figure below, I have labeled the leads of the diodes as anode and cathode, and have also labeled the leads of the “transistor”.

Putting it all together in the same way:

Since our KT815B transistor was silicon, and the 1N4007 diodes were also silicon, then, in theory, the diode transistor should open at a voltage of 0.6-0.7 V. Add the voltage to Bat1 to 0.7 V ...

and…

no, the light is not on

If you pay attention to the Bat1 power supply, you can see that the consumption at 0.7 V was already 0.14 A.

Simply put, if we had energized a little more, we would have burned the base-emitter diode, if, of course, we recall the current-voltage characteristic (CVC) of the diode.

But why, what's the matter? Why does the KT815B transistor, which essentially consists of the same silicon diodes, pass an electric current through the collector-emitter, and two diodes soldered also do not work as a transistor? Where is the dog buried?

Do you know how these “diodes” are located in the transistor? If we take into account that the N semiconductor is bread, and the thin layer of ham is the P semiconductor, then in the transistor they are located something like this (we don’t look at the salad):

The point is that the base in the transistor is very thin in width, like this ham, and the collector and emitter are as wide as these halves of bread (I exaggerate a little of course, they are slightly smaller), therefore, the transistor behaves like a transistor :-), that is, it opens and passes current through the collector-emitter.

Due to the fact that the base is very thin in width, it means that two P-N junctions are at a very small distance from each other and interaction occurs between them. This interaction is called transistor effect. And what can be the transistor effect between diodes, in which the distance between two P-N transitions how about the moon?

In all experiments, transistors KT315B, diodes D9B, miniature incandescent lamps 2.5V x 0.068A are used. Headphones - high-resistance type TON-2. Variable capacitor - any, with a capacity of 15 ... 180 pF. The power supply battery consists of two 4.5V 3R12 batteries connected in series. The lamps can be replaced with series-connected LED type AL307A and a resistor with a nominal value of 1 kOhm.

EXPERIMENT 1
ELECTRICAL DIAGRAM (conductors, semiconductors and insulators)

Electric current is the directed movement of electrons from one pole to another under the influence of voltage (9 V battery).

All electrons have the same negative charge. Atoms of different substances have different numbers of electrons. Most electrons are firmly bound to atoms, but there are also so-called "free", or valence, electrons. If a voltage is applied to the ends of the conductor, then free electrons will begin to move towards the positive pole of the battery.

In some materials, the movement of electrons is relatively free, they are called conductors; in others, movement is difficult, they are called semiconductors; thirdly, it is generally impossible; such materials are called insulators, or dielectrics.

Metals are good current conductors. Substances such as mica, porcelain, glass, silk, paper, cotton are insulators.

Semiconductors include germanium, silicon, etc. These substances become conductors under certain conditions. This property is used in the production of semiconductor devices - diodes, transistors.

Rice. 1. Determination of water conductivity

This experiment demonstrates the operation of a simple electrical circuit and the difference in conductance between conductors, semiconductors, and dielectrics.

Assemble the circuit as shown in Fig. 1, and bring the bare ends of the wires to the front of the board. Connect the bare ends together, the bulb will light up. This indicates that an electric current is passing through the circuit.

With two wires, you can test the conductivity of various materials. To accurately determine the conductivity of certain materials, special instruments are needed. (By the brightness of the lamp, one can only determine whether the material under study is a good or bad conductor.)

Attach the bare ends of the two conductors to a piece of dry wood at a short distance from each other. The light bulb will not light. This means that dry wood is a dielectric. If the bare ends of two conductors are attached to aluminum, copper or steel, the light bulb will burn. This suggests that metals are good conductors of electric current.

Dip the bare ends of the conductors into a glass of tap water (Fig. 1, a). The lamp does not light. This means that water is a poor conductor of current. If you add a little salt to the water and repeat the experiment (Fig. 1, b), the bulb will light up, which indicates the flow of current in the circuit.

The 56 ohm resistor in this circuit and in all subsequent experiments serves to limit the current in the circuit.

EXPERIMENT 2
DIODE ACTION

The purpose of this experiment is to demonstrate that a diode conducts well in one direction and does not conduct in the opposite direction.

