Transistor and bipolar transistor, calculation of the transistor cascade. Bipolar transistors: switching circuits. Connection circuit for a bipolar transistor with a common emitter
Bipolar transistor.
Bipolar transistor- an electronic semiconductor device, one of the types of transistors, designed to amplify, generate and convert electrical signals. The transistor is called bipolar, since two types of charge carriers simultaneously participate in the operation of the device - electrons And holes. This is how it differs from unipolar(field-effect) transistor, in which only one type of charge carrier is involved.
The principle of operation of both types of transistors is similar to the operation of a water tap that regulates the flow of water, only a flow of electrons passes through the transistor. In bipolar transistors, two currents pass through the device - the main “large” current, and the control “small” current. The main current power depends on the control power. With field-effect transistors, only one current passes through the device, the power of which depends on the electromagnetic field. In this article we will take a closer look at the operation of a bipolar transistor.
Bipolar transistor design.
A bipolar transistor consists of three semiconductor layers and two PN junctions. PNP and NPN transistors are distinguished according to the type of alternation hole and electron conductivities. It's like two diode, connected face to face or vice versa.
A bipolar transistor has three contacts (electrodes). The contact coming out of the central layer is called base. The extreme electrodes are called collector And emitter (collector And emitter). The base layer is very thin relative to the collector and emitter. In addition to this, the semiconductor regions at the edges of the transistor are asymmetrical. The semiconductor layer on the collector side is slightly thicker than on the emitter side. This is necessary for the transistor to operate correctly.
Operation of a bipolar transistor.
Let's consider the physical processes occurring during operation of a bipolar transistor. Let's take the NPN model as an example. The principle of operation of a PNP transistor is similar, only the polarity of the voltage between the collector and emitter will be opposite.
As already stated in article on types of conductivity in semiconductors, in a P-type substance there are positively charged ions - holes. N-type substance is saturated with negatively charged electrons. In a transistor, the concentration of electrons in the N region significantly exceeds the concentration of holes in the P region.
Let's connect a voltage source between the collector and emitter V CE (V CE). Under its action, electrons from the upper N part will begin to be attracted to the plus and collect near the collector. However, current will not be able to flow because the electric field of the voltage source does not reach the emitter. This is prevented by a thick layer of collector semiconductor plus a layer of base semiconductor.
Now let's connect the voltage between base and emitter V BE , but significantly lower than V CE (for silicon transistors the minimum required V BE is 0.6V). Since the layer P is very thin, plus a voltage source connected to the base, it will be able to “reach” with its electric field the N region of the emitter. Under its influence, electrons will be directed to the base. Some of them will begin to fill the holes located there (recombine). The other part will not find a free hole, because the concentration of holes in the base is much lower than the concentration of electrons in the emitter.
As a result, the central layer of the base is enriched with free electrons. Most of them will go towards the collector, since the voltage is much higher there. This is also facilitated by the very small thickness of the central layer. Some part of the electrons, although much smaller, will still flow towards the plus side of the base.
As a result, we get two currents: a small one - from the base to the emitter I BE, and a large one - from the collector to the emitter I CE.
If you increase the voltage at the base, then even more electrons will accumulate in the P layer. As a result, the base current will increase slightly and the collector current will increase significantly. Thus, with a slight change in base current I B , the collector current I changes greatly WITH. That's what happens signal amplification in a bipolar transistor. The ratio of the collector current I C to the base current I B is called the current gain. Designated β , hfe or h21e, depending on the specifics of the calculations carried out with the transistor.
The simplest bipolar transistor amplifier
Let us consider in more detail the principle of signal amplification in the electrical plane using the example of a circuit. Let me make a reservation in advance that this scheme is not entirely correct. No one connects a DC voltage source directly to an AC source. But in this case, it will be easier and more clear to understand the amplification mechanism itself using a bipolar transistor. Also, the calculation technique itself in the example below is somewhat simplified.
1.Description of the main elements of the circuit
So, let’s say we have a transistor with a gain of 200 (β = 200). On the collector side, we will connect a relatively powerful 20V power source, due to the energy of which amplification will occur. From the base of the transistor we connect a weak 2V power source. We will connect to it in series an alternating voltage source in the form of a sine wave, with an oscillation amplitude of 0.1V. This will be a signal that needs to be amplified. The resistor Rb near the base is necessary in order to limit the current coming from the signal source, which usually has low power.
2. Calculation of base input current I b
Now let's calculate the base current I b. Since we are dealing with alternating voltage, we need to calculate two current values - at the maximum voltage (V max) and the minimum (V min). Let's call these current values respectively - I bmax and I bmin.
Also, in order to calculate the base current, you need to know the base-emitter voltage V BE. There is one PN junction between the base and emitter. It turns out that the base current “meets” the semiconductor diode on its path. The voltage at which a semiconductor diode begins to conduct is about 0.6V. Let's not go into details current-voltage characteristics of the diode, and for simplicity of calculations, we will take an approximate model, according to which the voltage on the current-carrying diode is always 0.6V. This means that the voltage between the base and emitter is V BE = 0.6V. And since the emitter is connected to ground (V E = 0), the voltage from base to ground is also 0.6V (V B = 0.6V).
Let's calculate I bmax and I bmin using Ohm's law:
2. Calculation of output collector current I WITH
Now, knowing the gain (β = 200), you can easily calculate the maximum and minimum values of the collector current (I cmax and I cmin).
3. Calculation of output voltage V out
The collector current flows through the resistor Rc, which we have already calculated. It remains to substitute the values:
4. Analysis of results
As can be seen from the results, V Cmax turned out to be less than V Cmin. This is due to the fact that the voltage across the resistor V Rc is subtracted from the supply voltage VCC. However, in most cases this does not matter, since we are interested in the variable component of the signal - the amplitude, which has increased from 0.1V to 1V. The frequency and sinusoidal shape of the signal have not changed. Of course, the V out / V in ratio of ten times is far from the best indicator for an amplifier, but it is quite suitable for illustrating the amplification process.
