Transistors are at the heart of most electronic devices. It can be in the form of separate radio components, or as part of microcircuits. Even the most complex microprocessor consists of a great many tiny transistors packed tightly into its mighty crystal.
Transistors are different:
The two main groups are bipolar and field. A bipolar transistor is indicated in the diagram, as shown in Figure 1. It comes in forward (p-p-p) and reverse (p-p-p) conductivity. The structure of the transistor and the physical processes occurring in it are studied at school, so we won’t talk about it here - so to speak, closer to practice. In essence, the difference is that pnp transistors are connected so that their emitter receives a positive voltage potential, and the collector receives a negative one. For n-p-n transistors, the opposite is true: the emitter is given a negative potential, and the collector is given a positive potential.
Why do you need a transistor?
It is mainly used to amplify current, signals, and voltage. And the amplification occurs due to the power source. I’ll try to explain the principle of working “on the fingers”. The car has a vacuum brake booster. When the driver presses the brake pedal, its membrane moves and a valve opens through which the car's engine sucks in this membrane, adding force to it. As a result, weak pressure on the brake pedal results in strong force on the brake pads. And the addition of force occurs due to the power of the running motor of the machine.
It's similar with a transistor. A weak current is supplied to the base (Fig. 2). Under the influence of this current, the collector-emitter conductivity increases and a much stronger current flows through the collector, coming from the power source. As the weak base current changes, the strong collector current changes accordingly. Ideally, the collector current graph looks like an enlarged copy of the base current graph.
This difference between the weak base current and the strong collector current is called the transistor current gain, and is denoted I21e. It is determined as follows: h21e = Ik / I6 (collector current divided by base current). The larger this parameter, the better the amplification properties of the transistor.
But this is all ideal. In fact, the dependence of collector current on base voltage is not so linear. You should remember the BAX diode, where at the very bottom of the current characteristics it is very small, and begins to increase sharply when the voltage reaches a certain value. Since the transistor is based on the same physical processes, there is a similar “defect” here too.
If we assemble the amplifier circuit shown in Figure 3 and speak into the microphone, there will be no sound in the speaker. Because the voltage on the microphone is very low, it is below the opening threshold of the transistor. Not only will there be no amplification here, but on the contrary, there will be a weakening of the signal.
In order for the transistor to work as an amplifier, you need to increase the voltage at its base. This can be done by somehow increasing the voltage at the microphone output. But then the meaning of the amplifier is lost. Or you need to apply some constant voltage to the base of the transistor (Fig. 4) through a resistor, such that the transistor opens slightly. And apply a weak alternating voltage to the base of this transistor through a capacitor. Now the most important thing is that a weak alternating voltage will combine with a constant voltage at the base. The voltage at the base will change in time with the weak alternating voltage. But since the constant voltage has shifted the operating point of the transistor to a steep linear portion of the characteristic, amplification occurs.
Simply put, the weak voltage did not have the strength to open the transistor, and we added a constant voltage to help it, which opened the transistor slightly.
The constant voltage that is applied to the base of the transistor to shift its operating mode to a region with a steeper and more linear characteristic is called bias voltage. By changing this voltage we can even adjust the gain of the amplifier stage.
Transistors are not always used with bias voltage. For example, in the amplification stages of transmitters, the bias voltage may not be applied to the bases of the transistors, since the amplitude of the input alternating voltage there is quite sufficient to “drive” the transistor.
If the transistor is used not as an amplifier, but as a switch, then the bias voltage is also not applied to the base. Simply, when the key should be closed, the voltage on the base is zero, and when it should be open, sufficient voltage is supplied to the base to open the transistor. This is commonly used in digital electronics, where there are only zeros (no voltage) and ones (voltage present) and no values in between.
Figure 5 shows a practical diagram of how to make a computer speaker from a radio loudspeaker. You need a simple single-program loudspeaker with only one plug for connecting to the radio network (the multi-program loudspeaker has a second plug for the electrical network). There is no need to make any changes to the loudspeaker circuit. It is connected to the collector of the transistor in the same way as to a radio network.