Assemble the circuit as shown in Fig. 2, a. The lamp will be on. Rotate the diode by 180° (Fig. 2, b). The light bulb will not light.

And now let's try to understand the physical essence of the experiment.

Rice. 2. The action of a semiconductor diode in an electronic circuit.

The semiconductor substances germanium and silicon each have four free, or valence, electrons. Semiconductor atoms bind into dense crystals (crystal lattice) (Fig. 3, a).

Rice. 3. Crystal lattice of semiconductors.

If an impurity is introduced into a semiconductor having four valence electrons, for example, arsenic, which has five valence electrons (Fig. 3, b), then the fifth electron in the crystal will be free. Such impurities provide electronic conductivity, or n-type conductivity.

Impurities having a lower valency than semiconductor atoms have the ability to attach electrons to themselves; such impurities provide hole or p-type conductivity (Fig. 3c).

Rice. 4. pn junctions in a semiconductor diode.

A semiconductor diode consists of a junction of p- and n-type materials (p-n-junction) (Fig. 4, a). Depending on the polarity of the applied voltage, the p-n junction can either promote (Fig. 4, d) or prevent (Fig. 4, c) the passage of electric current. At the boundary of two semiconductors, even before the application of an external voltage, a binary electric layer is created with a local electric field of strength E 0 (Fig. 4, b).

If an alternating current is passed through the diode, then the diode will pass only the positive half-wave (Fig. 4d), and the negative will not pass (see Fig. 4, c). The diode thus converts or "rectifies" the AC to DC.

EXPERIMENT 3
HOW A TRANSISTOR WORKS

This experiment clearly demonstrates the main function of the transistor, which is a current amplifier. A small drive current in the base circuit can cause a large current in the emitter-collector circuit. By changing the resistance of the base resistor, you can change the collector current.

Assemble the circuit (Fig. 5). Put resistors into the circuit in turn: 1 MΩ, 470 kΩ, 100 kΩ, 22 kΩ, 10 kΩ. You may notice that with 1 MΩ and 470 kΩ resistors, the light does not light; 100 kOhm - the light bulb barely burns; 22 kOhm - the light bulb burns brighter; full brightness is observed when a 10 kΩ base resistor is connected.

Rice. 6. Transistor with n-p-n structure.

Rice. 7. Transistor with p-n-p structure.

A transistor is essentially two semiconductor diodes that have one common area - the base. If, in this case, the region with p-conductivity turns out to be common, then a transistor with an n-p-n structure will be obtained (Fig. 6); if the common area is with n-conductivity, then the transistor will be with the p-n-p structure (Fig. 7).

The area of ​​the transistor that emits (emigrates) current carriers is called the emitter; the area that collects current carriers is called the collector. The zone enclosed between these areas is called the base. The transition between the emitter and the base is called the emitter, and between the base and the collector - the collector.

On fig. 5 shows the inclusion of an n-p-n type transistor in electrical circuit.

When a p-n-p transistor is connected to the circuit, the polarity of the battery B is reversed.

For currents flowing through a transistor, there is a dependence

I e \u003d I b + I to

Transistors are characterized by a current gain, denoted by the letter β, which is the ratio of the increase in collector current to the change in base current.

The value of β ranges from several tens to several hundreds of units, depending on the type of transistor.

EXPERIMENT 4
PROPERTIES OF THE CAPACITOR

By studying the principle of operation of a transistor, you can demonstrate the properties of a capacitor. Assemble the circuit (Figure 8), but do not attach the 100uF electrolytic capacitor. Then connect it for a while to position A (Fig. 8, a). The lamp will turn on and off. This indicates that a capacitor charge current was flowing in the circuit. Now place the capacitor in position B (Fig. 8, b), while not touching the terminals with your hands, otherwise the capacitor may be discharged. The lamp will light up and go out, the capacitor has discharged. Now place the capacitor again in position A. It has been charged. Lay the capacitor aside for a while (10 seconds) on the insulating material, then place it in position B. The light will turn on and off. From this experiment it can be seen that the capacitor is able to accumulate and store electric charge for a long time. The accumulated charge depends on the capacitance of the capacitor.

Rice. 8. Scheme explaining the principle of the capacitor.

Rice. 9. Change in voltage and current on the capacitor over time.