So, let's summarize the principle of operation of an amplifier based on a bipolar transistor. A current I b flows through the base, carrying constant and variable components. A constant component is needed so that the PN junction between the base and emitter begins to conduct - “opens”. The variable component is, in fact, the signal itself (useful information). The collector-emitter current inside the transistor is the result of the base current multiplied by the gain β. In turn, the voltage across the resistor Rc above the collector is the result of multiplying the amplified collector current by the resistor value.
Thus, the V out pin receives a signal with an increased oscillation amplitude, but with the same shape and frequency. It is important to emphasize that the transistor takes energy for amplification from the VCC power source. If the supply voltage is insufficient, the transistor will not be able to operate fully, and the output signal may be distorted.
Operating modes of a bipolar transistor
In accordance with the voltage levels on the electrodes of the transistor, there are four modes of its operation:
Cut off mode.
Active mode.
Saturation mode.
Reverse mode.
Cut-off mode
When the base-emitter voltage is lower than 0.6V - 0.7V, the PN junction between the base and emitter is closed. In this state, the transistor has no base current. As a result, there will be no collector current either, since there are no free electrons in the base ready to move towards the collector voltage. It turns out that the transistor is, as it were, locked, and they say that it is in cut-off mode.
Active mode
IN active mode The voltage at the base is sufficient for the PN junction between the base and the emitter to open. In this state, the transistor has base and collector currents. The collector current equals the base current multiplied by the gain. That is, the active mode is the normal operating mode of the transistor, which is used for amplification.
Saturation mode
Sometimes the base current may be too high. As a result, the supply power is simply not enough to provide such a magnitude of collector current that would correspond to the gain of the transistor. In saturation mode, the collector current will be the maximum that the power supply can provide and will not depend on the base current. In this state, the transistor is not able to amplify the signal, since the collector current does not respond to changes in the base current.
In saturation mode, the conductivity of the transistor is maximum, and it is more suitable for the function of a switch (switch) in the “on” state. Similarly, in the cut-off mode, the conductivity of the transistor is minimal, and this corresponds to the switch in the off state.
Inverse mode
In this mode, the collector and emitter change roles: the collector PN junction is biased in the forward direction, and the emitter junction is biased in the opposite direction. As a result, current flows from the base to the collector. The collector semiconductor region is asymmetrical to the emitter, and the gain in inverse mode is lower than in normal active mode. The transistor is designed in such a way that it operates as efficiently as possible in active mode. Therefore, the transistor is practically not used in inverse mode.
Basic parameters of a bipolar transistor.
Current gain– ratio of collector current I C to base current I B. Designated β , hfe or h21e, depending on the specifics of the calculations carried out with transistors.
β is a constant value for one transistor, and depends on the physical structure of the device. A high gain is calculated in hundreds of units, a low gain - in tens. For two separate transistors of the same type, even if they were “pipeline neighbors” during production, β may be slightly different. This characteristic of a bipolar transistor is perhaps the most important. If other parameters of the device can often be neglected in calculations, then the current gain is almost impossible.
Input impedance– resistance in the transistor that “meets” the base current. Designated R in (R input). The larger it is, the better for the amplification characteristics of the device, since on the base side there is usually a source of a weak signal, which needs to consume as little current as possible. The ideal option is when the input impedance is infinity.
R input for an average bipolar transistor is several hundred KΩ (kilo-ohm). Here the bipolar transistor loses very much to the field-effect transistor, where the input resistance reaches hundreds of GΩ (gigaohms).
Output conductivity- conductivity of the transistor between collector and emitter. The greater the output conductance, the more collector-emitter current will be able to pass through the transistor at less power.
Also, with an increase in output conductivity (or a decrease in output resistance), the maximum load that the amplifier can withstand with insignificant losses in overall gain increases. For example, if a transistor with low output conductivity amplifies the signal 100 times without a load, then when a 1 KΩ load is connected, it will already amplify only 50 times. A transistor with the same gain but higher output conductivity will have a smaller gain drop. The ideal option is when the output conductivity is infinity (or output resistance R out = 0 (R out = 0)).
If we consider mechanical analogues, the operation of transistors resembles the principle of operation of a hydraulic power steering in a car. But the similarity is only valid at a first approximation, since transistors do not have valves. In this article we will separately consider the operation of a bipolar transistor.
Bipolar transistor device
The basis of the bipolar transistor device is a semiconductor material. The first semiconductor crystals for transistors were made from germanium; today silicon and gallium arsenide are more often used. First, a pure semiconductor material with a well-ordered crystal lattice is produced. Then the crystal is given the required shape and a special impurity is introduced into its composition (the material is doped), which gives it certain properties of electrical conductivity. If conductivity is due to the movement of excess electrons, it is defined as n-type donor (electronic). If the conductivity of a semiconductor is due to the sequential replacement of vacant places, so-called holes, by electrons, then such conductivity is called acceptor (hole) and is designated p-type conductivity.
Figure 1.
The transistor crystal consists of three parts (layers) with sequential alternation of conductivity type (n-p-n or p-n-p). Transitions from one layer to another form potential barriers. The transition from base to emitter is called emitter(EP), to the collector – collector(KP). In Figure 1, the transistor structure is shown as symmetrical, idealized. In practice, during production, the sizes of the areas are significantly asymmetrical, approximately as shown in Figure 2. The area of the collector junction is significantly larger than the emitter junction. The base layer is very thin, on the order of several microns.
Figure 2.
