Bipolar transistor - a detailed description of all parameters of a semiconductor. Bipolar transistors. Types and characteristics. Work and device
A transistor is a semiconductor device that can amplify, convert and generate electrical signals. The first functional bipolar transistor was invented in 1947. The material for its manufacture was germanium. And already in 1956, the silicon transistor was born.
A bipolar transistor uses two types of charge carriers - electrons and holes, which is why such transistors are called bipolar. In addition to bipolar ones, there are unipolar (field-effect) transistors, which use only one type of carrier - electrons or holes. This article will discuss.
Most silicon transistors have an n-p-n structure, which is also explained by production technology, although there are also silicon transistors of the p-n-p type, but there are slightly fewer of them than n-p-n structures. Such transistors are used as part of complementary pairs (transistors of different conductivity with the same electrical parameters). For example, KT315 and KT361, KT815 and KT814, and in the output stages of transistor UMZCH KT819 and KT818. Imported amplifiers often use the powerful complementary pair 2SA1943 and 2SC5200.
Transistors with a p-n-p structure are often called forward conduction transistors, and n-p-n structures are called reverse conduction transistors. For some reason, this name almost never appears in the literature, but among radio engineers and radio amateurs it is used everywhere, everyone immediately understands what we are talking about. Figure 1 shows a schematic design of transistors and their graphic symbols.
Picture 1.
In addition to differences in type of conductivity and material, bipolar transistors are classified by power and operating frequency. If the power dissipation on a transistor does not exceed 0.3 W, such a transistor is considered low-power. With a power of 0.3...3 W, the transistor is called a medium-power transistor, and with a power of over 3 W, the power is considered high. Modern transistors are able to dissipate power of several tens and even hundreds of watts.
Transistors do not amplify electrical signals equally well: as the frequency increases, the gain of the transistor cascade decreases, and at a certain frequency it stops altogether. Therefore, to operate over a wide frequency range, transistors are produced with different frequency properties.
Based on the operating frequency, transistors are divided into low-frequency - operating frequency not exceeding 3 MHz, mid-frequency - 3...30 MHz, high-frequency - over 30 MHz. If the operating frequency exceeds 300 MHz, then these are already ultra-high-frequency transistors.
In general, serious thick reference books list over 100 different parameters of transistors, which also indicates a huge number of models. And the number of modern transistors is such that it is no longer possible to place them in full in any reference book. And the model range is constantly increasing, allowing us to solve almost all the tasks set by the developers.
There are many transistor circuits (just remember the number of household equipment) for amplifying and converting electrical signals, but, despite all the diversity, these circuits consist of separate cascades, the basis of which are transistors. To achieve the required signal amplification, it is necessary to use several amplification stages connected in series. To understand how amplifier stages work, you need to become more familiar with the transistor switching circuits.
The transistor itself cannot amplify anything. Its amplifying properties lie in the fact that small changes in the input signal (current or voltage) lead to significant changes in voltage or current at the output of the stage due to the expenditure of energy from an external source. It is this property that is widely used in analog circuits - amplifiers, television, radio, communications, etc.
To simplify the presentation, circuits based on n-p-n transistors will be considered here. Everything that will be said about these transistors applies equally to pnp transistors. It is enough just to change the polarity of the power supplies, and, if any, to get a working circuit.
In total, three such circuits are used: a circuit with a common emitter (CE), a circuit with a common collector (OC) and a circuit with a common base (CB). All these schemes are shown in Figure 2.
Figure 2.
But before moving on to considering these circuits, you should get acquainted with how a transistor works in switching mode. This introduction should make it easier to understand in boost mode. In a certain sense, the key circuit can be considered as a type of circuit with OE.
Transistor operation in switching mode
Before studying the operation of a transistor in signal amplification mode, it is worth remembering that transistors are often used in switching mode.
This mode of operation of the transistor has been considered for a long time. The August 1959 issue of Radio magazine published an article by G. Lavrov, “Semiconductor triode in switch mode.” The author of the article proposed changing the duration of the pulses in the control winding (OC). Now this control method is called PWM and is used quite often. A diagram from a magazine of the time is shown in Figure 3.
Figure 3.
But key mode is used not only in PWM systems. Often a transistor simply turns something on and off.