Charge the capacitor by setting it to position A, then discharge it by connecting conductors with bare ends to the capacitor terminals (hold the conductor by the insulated part!), And place it in position B. The light will not light up. As can be seen from this experiment, a charged capacitor acts as a power source (battery) in the base circuit, but after use electric charge the light bulb goes out. On fig. 9 shows the dependences on time: capacitor charge voltage; charge current flowing in the circuit.

EXPERIMENT 5
TRANSISTOR AS A SWITCH

Assemble the circuit according to fig. 10, but do not install resistor R1 and transistor T1 into the circuit yet. Key B must be connected to the circuit at points A and E so that the connection point of resistors R3, R1 can be closed to a common wire (negative bus of the printed circuit board).

Rice. 10. The transistor in the circuit works like a switch.

Connect the battery, the lamp in the T2 collector circuit will be on. Now close the circuit with switch B. The light will go out, as the switch connects point A to the negative bus, thereby reducing the potential of point A, and therefore the potential of the base T2. If the switch is returned to its original position, the light will turn on. Now disconnect the battery and connect T1, do not connect the resistor R1. Connect the battery, the light will turn on again. As in the first case, the transistor T1 is open and an electric current passes through it. Now put a resistor R1 (470 kOhm) at points C and D. The light will go out. Remove the resistor and the bulb will light up again.

When the voltage at the collector T1 drops to zero (when a 470 kΩ resistor is installed), the transistor opens. The base of the transistor T2 is connected through T1 to the negative bus, and T2 is closed. The lamp goes out. Thus, the transistor T1 acts as a switch.

In previous experiments, the transistor was used as an amplifier, now it is used as a switch.

The possibilities of using a transistor as a key (switch) are given in experiments 6, 7.

EXPERIMENT 6
ALARM

A feature of this circuit is that the transistor T1, used as a key, is controlled by a photoresistor R2.

The photoresistor included in this kit changes its resistance from 2 kOhm in strong light to several hundred kOhm in the dark.

Assemble the circuit according to fig. 11. Depending on the lighting of the room where you are conducting the experiment, select the resistor R1 so that the bulb burns normally without dimming the photoresistor.

Rice. 11. Alarm circuit based on a photoresistor.

The state of the transistor T1 is determined by a voltage divider consisting of a resistor R1 and a photoresistor R2.

If the photoresistor is illuminated, its resistance is low, transistor T1 is closed, there is no current in its collector circuit. The state of the transistor T2 is determined by applying a positive potential by resistors R3 and R4 to the base of T2. Consequently, the transistor T2 opens, the collector current flows, the light is on.

When the photoresistor is darkened, its resistance increases greatly and reaches a value when the divider supplies voltage to the T1 base, sufficient to open it. The voltage at the collector T1 drops to almost zero, through the resistor R4 it closes the transistor T2, the light goes out.

In practice, in such circuits, other actuators (bell, relay, etc.) can be installed in the collector circuit of the transistor T2.

In this and subsequent circuits, a photoresistor of the SF2-9 type or similar can be used.

EXPERIMENT 7
AUTOMATIC LIGHT SWITCH

In contrast to experiment 6, in this experiment, when the photoresistor R1 is dimmed, the lamp lights up (Fig. 12).

Rice. 12. Scheme that turns on the light automatically.

When light hits the photoresistor, its resistance decreases greatly, which leads to the opening of the transistor T1, and consequently, to the closing of T2. The lamp does not light.

In the dark, the light turns on automatically.

This property can be used to turn lamps on and off depending on the amount of light.

EXPERIMENT 8
SIGNAL DEVICE

A distinctive feature of this scheme is its high sensitivity. In this and a number of subsequent experiments, a combined connection of transistors (composite transistor) is used (Fig. 13).

Rice. 13. Optoelectronic signaling device.

The principle of operation of this scheme does not differ from the scheme. At certain value resistance of resistors R1 + R2 and resistance of photoresistor R3 current flows in the base circuit of transistor T1. A current also flows in the collector circuit T1, but (3 times the current of the base T1. Let's assume that (β \u003d 100. All the current going through the emitter T1 must pass through the emitter-base T2 junction. Then the collector current T2 is β times more than the collector current T1, the collector current T1 is β times the base current T1, the collector current T2 is approximately 10,000 times the base current T1. Thus, the composite transistor can be considered as a single transistor with a very high gain and high sensitivity. of the composite transistor is that the transistor T2 must be powerful enough, while the transistor T1 controlling it can be low-power, since the current passing through it is 100 times less than the current passing through T2.