Operating principle of a bipolar transistor
Any p-n junction of a transistor works similarly. When a potential difference is applied to its poles, it is “displaced.” If the applied potential difference is conditionally positive, and the pn junction opens, the junction is said to be forward biased. When a conditionally negative potential difference is applied, a reverse bias of the junction occurs, at which it is locked. A feature of the operation of the transistor is that with a positive bias of at least one transition, the general area, called the base, is saturated with electrons or electron vacancies (depending on the type of conductivity of the base material), which causes a significant reduction in the potential barrier of the second transition and, as a consequence, its conductivity under reverse bias.
Operating modes
All transistor connection circuits can be divided into two types: normal And inverse.
Figure 3.
Normal transistor switching circuit involves changing the electrical conductivity of the collector junction by controlling the bias of the emitter junction.
Inverse scheme, as opposed to normal, allows you to control the conductivity of the emitter junction by controlling the bias of the collector junction. The inverse circuit is a symmetrical analogue of the normal one, but due to the structural asymmetry of the bipolar transistor, it is ineffective for use, has more stringent restrictions on the maximum permissible parameters and is practically not used.
With any switching circuit, the transistor can operate in three modes: Cut-off mode, active mode And saturation mode.
To describe the work, the direction of the electric current in this article is conventionally taken to be the direction of the electrons, i.e. from the negative pole of the power supply to the positive pole. Let's use the diagram in Figure 4 for this.
Figure 4.
Cut-off mode
For a pn junction, there is a minimum forward bias voltage at which electrons are able to overcome the potential barrier of this junction. That is, at a forward bias voltage up to this threshold value, no current can flow through the junction. For silicon transistors, the value of this threshold is approximately 0.6 V. Thus, with a normal switching circuit, when the forward bias of the emitter junction does not exceed 0.6 V (for silicon transistors), no current flows through the base, it is not saturated with electrons, and as a result, there is no emission of base electrons into the collector region, i.e. There is no collector current (zero).
Thus, for the cutoff mode the necessary conditions are the identities:
U BE<0,6 В
I B =0
Active mode
In the active mode, the emitter junction is biased in the forward direction until the moment of unlocking (the current begins to flow) with a voltage greater than 0.6 V (for silicon transistors), and the collector junction in the reverse direction. If the base has p-type conductivity, electrons are transferred (injected) from the emitter into the base, which are instantly distributed in a thin layer of the base and almost all reach the collector boundary. Saturation of the base with electrons leads to a significant reduction in the size of the collector junction, through which electrons, under the influence of a negative potential from the emitter and base, are forced into the collector area, flowing through the collector terminal, thereby causing the collector current. The very thin layer of the base limits its maximum current passing through a very small cross section in the direction of the base exit. But this small thickness of the base causes its rapid saturation with electrons. The junction area is significant, which creates conditions for the flow of significant emitter-collector current, tens and hundreds of times greater than the base current. Thus, by passing small currents through the base, we can create conditions for the passage of much larger currents through the collector. The greater the base current, the greater its saturation, and the greater the collector current. This mode allows you to smoothly control (regulate) the conductivity of the collector junction by correspondingly changing (regulating) the base current. This property of the active mode of the transistor is used in various amplifier circuits.
In active mode, the transistor emitter current is the sum of the base and collector current:
I E = I K + I B
The collector current can be expressed as:
I K = α I E
where α is the emitter current transfer coefficient
From the above equalities we can obtain the following:
where β is the base current amplification factor.
Saturation mode
The limit for increasing the base current until the moment when the collector current remains unchanged determines the point of maximum saturation of the base with electrons. A further increase in the base current will not change the degree of its saturation, and will not affect the collector current in any way; it can lead to overheating of the material in the base contact area and failure of the transistor. The reference data for transistors can indicate the values of the saturation current and the maximum permissible base current, or the emitter-base saturation voltage and the maximum permissible emitter-base voltage. These limits determine the saturation mode of the transistor under normal operating conditions.
The cutoff mode and saturation mode are effective when transistors operate as electronic switches for switching signal and power circuits.
The difference in the principle of operation of transistors with different structures
The case of operation of an n-p-n transistor was considered above. Transistors of pnp structures work similarly, but there are fundamental differences that you should know. A semiconductor material with p-type acceptor conductivity has a relatively low electron throughput, since it is based on the principle of electron transition from one vacant site (hole) to another. When all vacancies are replaced by electrons, their movement is possible only as vacancies appear in the direction of movement. With a significant area of such material, it will have significant electrical resistance, which leads to greater problems when used as the most massive collector and emitter of p-n-p bipolar transistors than when used in a very thin base layer of n-p-n transistors. A semiconductor material with n-type donor conductivity has the electrical properties of conductive metals, making it more advantageous for use as an emitter and collector, as in n-p-n transistors.
This distinctive feature of different bipolar transistor structures leads to great difficulties in producing pairs of components with different structures and electrical characteristics similar to each other. If you pay attention to the reference data for the characteristics of pairs of transistors, you will notice that when the same characteristics are achieved for two transistors of different types, for example KT315A and KT361A, despite their identical collector power (150 mW) and approximately the same current gain (20-90) , they have different maximum permissible collector currents, emitter-base voltages, etc.
P.S. This description of the principle of operation of the transistor was interpreted from the position of Russian Theory, therefore there is no description of the action of electric fields on fictitious positive and negative charges. Russian Physics makes it possible to use simpler, understandable mechanical models that are closer to reality than abstractions in the form of electric and magnetic fields, positive and electric charges, which the traditional school treacherously palms off on us. For this reason, I do not recommend using the stated theory without preliminary analysis and comprehension when preparing to take tests, coursework and other types of work; your teachers may simply not accept dissent, even competitive and quite consistent from the point of view of common sense and logic. In addition, on my part, this is the first attempt to describe the operation of a semiconductor device from the position of Russian Physics, it can be refined and supplemented in the future.
Bipolar transistors are semiconductor devices with three electrodes connected to three layers in series, with different conductivities. Unlike other transistors, which carry one type of charge, it is capable of carrying two types at once.