In this case, a relay can be used as a load: if an input signal is given, the relay is turned on; if not, the relay signal is turned off. Instead of relays, light bulbs are often used in key mode. This is usually done to indicate: the light is either on or off. The diagram of such a key stage is shown in Figure 4. Key stages are also used to work with LEDs or optocouplers.
Figure 4.
In the figure, the cascade is controlled by a regular contact, although there may be a digital chip or instead. A car light bulb, this one is used to illuminate the dashboard in Zhiguli cars. You should pay attention to the fact that the control voltage is 5V, and the switched collector voltage is 12V.
There is nothing strange about this, since voltages do not play any role in this circuit, only currents matter. Therefore, the light bulb can be at least 220V, if the transistor is designed to operate at such voltages. The collector source voltage must also match the operating voltage of the load. Using such cascades, the load is connected to digital chips or microcontrollers.
In this circuit, the base current controls the collector current, which, due to the energy of the power source, is several tens or even hundreds of times greater (depending on the collector load) than the base current. It is easy to see that current amplification occurs. When a transistor operates in switching mode, a value is usually used to calculate the cascade, which is called in reference books “current gain in large-signal mode” - in reference books it is denoted by the letter β. This is the ratio of the collector current, determined by the load, to the minimum possible base current. In the form of a mathematical formula, it looks like this: β = Ik/Ib.
For most modern transistors, the coefficient β is quite large, as a rule, from 50 and above, so when calculating the key stage it can be taken equal to only 10. Even if the base current turns out to be greater than the calculated one, then the transistor will not open stronger because of this; and key mode.
To light the light bulb shown in Figure 3, Ib = Ik/β = 100mA/10 = 10mA, this is the minimum. With a control voltage of 5V across the base resistor Rb, minus the voltage drop in section B-E, there will remain 5V - 0.6V = 4.4V. The resistance of the base resistor will be: 4.4V / 10mA = 440 Ohms. A resistor with a resistance of 430 Ohms is selected from the standard range. A voltage of 0.6V is the voltage at the B-E junction, and you should not forget about it when calculating!
To ensure that the base of the transistor does not remain “hanging in the air” when the control contact opens, the B-E junction is usually shunted by a resistor Rbe, which reliably closes the transistor. This resistor should not be forgotten, although in some circuits for some reason it is not present, which can lead to false operation of the cascade due to interference. Actually, everyone knew about this resistor, but for some reason they forgot, and once again stepped on the “rake”.
The value of this resistor must be such that when the contact opens, the voltage at the base would not be less than 0.6V, otherwise the cascade will be uncontrollable, as if section B-E was simply short-circuited. In practice, resistor Rbe is installed with a nominal value approximately ten times greater than Rb. But even if the Rb rating is 10K, the circuit will work quite reliably: the base and emitter potentials will be equal, which will lead to the transistor closing.
Such a key cascade, if it is working properly, can turn the light bulb on at full intensity, or turn it off completely. In this case, the transistor can be completely open (saturation state) or completely closed (cut-off state). Immediately, the conclusion suggests itself that between these “boundary” states there is such a thing when the light bulb shines at full intensity. In this case, is the transistor half open or half closed? It's like the filling-a-glass problem: an optimist sees the glass as half full, while a pessimist sees it as half empty. This mode of operation of the transistor is called amplification or linear.
Transistor operation in signal amplification mode
Almost all modern electronic equipment consists of microcircuits in which transistors are “hidden”. It is enough to simply select the operating amplifier mode to obtain the required gain or bandwidth. But, despite this, cascades on discrete (“scattered”) transistors are often used, and therefore an understanding of the operation of the amplifier stage is simply necessary.
The most common connection of a transistor compared to OK and OB is a common emitter (CE) circuit. The reason for this prevalence is, first of all, the high voltage and current gain. The highest gain of the OE cascade is achieved when half the power supply voltage Epit/2 drops at the collector load. Accordingly, the second half falls in the K-E section of the transistor. This is achieved by setting up the cascade, which will be discussed below. This amplification mode is called class A.
When the OE transistor is turned on, the output signal at the collector is out of phase with the input. As disadvantages, it can be noted that the input impedance of the OE is small (no more than a few hundred ohms), and the output impedance is within tens of kOhms.
If in the switching mode the transistor is characterized by a current gain in large-signal mode β, then in amplification mode the “current gain in small-signal mode” is used, designated h21e in reference books. This designation comes from the representation of a transistor as a four-terminal network. The letter “e” indicates that the measurements were made when a transistor with a common emitter was turned on.