Inside the single-program loudspeaker there is a speaker, a variable resistor for adjusting the volume, and a transformer. All this is needed, and it remains. When you open the speaker housing, solder the collector of the transistor and the plus of the power supply to the places to which its wire and plug are soldered. The wire itself can be removed.
To connect to a computer you need a shielded wire with a corresponding plug at the end. Or a regular two-wire wire. If the wire is shielded, connect the braid to the emitter of the transistor, and the central core to capacitor C1.
The signal from the computer sound card is supplied through the plug to capacitor C1. The supply voltage is supplied from the mains power supply. The best choice is a power supply from a game console to a TV, such as “Dandy” or “Kanga”. In general, any power supply with an output voltage from 7V to 12V is suitable. To connect to the power supply, you will need a corresponding socket; it must be installed on the speaker body by drilling a hole for it. Although, of course, you can solder the wires from the power supply directly to the circuit. When connecting the power source, you must observe polarity. The VD1 diode is not needed in principle, but it protects the circuit from failure if you confuse the plus with the minus of the power supply. Without it, if the power supply is connected incorrectly, the transistor can be burned, but with a diode, if the poles of the power supply are mixed up, the circuit simply will not turn on.
The KT315 transistor is in a rectangular case, which has a bevel on one side (shown in the figure). Now, if you turn it away from you with this bevel, and the leads up, then there will be a base on the left, an emitter on the right, and a collector in the middle. The KT315 transistor with any letter will do (KT315A, KT315B...). The transistor must be soldered correctly, without mixing up its terminals. If you make a mistake and turn on the power, it may die. Therefore, after you have soldered everything, don’t be too lazy to check three times for correct installation, whether the terminals of the transistor, capacitors, and diode are soldered correctly. And only when you are 100% sure, turn it on.
Diode VD1 type KD209. The anode is marked on it. You can also install another diode, for example, 1N4004 or something else. If you solder the diode incorrectly, the circuit will not work. So, if everything is turned on, but does not work, start by checking that the diode is connected correctly.
Capacitors are electrolytic, with a voltage of at least 12V. Our K50-16, K50-35 or imported analogues will do. It should be noted that our capacitors have a plus on the body near the positive terminal, while imported ones have a minus or a wide vertical stripe near the negative terminal. Instead of a 10 µF capacitor, you can choose any capacitance from 2 µF to 20 µF. Instead of a 100 µF capacitor, a capacitor of any capacity of at least 100 µF will do.
The figure below the diagram shows the wiring diagram, where soldering points are marked with dots. Do not confuse soldering points with wire crossings. The installation is done in a hinged manner, using parts leads and wiring harnesses. It is advisable to place the entire circuit inside the speaker housing (there is usually a lot of space there).
If everything works, but there is a lot of noise, it means you have mixed up the wires going to the sound card. Swap them.
The circuit should NOT be powered from a computer power supply!
For the stereo option, you can make two speakers, combining the inputs into one stereo cable to connect to the sound card, and power both speakers from the same power supply.
With one transistor cascade, the speaker will sound quiet, but enough for listening in a small room. The volume can be adjusted either with the computer regulator or with the knob found on the loudspeaker.
A transistor, otherwise called a semiconductor triode, is an electronic device based on semiconductor materials. The main purpose of the device is the ability, by changing a weak current in the control circuit, to obtain an amplified signal at the output. The semiconductor triode is one of the main components of the circuits of many electronic devices, from radios to computers.
The definition of “transistor” is closely related to the etymology of this word. It is formed from two English words: transfer (transfer) and resistor (resistance). Indeed, the operating principle of the device is associated with the transfer (change) of resistance in the electrical circuit.
- bipolar;
- field (unipolar).
Each class, in turn, is divided into several varieties.