Connection diagrams using bipolar transistors depend on the work performed and the type of conduction. Conduction can be electronic or hole.
Types of bipolar transistors
Bipolar transistors are divided according to various criteria into types according to:
- Material of manufacture: silicon or gallium arsenide.
- Frequency value: up to 3 MHz – low, up to 30 MHz – medium, up to 300 MHz – high, more than 300 MHz – ultra-high.
- Highest power dissipation: 0-0.3 W, 0.3-3 W, over 3 W.
- Device type: 3 layers of semiconductor with sequential order of conduction type.
Design and operation
The transistor layers, both inner and outer, are combined with built-in electrodes, which are named base, emitter and collector.
There are no significant differences in the types of conductivity between the collector and emitter, however, the percentage of inclusion of impurities in the collector is much lower, which makes it possible to increase the permissible voltage at the output.
The middle layer of the semiconductor (base) has a high resistance value, since it is made of lightly doped material. It is in contact with the collector over a large area. This makes it possible to increase heat dissipation, which is necessary due to the release of heat from the displacement of the junction in the other direction. Good base-collector contact allows electrons, which are minority carriers, to easily pass through.
The transition layers are made according to the same principle. However, bipolar transistors are considered as unbalanced devices. When alternating the outer layers in places with the same conductivity, it is impossible to form similar semiconductor parameters.
The transistor connection circuits are designed in such a way that they can provide it with both a closed and an open state. During active operation, when the semiconductor is open, the emitter is biased in the forward direction. To fully understand this design, you need to connect the supply voltage according to the diagram shown.
In this case, the boundary at the 2nd junction of the collector is closed, no current flows through it. In practice, the opposite phenomenon occurs due to adjacent transitions and their influence on each other. Since the minus pole of the battery is connected to the emitter, the open-type transition allows electrons to pass to the base, where they recombine with holes, which are the main carriers. Base current I b appears. The higher the base current, the higher the output current. This is the principle of operation of amplifiers.
Only diffusion movement of electrons occurs through the base, since there is no electric field work. Due to the small thickness of this layer and the significant gradient of particles, almost all of them enter the collector, although the base has high resistance. At the junction there is an electric field that promotes the transfer and draws them in. The emitter and collector currents are the same, except for a small loss of charge from redistribution at the base: I e = I b + I k.
Characteristics
- Current gain β = I k / I b.
- Voltage Gain U eq / U be.
- Input resistance.
- Frequency characteristic is the ability of a transistor to operate up to a certain frequency, beyond which the transition processes lag behind the change in the signal.
Operating modes and schemes
The type of circuit affects the mode of operation of the bipolar transistor. The signal can be collected and transmitted in two places for different cases, and there are three electrodes. Consequently, one arbitrary electrode must be both an output and an input. All bipolar transistors are connected according to this principle, and they have three types of circuits, which we will consider below.
Common collector circuit
The signal passes through the resistance R L, which is also included in the collector circuit.
This connection diagram makes it possible to create just a current amplifier. The advantage of such an emitter follower is the formation of significant resistance at the input. This makes it possible to match the amplification stages.
Scheme with a common base
The circuit has a drawback in the form of low input resistance. A common base circuit is most often used as an oscillator.
Common emitter circuit
Most often, when using bipolar transistors, a circuit with a common emitter is used. The voltage passes through the load resistance R L, and the power is connected to the emitter with the negative pole.
A variable value signal arrives at the base and emitter. In the collector circuit it becomes larger in value. The main elements of the circuit are a resistor, a transistor and an amplifier output circuit with a power supply. Additional steel elements: container C 1, which prevents current from passing to the input, resistance R 1, thanks to which the transistor opens.
In the collector circuit, the transistor voltage and resistance are equal to the EMF value: E= Ik Rk +Vke.
It follows that the small signal Ec determines the rule for changing the potential difference into the variable output of the transistor converter. This circuit makes it possible to increase the input current many times, as well as voltage and power.
One of the disadvantages of such a circuit is the low input resistance (up to 1 kOhm). As a consequence, problems arise in the formation of cascades. The output resistance is from 2 to 20 kOhm.
The considered circuits show the action of a bipolar transistor. Its operation is affected by signal frequency and overheating. To resolve this issue, additional separate measures are applied. Emitter grounding produces distortion at the output. To create the reliability of the circuit, filters, feedbacks, etc. are connected. After such measures, the circuit works better, but the gain decreases.
Operating modes
The speed of the transistor is influenced by the magnitude of the connected voltage. Let's consider different operating modes using the example of a circuit in which bipolar transistors are connected to a common emitter.
Cutoff
This mode is formed when the voltage V BE decreases to 0.7 volts. In this case, the emitter junction closes and there is no current at the collector, since there are no electrons in the base, and the transistor remains closed.
Active mode
When a voltage sufficient to turn on the transistor is applied to the base, a small input current and a large output current occur. This depends on the size of the gain. In this case, the transistor works as an amplifier.
Saturation mode
This work has its differences from the active mode. The semiconductor opens to the end, the collector current reaches its greatest value. Its increase can be achieved only by changing the load or the EMF of the output circuit. When adjusting the base current, the collector current does not change. The saturation mode has the peculiarities that the transistor is fully open and operates as a switch. If you combine the saturation and cutoff modes of bipolar transistors, you can create switches.
The properties of the output characteristics affect the modes. This is shown in the graph.
When plotting segments corresponding to the highest collector current and voltage size on the coordinate axes, and then connecting the ends with each other, a red load line is formed. The graph shows that the current and voltage point will move upward along the load line as the base current increases.
The area between the shaded output characteristic and the Vke axis is the cutoff work. In this case, the transistor is closed, and the reverse current is small. The characteristic at point A at the top intersects with the load, after which with a subsequent increase in I B the collector current no longer changes. On the graph, the saturation area is the shaded part between the Ik axis and the steepest graph.