The performance of the circuit shown in Fig. 13 is determined by the illumination of the room where the experiment is being carried out, therefore it is important to select the resistance R1 of the divider of the upper arm so that the lamp does not burn in the illuminated room, but burns when the photoresistor is darkened by hand, the room is darkened with curtains, or when the light is turned off if the experiment is carried out in the evening.

EXPERIMENT 9
HUMIDITY SENSOR

In this circuit (Fig. 14), a compound transistor with high sensitivity is also used to determine the moisture content of the material. Base bias T1 is provided by resistor R1 and two bare-ended conductors.

Check the electrical circuit by lightly squeezing the bare ends of the two conductors with the fingers of both hands, without connecting them to each other. The resistance of the fingers is enough to trigger the circuit, and the bulb lights up.

Rice. 14. Scheme of the humidity sensor. The bare ends of the conductors penetrate the blotting paper.

Now pass the bare ends through blotting paper at a distance of about 1.5-2 cm, attach the other ends to the diagram according to fig. 14. Then moisten the blotting paper between the wires with water. The light bulb lights up (In this case, the decrease in resistance occurred due to the dissolution of salts in the paper with water.).

If the blotting paper is impregnated with saline, then dried and the experiment repeated, the efficiency of the experiment increases, the ends of the conductors can be separated to a greater distance.

EXPERIMENT 10
SIGNAL DEVICE

This scheme is similar to the previous one, the only difference is that the lamp lights up when the photoresistor is illuminated and goes out when darkened (Fig. 15).

Rice. 15. Signaling device on a photoresistor.

The circuit works as follows: with normal illumination of the photoresistor R1, the bulb will light up, since the resistance R1 is low, the transistor T1 is open. When the light is turned off, the lamp will turn off. The light of a flashlight or lit matches will cause the light bulb to burn again. The sensitivity of the circuit is adjusted by increasing or decreasing the resistance of the resistor R2.

EXPERIMENT 11
PRODUCT COUNTER

This experiment should be carried out in a semi-dark room. All the time when light falls on the photoresistor, indicator light L2 is on. If you place a piece of cardboard between the light source (light bulb L1 and photoresistor, light bulb L2 goes out. If you remove the cardboard, light bulb L2 lights up again (Fig. 16).

Rice. 16. Product counter.

In order for the experiment to be successful, it is necessary to adjust the circuit, i.e., select the resistance of the resistor R3 (the most suitable in this case is 470 ohms).

This scheme can practically be used to count a batch of products on a conveyor. If the light source and photoresistor are placed in such a way that a batch of products passes between them, the circuit turns on and off, as the light flow is interrupted by passing products. Instead of the L2 indicator light, a special counter is used.

EXPERIMENT 12
SIGNAL TRANSMISSION USING LIGHT

Rice. 23. Frequency divider on transistors.

Transistors T1 and T2 open in turn. The control signal is sent to the flip-flop. When transistor T2 is open, light L1 is off. Light bulb L2 lights up when transistor T3 is open. But transistors T3 and T4 open and close in turn, therefore, the light bulb L2 lights up with every second control signal sent by the multivibrator. Thus, the burning frequency of the light bulb L2 is 2 times less than the burning frequency of the light bulb L1.

This property can be used in an electric organ: the frequencies of all notes of the upper octave of the organ are divided in half and a tone is created an octave lower. The process may be repeated.

EXPERIMENT 18
SCHEME "AND" BY UNITS

In this experiment, the transistor is used as the key and the light bulb is the output indicator (Figure 24).

This circuit is logical. The bulb will light up if there is a high potential at the base of the transistor (point C).

Suppose points A and B are not connected to the negative bus, they have a high potential, therefore, there is also a high potential at point C, the transistor is open, the light is on.

Rice. 24. Logic element 2I on a transistor.

We accept conditionally: high potential - logical "1" - the light is on; low potential - logical "0" - the light is off.

Thus, if there are logical "1" at points A and B, there will also be "1" at point C.