Bipolar:
Both of these types of triodes can be used in the same electronic circuit. Therefore, in order not to confuse which part should be used in a particular place in the circuit, the images of p-n-p and n-p-n triodes differ from each other.
Field:
- unipolar with p-n junction;
- MOS transistors with insulated gate.
Operating principle of the device
In electronics, semiconductors with electron (n) or hole (p) conductivity are used. These designations indicate that in the first case, negatively charged electrons predominate in the semiconductor, and in the second, positively charged holes.
Let's look at how a transistor works using the example of a bipolar semiconductor triode. Externally, the device looks like a small part in a metal or plastic case with three terminals. Inside is a kind of sandwich made of three layers of semiconductor. If the central layer is p-type, then the surrounding layers are n-type. The result is an n-p-n triode. If the center, also called the base, is n-type, then the plates are made of a semiconductor with hole conductivity, and the structure of the device is p-n-p. One of the outer layers is called the emitter, the other the collector. Each of these three parts of the device has a corresponding output.
A brief explanation of how a transistor works for dummies looks like this. Let's take an n-p-n transistor as an example, where the emitter and collector are layers with predominantly electronic conductivity, and the base with hole conductivity.
We connect the emitter to the negative terminal of the electric battery, and the base and collector to the positive terminal. A novice electronics enthusiast can imagine that a triode consists of two diodes, and the emitter-base diode is turned on in the forward direction, and current flows through it, and the base-collector diode is turned on in the opposite direction, and there is no current.
Let's assume that we have included a variable resistor in the base circuit, with which we can regulate the voltage supplied to the base. What effect do we get when the voltage is reduced to zero? The current in the emitter-base circuit will stop flowing. Let's increase the voltage a little. Electrons from the n - region of the emitter will rush to the base connected to the positive of the battery.
An important detail is that the base is made as thin as possible. Therefore, a mass of electrons passes through this layer and ends up in the collector under the influence of the positive pole of the battery, to which it is attracted. Thus, the current begins to pass not only between the emitter and the base, but also between the emitter and the collector. In this case, the collector current is significantly greater than the base current.
Another important circumstance: A small change in base current causes a much larger change in collector current. Thus, a semiconductor triode serves to amplify various signals. Typically, bipolar triodes are more often used in analog technology.
Field effect transistors
This type of triode differs from a bipolar triode not in properties or functions, but in its operating principle. In a field-effect triode, current flows from a terminal, called the source, to a terminal, called the drain, through a semiconductor of one type of conductivity, for example, p. And the strength of this current is controlled by changing the voltage at the third terminal - the gate.
This structure more accurately meets the requirements of modern digital technology, where field-effect triodes are mainly used. Today's technological capabilities make it possible to place several billion MIS elements with an insulated gate on a semiconductor chip with an area of 1-2 square centimeters. This is how central processors for personal computers are created.
Prospects for the development of devices
Prospects lie primarily in the area of further miniaturization of devices. Thus, American scientists are today developing a so-called single-molecule transistor. The main element of such a device is a benzene molecule, to which three electrodes are attached.
If the idea proves itself, it will be possible to create super-powerful computing systems. After all, the size of the molecule is much smaller than the size of today's MOS triodes on a silicon chip.
There are tens of thousands of transistors. They can all be divided into several types according to their characteristics. I will tell you what types of transistors exist and how they differ from each other.
Transistors can be divided into types according to such characteristics as:
- Physical structure
- Operating principle
- Power
- Frequency bandwidth
- Current gain
- etc.
But there are four main ones: the physical structure of the transistor, the operating principle of the transistor, the power and the operating frequency band of the transistor.
According to the principle of operation, all transistors can be divided into two large groups: bipolar transistors and field-effect transistors. They differ both in the principle of operation and in their physical structure. At the same time, both the structure of the transistor and the principle of their operation differ. Externally, both types perform the same functions, but inside field-effect and bipolar transistors, everything works differently.
Look at the diagram above. As you have already noticed, both bipolar and field-effect transistors have common characteristics: power and frequency. Which can be small, medium, high.