The coefficient h21e, as a rule, is slightly larger than β, although it can also be used in calculations as a first approximation. All the same, the spread of parameters β and h21e is so large even for one type of transistor that the calculations are only approximate. After such calculations, as a rule, configuration of the circuit is required.
The gain of the transistor depends on the thickness of the base, so it cannot be changed. Hence the large spread in the gain of transistors taken even from the same box (read one batch). For low-power transistors this coefficient ranges from 100...1000, and for high-power ones 5...200. The thinner the base, the higher the coefficient.
The simplest circuit for switching on an OE transistor is shown in Figure 5. This is just a small piece from Figure 2, shown in the second part of the article. This type of circuit is called a fixed base current circuit.
Figure 5.
The scheme is extremely simple. The input signal is fed to the base of the transistor through the decoupling capacitor C1, and, being amplified, is removed from the collector of the transistor through capacitor C2. The purpose of capacitors is to protect the input circuits from the constant component of the input signal (just remember a carbon or electret microphone) and provide the necessary cascade bandwidth.
Resistor R2 is the collector load of the cascade, and R1 supplies a constant bias to the base. Using this resistor, they try to make sure that the voltage at the collector is Epit/2. This state is called the operating point of the transistor; in this case, the gain of the cascade is maximum.
Approximately the resistance of resistor R1 can be determined by the simple formula R1 ≈ R2 * h21e / 1.5...1.8. The coefficient 1.5...1.8 is adjusted depending on the supply voltage: at low voltage (no more than 9V) the coefficient value is no more than 1.5, and starting from 50V it approaches 1.8...2.0. But, indeed, the formula is so approximate that resistor R1 most often has to be selected, otherwise the required value of Epit/2 at the collector will not be obtained.
Collector resistor R2 is specified as a condition of the problem, since the collector current and the gain of the cascade as a whole depend on its value: the greater the resistance of resistor R2, the higher the gain. But you need to be careful with this resistor; the collector current must be less than the maximum permissible for this type of transistor.
The circuit is very simple, but this simplicity also gives it negative properties, and you have to pay for this simplicity. Firstly, the gain of the cascade depends on the specific instance of the transistor: if you replaced the transistor during repair, select the bias again, bring it to the operating point.
Secondly, it depends on the ambient temperature - with increasing temperature, the reverse collector current Iko increases, which leads to an increase in the collector current. And where then is half the supply voltage at the collector Epit/2, that same operating point? As a result, the transistor heats up even more, after which it fails. To get rid of this dependence, or at least reduce it to a minimum, additional negative feedback elements - OOS - are introduced into the transistor cascade.
Figure 6 shows a circuit with a fixed bias voltage.
Figure 6.
It would seem that the voltage divider Rb-k, Rb-e will provide the required initial bias of the cascade, but in fact, such a cascade has all the disadvantages of a circuit with a fixed current. Thus, the circuit shown is just a variation of the fixed current circuit shown in Figure 5.
Temperature-stabilized circuits
The situation is somewhat better when using the circuits shown in Figure 7.
Figure 7.
In a collector-stabilized circuit, bias resistor R1 is connected not to the power source, but to the collector of the transistor. In this case, if the reverse current increases as the temperature increases, the transistor opens more strongly, and the voltage on the collector decreases. This reduction results in a decrease in the bias voltage supplied to the base through R1. The transistor begins to close, the collector current decreases to an acceptable value, and the position of the operating point is restored.
It is quite obvious that such a stabilization measure leads to some reduction in the gain of the cascade, but this does not matter. The missing gain is usually added by increasing the number of amplification stages. But such environmental protection allows you to significantly expand the range of operating temperatures of the cascade.
The circuit design of a cascade with emitter stabilization is somewhat more complex. The amplifying properties of such cascades remain unchanged over an even wider temperature range than that of a collector-stabilized circuit. And one more undeniable advantage is that when replacing a transistor, you do not have to re-select the operating modes of the cascade.
Emitter resistor R4, while providing temperature stabilization, also reduces the cascade gain. This is for DC. In order to eliminate the influence of resistor R4 on the amplification of alternating current, resistor R4 is shunted by capacitor Ce, which for alternating current represents insignificant resistance. Its value is determined by the frequency range of the amplifier. If these frequencies lie in the audio range, then the capacitance of the capacitor can be from units to tens and even hundreds of microfarads. For radio frequencies this is already hundredths or thousandths, but in some cases the circuit works fine without this capacitor.