Bipolar transistors in various modes
The transistor interacts with signals of different types in the input circuit. The transistor is mainly used in amplifiers. The input AC signal changes the output current. In this case, circuits with a common emitter or collector are used. A load is required in the output circuit for the signal.
Most often, a resistance installed in the collector output circuit is used for this. If it is chosen correctly, the voltage value at the output will be much greater than at the input.
During pulse signal conversion, the mode remains the same as for sinusoidal signals. The quality of the harmonic change is determined by the frequency characteristics of the semiconductors.
Switching mode
Transistor switches are used for contactless switching in electrical circuits. This work consists of intermittently adjusting the resistance value of the semiconductor. Bipolar transistors are most used in switching devices.
Semiconductors are used in signal modification circuits. Their universal operation and wide classification makes it possible to use transistors in various circuits, which determine their operating capabilities. The main circuits used are amplifying and switching circuits.
TOPIC 4. BIPOLAR TRANSISTORS
4.1 Design and principle of operation
A bipolar transistor is a semiconductor device consisting of three regions with alternating types of electrical conductivity and is suitable for power amplification.
Currently produced bipolar transistors can be classified according to the following criteria:
By material: germanium and silicon;
According to the type of conductivity of the areas: p-n-p and n-p-n types;
By power: low (Pmax £ 0.3 W), medium (Pmax £ 1.5 W) and high power (Pmax > 1.5 W);
By frequency: low-frequency, mid-frequency, high-frequency and microwave.
In bipolar transistors, the current is determined by the movement of charge carriers of two types: electrons and holes (or majority and minority). Hence their name - bipolar.
Currently, only transistors with planar p-n junctions are manufactured and used.
The structure of a planar bipolar transistor is shown schematically in Fig. 4.1.
It is a plate of germanium or silicon in which three regions with different electrical conductivities are created. In an n-p-n transistor, the middle region has hole, and the outer regions have electronic conductivity.
Transistors of the pnp type have a middle region with electronic conductivity, and outer regions with hole electrical conductivity.
The middle region of the transistor is called the base, one extreme region is the emitter, and the other is the collector. Thus, the transistor has two p-n junctions: the emitter - between the emitter and the base and the collector - between the base and the collector. The area of the emitter junction is smaller than the area of the collector junction.
The emitter is the region of the transistor whose purpose is to inject charge carriers into the base. A collector is a region whose purpose is to extract charge carriers from the base. The base is the region into which the emitter injects charge carriers that are non-majority for this region.
The concentration of the main charge carriers in the emitter is many times greater than the concentration of the main charge carriers in the base, and their concentration in the collector is somewhat less than the concentration in the emitter. Therefore, the emitter conductivity is several orders of magnitude higher than the base conductivity, and the collector conductivity is somewhat less than the emitter conductivity.
Conclusions are drawn from the base, emitter and collector. Depending on which of the terminals is common to the input and output circuits, there are three circuits for connecting the transistor: with a common base (CB), a common emitter (CE), and a common collector (CC).
The input, or control, circuit serves to control the operation of the transistor. In the output, or controlled, circuit, amplified oscillations are obtained. The source of amplified oscillations is included in the input circuit, and the load is connected to the output circuit.
Let's consider the principle of operation of a transistor using the example of a pnp type transistor connected according to a circuit with a common base (Fig. 4.2).
Figure 4.2 – Operating principle of a bipolar transistor (pnp type)
The external voltages of two power sources EE and Ek are connected to the transistor in such a way that the emitter junction P1 is biased in the forward direction (forward voltage), and the collector junction P2 is biased in the reverse direction (reverse voltage).
If a reverse voltage is applied to the collector junction and the emitter circuit is open, then a small reverse current Iko (units of microamps) flows in the collector circuit. This current arises under the influence of reverse voltage and is created by the directional movement of minority charge carriers, base holes and collector electrons through the collector junction. The reverse current flows through the circuit: +Ek, base-collector, -Ek. The magnitude of the reverse collector current does not depend on the collector voltage, but depends on the temperature of the semiconductor.
When a constant voltage EE is connected to the emitter circuit in the forward direction, the potential barrier of the emitter junction decreases. The injection of holes into the base begins.
The external voltage applied to the transistor turns out to be applied mainly to the transitions P1 and P2, because they have high resistance compared to the resistance of the base, emitter and collector regions. Therefore, holes injected into the base move through it through diffusion. In this case, the holes recombine with the electrons of the base. Since the carrier concentration in the base is much lower than in the emitter, very few holes recombine. With a small base thickness, almost all holes will reach the collector junction P2. In place of the recombined electrons, electrons from the power source Ek enter the base. Holes that recombine with electrons in the base create a base current IB.
Under the influence of reverse voltage Ek, the potential barrier of the collector junction increases, and the thickness of the junction P2 increases. But the potential barrier of the collector junction does not prevent holes from passing through it. The holes entering the region of the collector junction fall into a strong accelerating field created at the junction by the collector voltage, and are extracted (retracted) by the collector, creating a collector current Ik. The collector current flows through the circuit: +Ek, base-collector, -Ek.
Thus, three currents flow in the transistor: emitter, collector and base current.
In the wire, which is the base terminal, the emitter and collector currents are directed in opposite directions. Therefore, the base current is equal to the difference between the emitter and collector currents: IB = IE − IK.
Physical processes in an n-p-n transistor proceed similarly to the processes in a p-n-p transistor.
The total emitter current IE is determined by the number of main charge carriers injected by the emitter. The main part of these charge carriers reaching the collector creates a collector current Ik. A small part of the charge carriers injected into the base recombine in the base, creating a base current IB. Consequently, the emitter current will be divided into base and collector currents, i.e. IE = IB + Ik.