Transistor power dissipation
In this case, low-power transistors are considered to be those that are able to dissipate no more than 0.3 W, while medium-power transistors are able to dissipate from 0.3 W to 1.5 W. Well, powerful transistors dissipate more than 1.5W.
Transistor Bandwidth
This is the name for the frequency range in which the transistor retains its qualities as a transistor. The choice of transistor by frequency is greatly influenced by the type of your device and what frequencies of the input signals it should be able to work with correctly.
Bipolar transistor
I will not describe the structure of the transistor; there are other articles for that. This time I want to draw your attention to the fact that there are two clans in the family of bipolar transistors. This is a clan of transistors with an N-P-N structure and a clan with a P-N-P structure. In addition to physical there are no other structural differences between them.
Field effect transistors
Field-effect transistors, like bipolar transistors, can be divided into P- and N-type transistors. But besides this, they are divided into two more types: MOSFET and JFET. MOSFET is an insulated gate field effect transistor and JFET is a single PN junction field effect transistor.
| Difference between field effect and bipolar transistors | ||
| Principle of operation | bipolar | field |
| Controlled by current. To operate, an initial bias current must be supplied to the base. | Voltage controlled. All they need to operate is voltage applied to the gate. | |
|
They have a relatively low input resistance, therefore they consume more current than polar |
They have a high input impedance, which means virtually no input current to the transistor. Allows less load on the power supply due to less current consumption from the source | |
| Current gain | Bipolar transistors have a higher coefficient. gain. | Coeff. the gain is less than in a bipolar transistor. |
| Size | They come in medium and large sizes. | Field effect transistors can be produced for surface mounting. And also used in integrated circuits. |
| Popularity | Today, bipolar transistors have begun to give way to FETs. | FET transistors are becoming increasingly popular and are actively used in commercial software. |
| Price | Bipolar transistors are cheap to produce. | FETs, and especially MOSFETs, are significantly more expensive to produce than bipolar transistors. |
That's all. Of course, the deep principles of how transistors operate are left behind the scenes. But this was done intentionally. I'll tell you about them some other time.
Before considering the types of transistors, you should find out what a transistor actually is and what it is used for.
What is a transistor
A transistor is a semiconductor triode, which is a component used in the field of radio electronics, made from semiconductor materials. It has three terminals that allow you to control electrical current in a circuit using an input signal.
Because of its properties, it is used in cases where it is necessary to convert, generate or amplify electrical signals. The name transistor is also used for other devices that imitate the main quality of a transistor - the ability to change the signal in two different states, while simultaneously changing the signal of the control electrode.
Types and characteristics
All transistors are divided into two types - NPN and PNP. There is nothing particularly complicated in these seemingly complex abbreviations. These letter designations determine the order in which specific layers are applied. Such layers are pn junctions in the semiconductor materials used for their manufacture. Looking visually at any semiconductor, it is impossible to determine the type of semiconductor structure located inside the package. This data is indicated by markings on the housing. The type of transistor must be known in advance, since its use in the circuit can be very different.
One thing to keep in mind is that NPN and PNP are completely different. Therefore, they cannot simply be confused or replaced with each other. It is possible to replace one with the other under certain conditions. The main condition is a significant change in the connection circuit of these transistors. Thus, for certain components of radio engineering devices, only their own specific brands are used, otherwise the device will simply fail and will not work.
Technological differences
In addition to the type of pn junction, they all differ in the technology used for their manufacture.
In this regard, two types of transistors can be noted, differing in parameters:
- Bipolar - differ in the supply of a small current to their base. This current, in turn, serves to control the amount of current passing between the emitter and collector.
- - equipped with three terminals, called gate, drain and source. In this case, the gate of the transistor is affected not by current, but by voltage. These transistors have different polarities.
At one time, transistors replaced vacuum tubes. This is due to the fact that they have smaller dimensions, high reliability and lower production costs. Now, bipolar transistorsare the basic elements in all amplification circuits.