In order to better understand how emitter stabilization works, we need to consider the connection circuit of a transistor with a common collector OK.
A circuit with a common collector (OC) is shown in Figure 8. This circuit is a piece of Figure 2, from the second part of the article, which shows all three circuits for connecting transistors.
Figure 8.
The load of the cascade is the emitter resistor R2, the input signal is supplied through capacitor C1, and the output signal is removed through capacitor C2. Here you can ask why this scheme is called OK? After all, if you recall the OE circuit, you can clearly see that the emitter is connected to the common wire of the circuit, relative to which the input signal is supplied and the output signal is removed.
In the OK circuit, the collector is simply connected to the power source, and at first glance it seems that it has nothing to do with the input and output signals. But in fact, the EMF source (battery) has a very small internal resistance; for the signal it is practically one point, the same contact.
The operation of the OK circuit can be examined in more detail in Figure 9.
Figure 9.
It is known that for silicon transistors the b-e transition voltage is in the range of 0.5...0.7 V, so you can take it on average 0.6 V, if you do not set out to carry out calculations with an accuracy of tenths of a percent. Therefore, as can be seen in Figure 9, the output voltage will always be less than the input voltage by the value Ub-e, namely by the same 0.6V. Unlike the OE circuit, this circuit does not invert the input signal, it simply repeats it, and even reduces it by 0.6V. This circuit is also called an emitter follower. Why is such a scheme needed, what is its benefit?
The OK circuit amplifies the current signal by h21e times, which indicates that the input resistance of the circuit is h21e times greater than the resistance in the emitter circuit. In other words, you can apply voltage directly to the base (without a limiting resistor) without fear of burning the transistor. Just take the base pin and connect it to the +U power bus.
High input impedance allows you to connect a high impedance (impedance) input source, such as a piezoelectric pickup. If such a pickup is connected to a cascade according to the OE circuit, then the low input impedance of this stage will simply “plant” the signal of the pickup - “the radio will not play.”
A distinctive feature of the OK circuit is that its collector current Ik depends only on the load resistance and the voltage of the input signal source. In this case, the transistor parameters do not play any role here at all. Such circuits are said to be covered by 100% voltage feedback.
As shown in Figure 9, the current in the emitter load (aka emitter current) Iн = Iк + Ib. Taking into account that the base current Ib is negligible compared to the collector current Ik, we can assume that the load current is equal to the collector current Il = Ik. The current in the load will be (Uin - Ube)/Rn. In this case, we will assume that Ube is known and is always equal to 0.6V.
It follows that the collector current Ik = (Uin - Ube)/Rn depends only on the input voltage and load resistance. The load resistance can be changed within wide limits, however, you don’t need to be particularly zealous. After all, if instead of Rn you put a nail - a hundred square meters, then no transistor will withstand it!
The OK circuit makes it quite easy to measure the static current transfer coefficient h21e. How to do this is shown in Figure 10.
Figure 10.
First, the load current should be measured as shown in Figure 10a. In this case, the base of the transistor does not need to be connected anywhere, as shown in the figure. After this, the base current is measured in accordance with Figure 10b. In both cases, measurements must be made in the same quantities: either in amperes or milliamps. The power supply voltage and load must remain the same for both measurements. To find out the static current transfer coefficient, it is enough to divide the load current by the base current: h21e ≈ In/Ib.
It should be noted that with increasing load current h21e decreases slightly, and with increasing supply voltage it increases. Emitter followers are often built in a push-pull circuit using complementary pairs of transistors, which increases the output power of the device. Such an emitter follower is shown in Figure 11.
Figure 11.
Figure 12.
Switching on transistors according to a circuit with a common OB base
This circuit provides only voltage gain, but has better frequency properties compared to the OE circuit: the same transistors can operate at higher frequencies. The main application of the OB circuit is antenna amplifiers for the UHF bands. The antenna amplifier circuit is shown in Figure 12.
Characteristics of bipolar transistors
Static mode transistor operation This is called a mode in which there is no load in the output circuit, and a change in input current or voltage does not cause a change in the output voltage Fig. 7.
Static characteristics There are two types of transistors: entrance and exit. In Fig.8. shows a diagram of an installation for measuring the static characteristics of a transistor connected according to a circuit with a common emitter.