Now let's connect point A to the negative bus. Its potential will become low (drop to "0" V). Point B has a high potential. Through the circuit R3 - D1 - the battery will flow current. Therefore, at point C there will be a low potential or "0". The transistor is closed, the light is off.

Let's connect point B to the ground. The current now flows through the circuit R3 - D2 - battery. The potential at point C is low, the transistor is closed, the light is off.

If both points are connected to ground, there will also be a low potential at point C.

Similar circuits can be used in an electronic examiner and other logic circuits ah, where the output signal will be only if there are simultaneous signals in two or more input channels.

Possible circuit states are shown in the table.

Truth table of the AND circuit

EXPERIMENT 19
SCHEME "OR" BY UNITS

This scheme is the opposite of the previous one. In order for there to be “0” at point C, it is necessary that there is also “0” at points A and B, that is, points A and B must be connected to the negative bus. In this case, the transistor will close, the light will go out (Fig. 25).

If now only one of the points, A or B, is connected to the negative bus, then at point C there will still be a high level, i.e. "1", the transistor is open, the light is on.

Rice. 25. Logic element 2OR on the transistor.

When connecting point B to the negative bus, current will flow through R2, D1 and R3. No current will flow through diode D2, since it is turned on in the opposite direction for conductivity. At point C, there will be about 9 V. The transistor is open, the light is on.

Now let's connect point A to the negative bus. The current will go through R1, D2, R3. The voltage at point C will be about 9 V, the transistor is open, the light is on.

OR circuit truth table

EXPERIMENT 20
"NOT" CIRCUIT (INVERTER)

This experiment demonstrates the operation of a transistor as an inverter - a device that can change the polarity of the output signal relative to the input to the opposite. In the experiments, the transistor was not part of the existing logic circuits, it only served to turn on the light bulb. If point A is connected to a negative bus, then its potential will drop to “0”, the transistor will close, the light will go out, at point B there is a high potential. This means a logical "1" (Fig. 26).

Rice. 26. The transistor works like an inverter.

If point A is not connected to the negative bus, that is, at point A - "1", then the transistor is open, the light is on, the voltage at point B is close to "0" or this is logical "0".

In this experiment, the transistor is integral part logic circuit and can be used to convert an OR circuit to a NOR circuit and an AND circuit to a NAND circuit.

NOT circuit truth table

EXPERIMENT 21
SCHEME "AND-NOT"

This experiment combines two experiments: 18 - scheme AND and 20 - scheme NOT (Fig. 27).

This circuit functions similarly to the circuit, forming "1" or "0" based on the transistor.

Rice. 27. Logic element 2I-NOT on a transistor.

The transistor is used as an inverter. If "1" appears on the base of the transistor, then the output point is "0" and vice versa.

If the potentials at point D are compared with the potentials at point C, it can be seen that they are inverted.

Truth table of the NAND circuit

EXPERIMENT 22
SCHEME "OR-NOT"

This experiment combines two experiments: - the OR circuit and - the NOT circuit (Fig. 28).

Rice. 28. Logic element 2OR-NOT on the transistor.

The circuit functions in exactly the same way as in experiment 20 (a “0” or “1” is generated based on the transistor). The only difference is that the transistor is used as an inverter: if “1” is at the input of the transistor, then “0” is at its output and vice versa.

Truth table of NOR circuit

EXPERIMENT 23
SCHEME "AND-NOT", ASSEMBLED ON TRANSISTORS

This circuit consists of two NOT logic circuits, the transistor collectors of which are connected at point C (Fig. 29).

If both points, A and B, are connected to the negative bus, then their potentials will become equal to "0". The transistors will close, there will be a high potential at point C, the bulb will not light.

Rice. 29. Logic element 2I-NOT.

If only point A is connected to the negative bus, at point B logical "1", T1 is closed, and T2 is open, collector current flows, the light is on, at point C logical "0".

If point B is connected to the negative bus, then the output will also be “0”, the light will be on, in this case T1 is open, T2 is closed.

And finally, if points A and B are logic "1" (not connected to the negative bus), both transistors are open. On their collectors "0", the current flows through both transistors, the light is on.