The emitter current is the input current, the collector current is the output current. The output current is part of the input current, i.e.
where a is the current transfer coefficient for the OB circuit;
Since the output current is less than the input current, the coefficient a<1. Он показывает, какая часть инжектированных в базу носителей заряда достигает коллектора. Обычно величина a составляет 0,95¸0,995.
In a common emitter circuit, the output current is the collector current, and the input current is the base current. Current gain for the OE circuit:
(4.3)
Consequently, the current gain for the OE circuit is tens of units.
The output current of the transistor depends on the input current. Therefore, a transistor is a current-controlled device.
Changes in emitter current caused by changes in emitter junction voltage are completely transmitted to the collector circuit, causing a change in collector current. And because The voltage of the collector power source Ek is significantly greater than the emitter Ee, then the power consumed in the collector circuit Pk will be significantly greater than the power in the emitter circuit Re. Thus, it is possible to control high power in the collector circuit of the transistor with low power spent in the emitter circuit, i.e. there is an increase in power.
4.2 Circuits for connecting bipolar transistors
The transistor is connected to the electrical circuit in such a way that one of its terminals (electrode) is the input, the second is the output, and the third is common to the input and output circuits. Depending on which electrode is common, there are three transistor switching circuits: OB, OE and OK. These circuits for a pnp transistor are shown in Fig. 4.3. For an n-p-n transistor in the switching circuits, only the polarity of the voltages and the direction of the currents change. For any transistor switching circuit (in active mode), the polarity of the power supplies must be selected so that the emitter junction is switched on in the forward direction, and the collector junction in the reverse direction.
Figure 4.3 – Connection circuits for bipolar transistors: a) OB; b) OE; c) OK
4.3 Static characteristics of bipolar transistors
The static mode of operation of the transistor is the mode when there is no load in the output circuit.
The static characteristics of transistors are the graphically expressed dependences of the voltage and current of the input circuit (input current-voltage characteristics) and the output circuit (output current-voltage characteristics). The type of characteristics depends on the method of switching on the transistor.
4.3.1 Characteristics of a transistor connected according to the OB circuit
IE = f(UEB) with UKB = const (Fig. 4.4, a).
IK = f(UKB) with IE = const (Fig. 4.4, b).
Figure 4.4 – Static characteristics of a bipolar transistor connected according to the OB circuit
The output current-voltage characteristics have three characteristic regions: 1 – strong dependence of Ik on UKB (nonlinear initial region); 2 – weak dependence of Ik on UKB (linear region); 3 – breakdown of the collector junction.
A feature of the characteristics in region 2 is their slight increase with increasing voltage UKB.
4.3.2 Characteristics of a transistor connected according to the OE circuit:
The input characteristic is the dependence:
IB = f(UBE) with UKE = const (Fig. 4.5, b).
The output characteristic is the dependence:
IK = f(UKE) with IB = const (Fig. 4.5, a).
Figure 4.5 – Static characteristics of a bipolar transistor connected according to the OE circuit
The transistor in the OE circuit provides current amplification. Current gain in the OE circuit: 
If coefficient a for transistors is a = 0.9¸0.99, then coefficient b = 9¸99. This is the most important advantage of connecting the transistor according to the OE circuit, which, in particular, determines the wider practical application of this connection circuit compared to the OB circuit.
From the principle of operation of the transistor, it is known that two current components flow through the base terminal in the opposite direction (Fig. 4.6): the reverse current of the collector junction IKO and part of the emitter current (1 - a)IE. In this regard, the zero value of the base current (IB = 0) is determined by the equality of the specified current components, i.e. (1 − a)IE = IKO. Zero input current corresponds to the emitter current IE=IKO/(1−a)=(1+b)IKO and the collector current. In other words, at zero base current (IB = 0), a current flows through the transistor in the OE circuit, called the initial or through current IKO(E) and equal to (1+ b) IKO.
Figure 4.6 – Connection circuit for a transistor with a common emitter (OE circuit)
4.4 Basic parameters
To analyze and calculate circuits with bipolar transistors, the so-called h - parameters of the transistor connected according to the OE circuit are used.
The electrical state of a transistor connected according to the OE circuit is characterized by the values IB, IBE, IK, UKE.
The system of h − parameters includes the following quantities:
1. Input impedance
h11 = DU1/DI1 at U2 = const. (4.4)
represents the transistor’s resistance to alternating input current at which a short circuit occurs at the output, i.e. in the absence of AC output voltage.
2. Voltage feedback coefficient:
h12 = DU1/DU2at I1= const. (4.5)
shows what proportion of the input AC voltage is transferred to the input of the transistor due to feedback in it.
3. Current force coefficient (current transfer coefficient):
h21 = DI2/DI1at U2= const. (4.6)
shows the amplification of alternating current by the transistor in no-load mode.
4. Output conductivity:
h22 = DI2/DU2 at I1 = const. (4.7)
represents the conductance for alternating current between the output terminals of the transistor.
Output resistance Rout = 1/h22.
For a common emitter circuit, the following equations apply:
(4.8)
To prevent overheating of the collector junction, it is necessary that the power released in it during the passage of the collector current does not exceed a certain maximum value:
(4.9)
In addition, there are limitations on collector voltage:
and collector current:
4.5 Operating modes of bipolar transistors
A transistor can operate in three modes depending on the voltage at its junctions. When operating in active mode, the voltage at the emitter junction is direct, and at the collector junction it is reverse.
The cut-off, or blocking, mode is achieved by applying reverse voltage to both junctions (both p-n junctions are closed).
If the voltage at both junctions is direct (both p-n junctions are open), then the transistor operates in saturation mode.
In cutoff mode and saturation mode, there is almost no control of the transistor. In the active mode, such control is carried out most efficiently, and the transistor can perform the functions of an active element of an electrical circuit (amplification, generation, etc.).