Fig.8. Scheme
static measurements
parameters of the transistor with OE.
Input staticcharacteristic I B from input voltage U BE at constant output voltage U CE. For a common emitter circuit:
I B = f (U BE) at U EC = const.
Since the branches of the input static characteristic for U FE > 0 are located very close to each other and practically merge into one, then in practice one averaged characteristic can be used with sufficient accuracy (Fig. 9 A). A feature of the input static characteristic is the presence in the lower part of a nonlinear section in the bend area U 1(approximately 0.2...0.3 V for germanium transistors and 0.3...0.4 V for silicon ones).
Day off static characteristic is the dependence of the output current I K from output voltage U CE at constant input current I B. For a connection circuit with a common emitter:
I K = f (U KE) at I B = const.
From Fig.9 b It can be seen that the output characteristics are straight lines, almost parallel to the voltage axis. This is explained by the fact that the collector junction is closed regardless of the magnitude of the base-collector voltage, and the collector current is determined only by the number of charge carriers passing from the emitter through the base to the collector, i.e., by the emitter current I E.
Dynamic mode operation of a transistor is called such a mode in which there is a load resistor in the output circuit R K, due to which the change in input current or voltage U VX will cause a change in output voltage U OUT = U CE(Fig. 10).
Fig.9. Static characteristics of a transistor with OE: A– input; b- weekend.
Input dynamiccharacteristic is the dependence of the input current I B from input voltage U BE when there is a load. For a common emitter circuit:
I B = f (U BE)
Since in static mode for U FE > 0 we use one averaged characteristic, then input dynamic characteristic coincides with input static(Fig.11 A).
Fig. 10. Scheme for switching on a transistor in dynamic mode with OE.
Day off dynamic (load) characteristic represents the dependence of the output voltage U CE from output current I K at fixed input current values I B(Fig.11 b):
U KE = E K – I K R K
Since this equation is linear, then output dynamic characteristic is straight line and is built on the output static characteristics at two points, for example: A, IN in Fig. 11 b.
Point coordinates A [U CE = 0; I K = E K⁄ R K] – on the axis I K.
Point coordinates IN [I K = 0; U KE = E K] – on the axis U CE.
Point coordinates R [U 0K; I 0 K] – correspond to the position of the RT operating point in rest mode (in the absence of a signal).
 |  |
Fig. 11. Dynamic characteristics of a transistor with OE: A)– entrance; b)- day off.
The load line is drawn through any two points A, B, or P, the coordinates of which are known.
Depending on the state of the p-n junctions of the transistors, several types of its operation are distinguished - cut-off mode, saturation mode, limit and linear modes (Fig. 11).
Cutoff mode. This is a mode in which both of its transitions are closed - the transistor is locked. The base current in this case is zero. The collector current will be equal to the reverse current I K0, and voltage U KE = E K.
Saturation mode- this is a mode when both transitions - the emitter and the collector are open, and a free transition of charge carriers occurs in the transistor. In this case, the base current will be maximum, the collector current will be equal to the saturation collector current, and the voltage between the collector and emitter will tend to zero.
I B = max; I K ≈ I KN; U KE = E K – I KN R N; U CE → 0.
Limit modes– these are modes in which operation is limited by the maximum permissible parameters: I K additional, U CE additional, P K additional(Fig.11 b) And I B us, U BE extra(Fig.11 A) and is associated with overheating of the transistor or its failure.
Linear mode- This is a mode in which sufficient linearity of characteristics is ensured and it can be used for active amplification.
The transistor is a ubiquitous and important component in modern microelectronics. Its purpose is simple: it allows you to control a much stronger one using a weak signal.
In particular, it can be used as a controlled “damper”: by the absence of a signal at the “gate”, block the flow of current, and by supplying it, allow it. In other words: this is a button that is pressed not by a finger, but by applying voltage. This is the most common application in digital electronics.
Transistors are available in different packages: the same transistor can look completely different in appearance. In prototyping, the most common cases are:
TO-92 - compact, for light loads
TO-220AB - massive, good heat dissipation, for heavy loads
The designation on the diagrams also varies depending on the type of transistor and the designation standard used in the compilation. But regardless of the variation, its symbol remains recognizable.