Truth table of the NAND circuit

EXPERIMENT 24
PHONE SENSOR AND AMPLIFIER

In the experimental scheme, both transistors are used as an amplifier sound signals(Fig. 30).

Rice. 30. Inductive phone sensor.

The signals are picked up and fed to the base of the transistor T1 with the help of an inductive coil L, then they are amplified and fed into the phone. When you have finished assembling the circuit on the board, position the ferrite rod near the phone, perpendicular to the incoming wires. Speech will be heard.

In this scheme and in the future, a ferrite rod with a diameter of 8 mm and a length of 100-160 mm, brand 600NN, is used as an inductive coil L. The winding contains approximately 110 turns of copper insulated wire with a diameter of 0.15..0.3 mm, type PEL or PEV.

EXPERIMENT 25
MICROPHONE AMPLIFIER

If an extra telephone is available (Figure 31), it can be used in place of the inductor in the previous experiment. As a result, we will have a sensitive microphone amplifier.

Rice. 31. Microphone amplifier.

Within assembled circuit you can get a semblance of a two-way communication device. Phone 1 can be used as the receiving device (connection at point A) and phone 2 as the output device (connection at point B). In this case, the second ends of both telephones must be connected to the negative bus.

EXPERIMENT 26
AMPLIFIER FOR PLAYER

With the help of a gramophone amplifier (Fig. 32), you can listen to recordings without disturbing the peace of those around you.

The circuit consists of two audio amplification stages. The input signal is the signal coming from the pickup.

Rice. 32. Amplifier for the player.

In the diagram, the letter A indicates the sensor. This sensor and capacitor C2 are a capacitive voltage divider to reduce the initial volume. Trimmer capacitor C3 and capacitor C4 are the secondary voltage divider. C3 controls the volume.

EXPERIMENT 27
"ELECTRONIC VIOLIN"

Here the multivibrator circuit is for making electronic music. The scheme is similar. The main difference is that the base bias resistor of transistor T1 is variable. A 22 kΩ resistor (R2) connected in series with a variable resistor provides the minimum base bias resistance T1 (Fig. 33).

Rice. 33. Multivibrator for creating music.

EXPERIMENT 28
FLASHING MORSE BUZZER

In this circuit, the multivibrator is designed to generate pulses with a tone frequency. The lamp lights up when the circuit is powered on (Fig. 34).

The phone in this circuit is connected to the circuit between the collector of the transistor T2 through the capacitor C4 and the negative bus of the board.

Rice. 34. Generator for learning Morse code.

With this scheme, you can practice learning Morse code.

If you are not satisfied with the tone of the sound, swap capacitors C2 and C1.

EXPERIMENT 29
METRONOME

A metronome is a device for setting the rhythm (tempo), for example, in music. For these purposes, a pendulum metronome was previously used, which gave both a visual and audible designation of the tempo.

In this scheme, these functions are performed by a multivibrator. The tempo frequency is approximately 0.5 s (Fig. 35).

Rice. 35. Metronome.

Thanks to the telephone and the indicator light, it is possible to hear and visually feel the set rhythm.

EXPERIMENT 30
AUTOMATIC ALARM DEVICE WITH AUTOMATIC RESET

This circuit (Fig. 36) demonstrates the use of a single vibrator, the operation of which is described in experiment 14. In the initial state, the transistor T1 is open, and T2 is closed. The phone is used as a microphone here. Whistling into the microphone (you can just blow on it) or light tapping excites an alternating current in the microphone circuit. Negative signals, arriving at the base of transistor T1, close it, and therefore open transistor T2, a current appears in the collector circuit T2, and the bulb lights up. At this time, the capacitor C1 is charged through the resistor R1. The voltage of the charged capacitor C2 is sufficient to open the transistor T1, i.e., the circuit returns to its original state spontaneously, while the light goes out. The burning time of the bulb is about 4 s. If the capacitors C2 and C1 are interchanged, then the burning time of the bulb will increase to 30 s. If the resistor R4 (1 kOhm) is replaced by 470 kOhm, then the time will increase from 4 to 12 s.

Rice. 36. Acoustic signaling device.

This experiment can be presented as a trick that can be shown in a circle of friends. To do this, you need to remove one of the microphones of the phone and put it under the board near the light bulb so that the hole in the board coincides with the center of the microphone. Now, if you blow on the hole in the board, it will seem that you are blowing on a light bulb and therefore it lights up.