4.6 Scope of application
Bipolar transistors are semiconductor devices for universal purposes and are widely used in various amplifiers, generators, pulse and switching devices.
4.7 The simplest amplifier stage using a bipolar transistor
The most widely used circuit is to switch on a transistor according to a circuit with a common emitter (Fig. 4.7)
The main elements of the circuit are the power supply Ek, the controlled element - transistor VT and resistor Rk. These elements form the main (output) circuit of the amplifier stage, in which, due to the flow of controlled current, an amplified alternating voltage is created at the output of the circuit.
The remaining elements play a supporting role. Capacitor Cp is a separating capacitor. In the absence of this capacitor in the input signal source circuit, a direct current would be created from the power source Ek.
Figure 4.7 – Diagram of the simplest amplifier stage on a bipolar transistor according to a common-emitter circuit
Resistor RB, connected to the base circuit, ensures operation of the transistor in rest mode, i.e. in the absence of an input signal. The quiescent mode is ensured by the quiescent base current IB » Ek/RB.
With the help of resistor Rk, an output voltage is created, i.e. Rк performs the function of creating a varying voltage in the output circuit due to the flow of current in it, controlled through the base circuit.
For the collector circuit of the amplifier stage, we can write the following equation of electrical state:
Ek = Uke + IkRk, (4.10)
that is, the sum of the voltage drop across the resistor Rk and the collector-emitter voltage Uke of the transistor is always equal to a constant value - the emf of the power source Ek.
The amplification process is based on the conversion of the energy of a constant voltage source Ek into the energy of an alternating voltage in the output circuit by changing the resistance of the controlled element (transistor) according to the law specified by the input signal.
When an alternating voltage uin is applied to the input of the amplifier stage, an alternating current component IB~ is created in the base circuit of the transistor, which means the base current will change. A change in the base current leads to a change in the value of the collector current (IK = bIB), and therefore to a change in the voltage values across the resistance Rk and Uke. The amplifying abilities are due to the fact that the change in the collector current values is b times greater than the base current.
4.8 Calculation of electrical circuits with bipolar transistors
For the collector circuit of the amplifier stage (Fig. 4.7), in accordance with Kirchhoff’s second law, equation (4.10) is valid.
The volt-ampere characteristic of the collector resistor RK is linear, and the volt-ampere characteristics of the transistor are non-linear collector characteristics of the transistor (Fig. 4.5, a) connected according to the OE circuit.
The calculation of such a nonlinear circuit, that is, the determination of IK, URK and UKE for various values of base currents IB and resistor resistance RK, can be carried out graphically. To do this, on the family of collector characteristics (Fig. 4.5, a) it is necessary to draw from point EK on the abscissa axis the volt-ampere characteristic of the resistor RK, satisfying the equation:
Uke = Ek − RkIk. (4.11)
This characteristic is built at two points:
Uke = Ek with Ik = 0 on the abscissa and Ik = Ek/Rk with Uke = 0 on the ordinate. The I-V characteristic of the collector resistor Rk constructed in this way is called the load line. The points where it intersects with the collector characteristics provide a graphic solution to equation (4.11) for a given resistance Rк and various values of the base current IB. From these points you can determine the collector current Ik, which is the same for the transistor and resistor Rk, as well as the voltage UKE and URK.
The point of intersection of the load line with one of the static current-voltage characteristics is called the operating point of the transistor. By changing IB, you can move it along the load line. The initial position of this point in the absence of an input alternating signal is called the resting point - T0.
a) b)
Figure 4.8 – Graphic-analytical calculation of the operating mode of a transistor using output and input characteristics.
The rest point (operating point) T0 determines the current ICP and the voltage UCP in rest mode. Using these values, you can find the power of the RCP released in the transistor in rest mode, which should not exceed the maximum power of the RK max, which is one of the parameters of the transistor:
RKP = IKP ×UKEP £ RK max. (4.12)
Reference books usually do not provide a family of input characteristics, but only characteristics for UKE = 0 and for some UKE > 0.
The input characteristics for various UCEs exceeding 1V are located very close to each other. Therefore, the calculation of input currents and voltages can be approximately done using the input characteristic for UCE > 0, taken from the reference book.
Points A, To and B of the output operating characteristic are transferred to this curve, and points A1, T1 and B1 are obtained (Fig. 4.8, b). Operating point T1 determines the constant base voltage UBES and the constant base current IUPS.
The resistance of the resistor RB (ensures the operation of the transistor in rest mode), through which a constant voltage will be supplied from the source EK to the base:
(4.13)
In the active (amplifying) mode, the rest point of the transistor To is located approximately in the middle of the AB load line section, and the operating point does not extend beyond the AB section.
Transistor
A transistor is a semiconductor device that allows you to control a stronger signal using a weak signal. Because of this property, they often talk about the ability of a transistor to amplify a signal. Although in fact, it does not enhance anything, but simply allows you to turn on and off a large current with much weaker currents. Transistors are very common in electronics, because the output of any controller can rarely produce a current of more than 40 mA, therefore, even 2-3 low-power LEDs cannot be powered directly from the microcontroller. This is where transistors come to the rescue. The article discusses the main types of transistors, the differences between P-N-P and N-P-N bipolar transistors, P-channel and N-channel field-effect transistors, discusses the main subtleties of connecting transistors and reveals their scope of application.
Do not confuse a transistor with a relay. A relay is a simple switch. The essence of its work is to close and open metal contacts. The transistor is more complex and its operation is based on an electron-hole transition. If you are interested in learning more about this, you can watch an excellent video that describes the operation of a transistor from simple to complex. Don’t be confused by the year the video was produced - the laws of physics have not changed since then, and a newer video that presents the material so well could not be found:
Types of transistors
Bipolar transistor
The bipolar transistor is designed to control weak loads (for example, low-power motors and servos). It always has three outputs:
- Base - current is supplied or turned off to open or close the transistor
Collector - high voltage is supplied, which the transistor controls
Emitter (English: emitter) - “output” output of a transistor. Current flows through it from the collector and base.