Bipolar transistors
Bipolar junction transistors (BJT, Bipolar Junction Transistors) have three contacts:
Collector - high voltage is applied to it, which you want to control
Base - a small amount is supplied through it current to unlock large; the base is grounded to block it
Emitter - current flows through it from the collector and base when the transistor is “open”
The main characteristic of a bipolar transistor is the indicator hfe also known as gain. It reflects how many times more current in the collector-emitter section the transistor can pass in relation to the base-emitter current.
For example, if hfe= 100, and 0.1 mA passes through the base, then the transistor will pass through itself a maximum of 10 mA. If in this case there is a component in the high current section that consumes, for example, 8 mA, it will be provided with 8 mA, and the transistor will have a “reserve”. If there is a component that draws 20 mA, it will only be provided with the maximum 10 mA.
Also, the documentation for each transistor indicates the maximum permissible voltages and currents at the contacts. Exceeding these values leads to excessive heating and reduced service life, and a strong excess can lead to destruction.
NPN and PNP
The transistor described above is a so-called NPN transistor. It is called that because it consists of three layers of silicon connected in the order: Negative-Positive-Negative. Where negative is a silicon alloy with an excess of negative charge carriers (n-doped), and positive is an alloy with an excess of positive charge carriers (p-doped).
NPNs are more effective and common in industry.
When designating PNP transistors, they differ in the direction of the arrow. The arrow always points from P to N. PNP transistors have an “inverted” behavior: current is not blocked when the base is grounded and blocked when current flows through it.
Field effect transistors
Field effect transistors (FET, Field Effect Transistor) have the same purpose, but differ in internal structure. A particular type of these components are MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) transistors. They allow you to operate with much greater power with the same dimensions. And the control of the “damper” itself is carried out exclusively using voltage: no current flows through the gate, unlike bipolar transistors.
Field effect transistors have three contacts:
Drain - high voltage is applied to it, which you want to control
Gate - voltage is applied to it to allow current to flow; the gate is grounded to block the current.
Source - current flows through it from the drain when the transistor is “open”
N-Channel and P-Channel
By analogy with bipolar transistors, field 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.
Connecting transistors to drive high-power components
A typical task of a microcontroller is to turn a specific circuit component on and off. The microcontroller itself usually has modest power handling characteristics. So Arduino, with 5 V output per pin, can withstand a current of 40 mA. Powerful motors or ultra-bright LEDs can draw hundreds of milliamps. When connecting such loads directly, the chip can quickly fail. In addition, for the operation of some components, a voltage greater than 5 V is required, and Arduino cannot produce more than 5 V from the digital output pin.
But it is easily enough to control a transistor, which in turn will control a large current. Let's say we need to connect a long LED strip that requires 12 V and consumes 100 mA:
Now, when the output is set to logical one (high), the 5 V entering the base will open the transistor and current will flow through the tape - it will glow. When the output is set to logic zero (low), the base will be grounded through the microcontroller and current flow will be blocked.
Pay attention to the current limiting resistor R. It is necessary so that when control voltage is applied, a short circuit does not form along the route microcontroller - transistor - ground. The main thing is not to exceed the permissible current through the Arduino contact of 40 mA, so you need to use a resistor with a value of at least:
Here Ud- this is the voltage drop across the transistor itself. It depends on the material from which it is made and is usually 0.3 – 0.6 V.
But it is absolutely not necessary to keep the current at the permissible limit. It is only necessary that the gain of the transistor allows it to control the required current. In our case it is 100 mA. Acceptable for the transistor used hfe= 100, then a control current of 1 mA will be enough for us
A resistor with a value from 118 Ohm to 4.7 kOhm is suitable for us. For stable operation on one side and light load on the chip on the other, 2.2 kOhm is a good choice.
If you use a field-effect transistor instead of a bipolar transistor, you can do without a resistor:
This is due to the fact that the gate in such transistors is controlled solely by voltage: there is no current in the microcontroller - gate - source section. And thanks to its high characteristics, a circuit using MOSFETs allows you to control very powerful components.
In this article we discussed such an important transistor parameter as the beta coefficient (β) . But there is another interesting parameter in the transistor. In itself he is insignificant, but he can do a lot of business! It’s like a pebble that gets into an athlete’s sneaker: it seems small, but it causes inconvenience when running. So what does this “pebble” interfere with the transistor? Let's find out...
Direct and reverse connection of PN junction
As we remember, a transistor consists of three semiconductors. , which we call base-emitter emitter junction, and the base-collector transition is collector transition.