EXPERIMENT 31
BUZZER WITH MANUAL RESET

This circuit (Fig. 37) is similar in principle to the previous one, with the only difference that when switching, the circuit does not automatically return to its original state, but this is done using switch B.

Rice. 37. Acoustic signaling device with manual reset.

The state of readiness of the circuit or the initial state will be when the transistor T1 is open, T2 is closed, the lamp is off.

A light whistle into the microphone gives a signal that turns off transistor T1, while opening transistor T2. The signal lamp lights up. It will burn until transistor T2 closes. To do this, it is necessary to short-circuit the base of the transistor T2 to the negative bus (“ground”) using key B. Other actuators, such as relays, can be connected to similar circuits.

EXPERIMENT 32
SIMPLE DETECTOR RECEIVER

For a beginner radio amateur, the design of radio receivers should begin with the simplest structures, for example, with a detector receiver, the diagram of which is shown in Fig. 38.

The detector receiver works as follows: electromagnetic waves sent on the air by radio stations, crossing the receiver antenna, induce voltage in it with a frequency corresponding to the frequency of the radio station signal. The induced voltage enters the input circuit L, C1. In other words, this circuit is called resonant, as it is pre-tuned to the frequency of the desired radio station. In the resonant circuit, the input signal is amplified tenfold and then fed to the detector.

Rice. 38. Detector receiver.

The detector is assembled on a semiconductor diode, which serves to rectify the modulated signal. The low frequency (audio) component will pass through the headphones and you will hear speech or music, depending on the transmission of that radio station. The high-frequency component of the detected signal, bypassing the headphones, will pass through the capacitor C2 to the ground. The capacitance of capacitor C2 determines the degree of filtering of the high-frequency component of the detected signal. Usually, the capacitance of the capacitor C2 is chosen in such a way that it represents a large resistance for audio frequencies, and its resistance is low for the high-frequency component.

As capacitor C1, you can use any small-sized variable capacitor with measurement limits of 10 ... 200 pF. In this constructor, a ceramic tuning capacitor of the KPK-2 type with a capacity of 25 to 150 pF is used to adjust the circuit.

The inductor L has the following parameters: number of turns - 110 ± 10, wire diameter - 0.15 mm, type - PEV-2, frame diameter of insulating material - 8.5 mm.

ANTENNA

A properly assembled receiver starts working immediately when an external antenna is connected to it, which is a piece copper wire 0.35 mm in diameter, 15-20 m long, suspended on insulators at a certain height above the ground. The higher the antenna is above the ground, the better the reception of radio signals will be.

GROUNDING

The reception volume increases if ground is connected to the receiver. The ground wire should be short and have little resistance. Its end is connected to a copper pipe going deep into the ground.

EXPERIMENT 33
DETECTOR RECEIVER WITH LOW FREQUENCY AMPLIFIER

This circuit (Fig. 39) is similar to the previous detector receiver circuit, with the only difference being that the simplest amplifier low frequency, assembled on a transistor T. The low frequency amplifier serves to increase the power of the signals detected by the diode. Tuning scheme oscillatory circuit is connected to the diode through capacitor C2 (0.1 uF), and resistor R1 (100 kOhm) provides the diode with a constant bias.

Rice. 39. Detector receiver with a single-stage ULF.

For normal operation of the transistor, a 9 V power supply is used. Resistor R2 is necessary in order to provide voltage to the base of the transistor to create the necessary mode of its operation.

For this circuit, as in the previous experiment, an external antenna and ground are required.

EXPERIMENT 34

SIMPLE TRANSISTOR RECEIVER

The receiver (Fig. 40) differs from the previous one in that instead of diode D, a transistor is installed, which simultaneously works both as a detector of high-frequency oscillations and as a low-frequency amplifier.

Rice. 40. Single transistor receiver.

The detection of a high-frequency signal in this receiver is carried out at the base-emitter section, therefore, such a receiver does not require a special detector (diode). The transistor with an oscillating circuit is connected, as in the previous circuit, through a 0.1 μF capacitor and is decoupling. Capacitor C3 serves to filter the high-frequency component of the signal, which is also amplified by the transistor.