It is a semiconductor element having a three-layer structure, which forms two electron-hole junctions. Therefore, the transistor can be represented as two back-to-back diodes. Depending on what will be the main charge carriers, they distinguish p-n-p And n-p-n transistors.
Base– a semiconductor layer, which is the basis of the transistor design.
Emitter called a semiconductor layer whose function is to inject charge carriers into the base layer.
Collector called a semiconductor layer, the function of which is to collect charge carriers passing through the base layer.
Typically, the emitter contains a much larger number of main charges than the base. This is the main condition for the operation of the transistor, because in this case, when the emitter junction is forward biased, the current will be determined by the main carriers of the emitter. The emitter will be able to perform its main function - injecting carriers into the base layer. They usually try to make the emitter reverse current as small as possible. An increase in emitter majority carriers is achieved using a high dopant concentration.
Make the base as thin as possible. This is due to the lifetime of the charges. The charge carriers must cross the base and recombine as little as possible with the main base carriers in order to reach the collector.
In order for the collector to be able to more fully collect media passing through the base, they try to make it wider.
Transistor operating principle
Let's look at the example of a p-n-p transistor.
In the absence of external voltages, a potential difference is established between the layers. Potential barriers are installed at crossings. Moreover, if the number of holes in the emitter and collector is the same, then the potential barriers will be the same width.
In order for the transistor to work correctly, the emitter junction must be forward biased and the collector junction must be reverse biased.. This will correspond to the active mode of operation of the transistor. In order to make such a connection, two sources are needed. A source with voltage Ue is connected with the positive pole to the emitter, and the negative pole to the base. A source with voltage Uк is connected with the negative pole to the collector, and the positive pole to the base. Moreover, Ue< Uк.
Under the influence of voltage Ue, the emitter junction is biased in the forward direction. As is known, when the electron-hole transition is forward biased, the external field is directed opposite to the transition field and therefore reduces it. The majority carriers begin to pass through the transition; in the emitter there are 1-5 holes, and in the base there are 7-8 electrons. And since the number of holes in the emitter is greater than the number of electrons in the base, the emitter current is mainly due to them.
The emitter current is the sum of the hole component of the emitter current and the electronic component of the base.
Since only the hole component is useful, they try to make the electronic component as small as possible. The qualitative characteristic of the emitter junction is injection ratio.
They try to bring the injection coefficient closer to 1.
Holes 1-5 that have passed into the base accumulate at the boundary of the emitter junction. Thus, a high concentration of holes is created near the emitter and a low concentration near the collector junction, as a result of which the diffusion movement of holes from the emitter to the collector junction begins. But near the collector junction, the hole concentration remains zero, because as soon as the holes reach the junction, they are accelerated by its internal field and are extracted (pulled) into the collector. Electrons are repelled by this field.
While the holes cross the base layer, they recombine with the electrons located there, for example, like hole 5 and electron 6. And since holes come constantly, they create an excess positive charge, therefore, electrons must also come in, which are drawn through the base terminal and form a base current Ibr. This is an important condition for the operation of the transistor – the concentration of holes in the base should be approximately equal to the concentration of electrons. In other words The electrical neutrality of the base must be ensured.
The number of holes reaching the collector is less than the number of holes leaving the emitter by the amount of recombined holes in the base. That is, The collector current differs from the emitter current by the amount of the base current.
From here it appears transfer coefficient carriers, which they also try to bring closer to 1.
The collector current of the transistor consists of the hole component Icr and the reverse collector current.
The reverse collector current arises as a result of the reverse bias of the collector junction, so it consists of minority carriers of hole 9 and electron 10. Precisely because the reverse current is formed by minority carriers, it depends only on the thermal generation process, that is, on temperature. Therefore it is often called thermal current.
The quality of the transistor depends on the magnitude of the thermal current; the smaller it is, the better the transistor.
The collector current is connected to the emitter current transfer coefficient.