Since in this case we have an NPN transistor, it means that the current will flow from the collector to the emitter, provided that we open the base by applying a voltage of more than 0.6 Volts to it (well, so that the transistor opens).
Let's hypothetically take a thin, thin knife and cut out the emitter directly along the PN junction. We'll end up with something like this:
Stop! Have we got a diode? Yes, he is the one! Remember, in the article current-voltage characteristic (CVC), we looked at the CVC of the diode:
On the right side of the current-voltage characteristic, we see how the branch of the graph very sharply flew up. In this case, we applied a constant voltage to the diode like this, that is, it was direct connection of the diode.
The diode passed electric current through itself. We even conducted experiments with direct and reverse connection of the diode. Those who don’t remember can read it.
But if you change the polarity
then our diode will not pass current. We have always been taught this way, and there is some truth in it, but... our world is not ideal).
How does a PN junction work? We imagined it as a funnel. So, for this drawing
our funnel will be turned upside down towards the stream
The direction of water flow is the direction of electric current. The funnel is the diode. But the water that got through the narrow neck of the funnel? What can it be called? And it's called reverse current of PN junction (I return).
What do you think, if you increase the speed of water flow, will the amount of water that passes through the narrow neck of the funnel increase? Definitely! This means that if you add voltage U arr., then the reverse current will increase I arr., which is what we see on the left side of the graph of the diode’s current-voltage characteristic:
But to what limit can the speed of water flow be increased? If it is very large, our funnel will not hold up, the walls will crack and it will fly into pieces, right? Therefore, for each diode you can find a parameter such as U rev.max, exceeding which for a diode is equivalent to death.
For example, for diode D226B:
U rev.max= 500 Volts, and the maximum reverse pulse U arr. imp.max= 600 Volts. But keep in mind that electronic circuits are designed, as they say, “with a 30% margin.” And even if in the circuit the reverse voltage on the diode is 490 Volts, then a diode that can withstand more than 600 Volts will be installed in the circuit. It is better not to play with critical values). Pulse reverse voltage is a sudden surge in voltage that can reach an amplitude of up to 600 volts. But here, too, it is better to take with a small margin.
So... why am I all this about the diode and about the diode... It’s like we’re studying transistors. But whatever one may say, a diode is a building block for building a transistor. So, if we apply a reverse voltage to the collector junction, then a reverse current will flow through the junction, as in a diode? Exactly. And this parameter in a transistor is called . We denote it as I KBO, among the bourgeoisie - I CBO. Stands for “current between collector and base, with emitter open”. Roughly speaking, the emitter leg does not cling anywhere and hangs in the air.
To measure the collector reverse current, it is enough to assemble these simple circuits:
For NPN transistor for PNP transistor
For silicon transistors, the reverse collector current is less than 1 µA, for germanium transistors: 1-30 µA. Since I only measure from 10 µA, and I don’t have germanium transistors at hand, I won’t be able to carry out this experiment, since the resolution of the device does not allow it.
We still haven’t answered the question, why is collector reverse current so important and is listed in reference books? The thing is that during operation the transistor dissipates some power into space, which means it heats up. The reverse collector current is very dependent on temperature and doubles its value for every 10 degrees Celsius. No, but what's wrong? Let it grow, it doesn’t seem to bother anyone.
Effect of reverse collector current
The thing is that in some switching circuits part of this current passes through the emitter junction. And as we remember, the base current flows through the emitter junction. The greater the control current (base current), the greater the controlled current (collector current). We discussed this in the article. Therefore, the slightest change in the base current leads to a large change in the collector current and the entire circuit begins to work incorrectly.
How to combat reverse collector current
This means that the most important enemy of the transistor is temperature. How do radio-electronic equipment (REA) developers fight it?
- use transistors in which the reverse collector current has a very small value. These are, of course, silicon transistors. A little hint - the marking of silicon transistors begins with the letters "KT", which means TO belt T transistor.
- use of circuits that minimize collector reverse current.
Reverse collector current is an important parameter of the transistor. It is given in the datasheet for each transistor. In circuits that are used under extreme temperature conditions, collector return current will play a very large role. Therefore, if you are assembling a circuit that does not use a radiator and fan, then, of course, it is better to take transistors with minimal reverse collector current.
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
The 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 RKP power released in the transistor in rest mode, which should not exceed the maximum RK power max, which is one of the transistor parameters:
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.