EXPERIMENT 35
REGENERATIVE RECEIVER

In this receiver (Fig. 41), regeneration is used to improve the sensitivity and selectivity of the circuit. This role is performed by the coil L2. The transistor in this circuit is turned on a little differently than in the previous one. The signal voltage from the input circuit is fed to the base of the transistor. The transistor detects and amplifies the signal. The high-frequency component of the signal does not immediately enter the filter capacitor C3, but first passes through the winding feedback L2, which is located on the same core with the contour coil L1. Due to the fact that the coils are placed on the same core, there is an inductive connection between them, and part of the amplified voltage of the high-frequency signal from the collector circuit of the transistor again enters the input circuit of the receiver. At correct inclusion at the ends of the coupling coil L2, the feedback voltage supplied to the circuit L1 due to inductive coupling coincides in phase with the signal coming from the antenna, and the signal increases, as it were. This increases the sensitivity of the receiver. However, with a large inductive coupling, such a receiver can turn into a undamped oscillation generator, and a sharp whistle is heard in telephones. To eliminate excessive excitation, it is necessary to reduce the degree of coupling between the coils L1 and L2. This is achieved either by removing the coils from each other, or by reducing the number of turns of the L2 coil.

Rice. 41. Regenerative receiver.

It may happen that the feedback does not give the desired effect and the reception of stations that were well audible earlier, when the feedback is introduced, stops altogether. This suggests that instead of a positive feedback, a negative one has formed and the ends of the L2 coil need to be swapped.

On the short distances from the radio station, the described receiver works well without external antenna, per magnetic antenna.

If the audibility of the radio station is low, you still need to connect an external antenna to the receiver.

The receiver with one ferrite antenna must be installed so that the electromagnetic waves coming from the radio station create the largest signal in the coil of the oscillatory circuit. Thus, when you have tuned in to the signal of the radio station with the help of a variable capacitor, if the audibility is poor, turn the circuit to receive signals in the phones at the volume you need.

EXPERIMENT 36
TWO-TRANSISTOR REGENERATIVE RECEIVER

This circuit (Fig. 42) differs from the previous one in that it uses a low-frequency amplifier assembled on T2 transistors.

With the help of a two-transistor regenerative receiver, you can receive a large number of radio stations.

Rice. 42. Regenerative receiver with a low frequency amplifier.

Although this kit (set no. 2) only has a long wave coil, the circuit can operate on both medium and short waves when using the appropriate tuning coils. You can make them yourself.

EXPERIMENT 37
"DIRECTION FINDER"

The scheme of this experiment is similar to the scheme of experiment 36 without antenna and ground.

Tune in to a powerful radio station. Take the board in your hands (it should be horizontal) and rotate until the sound (signal) disappears or at least decreases to a minimum. In this position, the axis of the ferrite points exactly to the transmitter. If you now turn the board 90°, the signals will be clearly audible. But more precisely, the location of the radio station can be determined by the graph-mathematical method, using a compass to determine the angle in azimuth.

To do this, you need to know the direction of the transmitter from different positions - A and B (Fig. 43, a).

Suppose we are at point A, we determined the direction of the transmitter, it is 60 °. Now let's move to point B, while measuring the distance AB. Let's determine the second direction of the transmitter location, it is 30°. The intersection of the two directions is the location of the transmitting station.

Rice. 43. Scheme of the direction finding of the radio station.

If you have a map with the location of broadcasting stations on it, then it is possible to accurately determine your location.

Tune into station A, let it be at a 45° angle, and then tune into station B; its azimuth is, say, 90°. Given these angles, draw lines on the map through points A and B, their intersection will give your location (Fig. 43, b).

In the same way, ships and planes orient themselves in the process of movement.

CHAIN ​​CONTROL

In order for the circuits to work reliably during experiments, you need to make sure that the battery is charged, all connections are clean, and all nuts are securely screwed. The battery leads must be properly connected; when connecting, it is necessary to strictly observe the polarity of electrolytic capacitors and diodes.

COMPONENT CHECK

Diodes can be tested in; transistors - in; electrolytic capacitors (10 and 100 microfarads) - c. The head phone can also be checked by connecting it to the battery - a “crackle” will be heard in the earpiece.