The bipolar transistor is controlled by current. The more current supplied to the base, the more current will flow from the collector to the emitter. The ratio of the current passing from the emitter to the collector to the current at the base of the transistor is called the gain. Denoted as h fe (in English literature it is called gain).
For example, if h fe= 150, and 0.2 mA passes through the base, then the transistor will pass a maximum of 30 mA through itself. If a component that draws 25 mA (such as an LED) is connected, 25 mA will be provided to it. If a component that draws 150 mA is connected, it will only be provided with the maximum 30 mA. The documentation for the contact indicates the maximum permissible values of currents and voltages base-> emitter And collector -> emitter . Exceeding these values leads to overheating and failure of the transistor.
Funny pictures:
NPN and PNP bipolar transistors
There are 2 types of polar transistors: NPN And PNP. They differ in the alternation of layers. N (from negative) is a layer with an excess of negative charge carriers (electrons), P (from positive) is a layer with an excess of positive charge carriers (holes). More information about electrons and holes is described in the video above.
The behavior of transistors depends on the alternation of layers. The animation above shows NPN transistor. IN PNP transistor control is the other way around - current flows through the transistor when the base is grounded and is blocked when current is passed through the base. As shown in the diagram PNP And NPN differ in the direction of the arrow. The arrow always points to the transition from N To P:
Designation of NPN (left) and PNP (right) transistors in the diagram
NPN transistors are more common in electronics because they are more efficient.
Field effect transistor
Field-effect transistors differ from bipolar transistors in their internal structure. MOS transistors are the most common in amateur electronics. MOS is an acronym for metal-oxide-conductor. The same in English: Metal-Oxide-Semiconductor Field Effect Transistor, abbreviated as MOSFET. MOS transistors allow you to control high powers with relatively small sizes of the transistor itself. The transistor is controlled by voltage, not current. Since the transistor is controlled by electrical field, the transistor got its name - field howl.
Field effect transistors have at least 3 terminals:
Drain - high voltage is applied to it, which you want to control
Gate - voltage is applied to it to control the transistor
Source - current flows through it from the drain when the transistor is “open”
There should be an animation with a field-effect transistor, but it will not differ in any way from a bipolar transistor except for the schematic display of the transistors themselves, so there will be no animation.
N channel and P channel field effect transistors
Field-effect transistors are also divided into 2 types depending on the device and behavior. N channel(N channel) opens when voltage is applied to the gate and closes. when there is no voltage. P channel(P channel) works the other way around: while there is no voltage at the gate, current flows through the transistor. When voltage is applied to the gate, the current stops. In the diagram, field-effect transistors are depicted slightly differently:
By analogy with bipolar transistors, field-effect transistors differ in polarity. The N-Channel transistor was described above. They are the most common.
P-Channel when designated differs in the direction of the arrow and, again, has an “inverted” behavior.
There is a misconception that a field effect transistor can control alternating current. This is wrong. To control alternating current, use a relay.
Darlington transistor
It is not entirely correct to classify the Darlington transistor as a separate type of transistor. However, it is impossible not to mention them in this article. The Darlington transistor is most often found in the form of a microcircuit that includes several transistors. For example, ULN2003. The Darlington transistor is characterized by the ability to quickly open and close (and therefore allows it to work with) and at the same time withstand high currents. It is a type of compound transistor and is a cascade connection of two or, rarely, more transistors connected in such a way that the load in the emitter of the previous stage is the base-emitter transition of the transistor of the next stage, that is, the transistors are connected by collectors, and the emitter of the input transistor is connected to the base day off. In addition, the resistive load of the emitter of the previous transistor can be used as part of the circuit to speed up the closing. Such a connection as a whole is considered as one transistor, the current gain of which, when the transistors are operating in the active mode, is approximately equal to the product of the gains of all transistors.
Transistor connection
It is no secret that the Arduino board is capable of supplying a voltage of 5 V to the output with a maximum current of up to 40 mA. This current is not enough to connect a powerful load. For example, if you try to connect an LED strip or motor directly to the output, you are guaranteed to damage the Arduino output. It is possible that the entire board will fail. Additionally, some connected components may require more than 5V to operate. The transistor solves both of these problems. It will help, using a small current from the Arduino pin, to control a powerful current from a separate power supply, or using a voltage of 5 V to control a higher voltage (even the weakest transistors rarely have a maximum voltage below 50 V). As an example, consider connecting a motor:
In the diagram above, the motor is connected to a separate power source. Between the motor contact and the power supply for the motor, we placed a transistor, which will be controlled using any Arduino digital pin. When we apply a HIGH signal to the controller output from the controller output, we will take a very small current to open the transistor, and a large current will flow through the transistor and will not damage the controller. Pay attention to the resistor installed between the Arduino pin and the base of the transistor. It is needed to limit the current flowing along the microcontroller - transistor - ground route and prevent short circuits. As mentioned earlier, the maximum current that can be drawn from the Arduino pin is 40 mA. Therefore, we will need a resistor of at least 125 Ohm (5V/0.04A=125 Ohm). You can safely use a 220 Ohm resistor. In fact, the resistor should be selected taking into account the current that must be supplied to the base to obtain the required current through the transistor. To select the correct resistor, you need to take into account the gain factor ( h fe).
IMPORTANT!! If you connect a powerful load from a separate power supply, then you need to physically connect the ground (“minus”) of the load power supply and the ground (“GND” pin) of the Arduino. Otherwise, you won’t be able to control the transistor.
When using a field effect transistor, a current limiting resistor on the gate is not needed. The transistor is controlled solely by voltage and no current flows through the gate.