Bipolar transistors. Bipolar transistors: switching circuits. Connection circuit for a bipolar transistor with a common emitter

Scheme with common emitter

The common emitter (CE) circuit is shown in Fig. 1.11. Transistor p-p-p in this scheme it works the same way as in the scheme with OB. Let us only note that the generally accepted direction of currents (from +E TO – voltage source), indicated in Fig. 1.11, A, opposite to the direction of electron motion. A characteristic feature of a circuit with OE is that the load is located in the collector circuit (Fig. 1.11.6).

Rice. 1.11. Connection diagram of a transistor with a common emitter (a); typical image in diagrams(b)

Just like for the circuit with OB, the input signal in this circuit is the voltage between the base and the emitter, and the output values ​​are the collector current I k and load voltage U out = I To R n A transistor in a circuit with an OE is characterized by the current transfer coefficient

having values ​​β = 10... 100, which is associated with the coefficient α for a circuit with the OB relation:

Let us estimate the values ​​of the gain factors of the circuit with OE (they are denoted by the index “E”).

The output current, as in the circuit with OB, is the current I k, flowing through the load, and the input current (unlike the circuit with OB) is the base current I B; the current gain of the circuit with OE is equal to

At α = 0.98 KIE = 0.98/(1 – 0.98) ≈ 50, i.e. several dozen, which is many times higher than the similar coefficient for the circuit with OB.

The input resistance in the circuit with OE is also significantly higher than in the circuit with OB, since in the circuit with OE the input current is the base current, and in the circuit with OB it is many times greater emitter current (namely 1/(1 – α ) ≈ β times):

The value of the input resistance in the circuit with OE is ≈ β times greater than in the circuit with OB and amounts to hundreds of ohms.

The voltage gain in the circuit with OE is comparable to the same coefficient in the circuit with OB:

In terms of power gain, the circuit with OE, due to the significantly higher current gain, is also many times superior to the circuit with OB:

and depends on the current transfer coefficient β and the ratio of the load resistance to the input resistance.

Thanks to the noted properties, the OE circuit has found very wide application.

Input and output characteristics of a common emitter circuit

The operation of a circuit is usually described using the input and output characteristics of the transistor in a particular circuit. For a circuit with an OE, the input characteristic is the dependence of the input current on the voltage at the input of the circuit, i.e. I B = f (UBE) at fixed collector-emitter voltage values ​​( U ke = const).

Output characteristics are the dependences of the output current, i.e. collector current, from the voltage drop between the collector and emitter of the transistor I k = f (And BE ) at base current I B = const.

The input characteristic essentially repeats the form of the diode characteristic when applying forward voltage (Fig. 1.12, b). With increasing tension U The KE input characteristic will shift slightly to the right.

Rice. 1.12. Output(s) and input(b ) characteristics of a transistor in a circuit with a common emitter

Type of output characteristics (Fig. 1.12, A) sharply different in the region of small (section OA) and relatively large values U ke. Let us recall that for normal operation transistor, it is necessary that a direct voltage is applied to the base-emitter junction, and a reverse voltage is applied to the base-collector junction. Therefore, while |1/ke|< 1/БЭ, напряжение на коллекторном переходе оказывается прямым, что резко уменьшает ток I j. With |UKE| > U BE voltage at the collector junction UБK = UКЭ – U The BE becomes reversed and, therefore, has little effect on the magnitude of the collector current, which is determined mainly by the emitter current. At this voltage, all carriers injected by the emitter into the base and passing through the base region rush to external source. At voltage UBE< 0 эмиттер носителей не инжектирует и ток базы I B = 0, but current flows in the collector circuit I K0 (lowest output characteristic). This current corresponds to the reverse current I 0 of a regular p-n junction.

When the transistor operates, its mode changes. Indeed, the greater the current flowing through the transistor, the greater the voltage drop across the load, and therefore, the less voltage will drop across the transistor itself. The characteristics presented in Fig. 1.12, a, b, describe only static mode operation of the circuit. To assess the dynamics and influence of the load on the operation of the circuit, a graphic-analytical calculation method is used based on input and output characteristics. Let's consider this method using the example of the input and output characteristics of a circuit with an OE.

Let us draw a straight line through the point Ek, plotted on the abscissa axis, and the point E To /R n plotted on the ordinate axis of the output characteristics of the transistor. The resulting straight line is called load. Dot E To /R n this straight line corresponds to the current that could flow through the load if the transistor is short-circuited. Dot E k corresponds to another extreme case - the circuit is open, the current through the load is zero, and the voltage Uke is equal to E j. Point r intersection of the load line with the static output characteristic corresponding to the input current I B, will determine the operating mode of the circuit, i.e. load current I k, voltage drop across it U n = I To R n and voltage drop (/ke on the transistor itself. In Fig. 1.12 , A dot r corresponds to the supply of base current to the transistor I B = 1 mA. It is easy to see that the supply of base current I B = 2 mA leads to a shift of the operating point to the point A and redistribution of voltages between the load and the transistor.

Example 1.1. Calculate the circuit with OE and R n =110 Ohm at input voltage UBE = +0.1 V, supply voltage E k = +25 V, using the characteristics of the transistor.

Solution. Let's find a relationship E K /R n = 25/110 = 228 mA and, putting the found point on the axis I k and value E k = +25 V on the Uke axis, let’s draw a load straight line.

By input characteristic for voltage 1/BE = 0.1 V, we determine the input current I B = 1 mL.

Intersection point r straight with characteristic corresponding I B = 1 mA, will determine the current I k = 150 mA.

The load voltage is

Voltage between collector and emitter of transistor

In conclusion, we note that the mode corresponding to the point A, called saturation mode (at given values R n and E k current I to at the point A reaches the highest possible value). Mode corresponding to point IN (input signal is zero), as well as the point WITH (the input signal is negative and turns off the transistor), called cut-off mode. All intermediate states of the transistor with a load between the points A And IN refer to active mode his work.

Greetings, dear friends! Today we will talk about bipolar transistors and the information will be useful primarily for beginners. So, if you are interested in what a transistor is, its operating principle and in general what it is used for, then take a more comfortable chair and come closer.

Let's continue, and we have content here, it will be more convenient to navigate the article :)

Types of transistors

Transistors are mainly of two types: bipolar transistors and field effect transistors. Of course, it was possible to consider all types of transistors in one article, but I don’t want to cook porridge in your head. Therefore, in this article we will look exclusively at bipolar transistors, and I will talk about field-effect transistors in one of the following articles. Let's not lump everything together, but pay attention to each one individually.

Bipolar transistor

The bipolar transistor is a descendant of tube triodes, those that were in televisions of the 20th century. Triodes went into oblivion and gave way to more functional brothers - transistors, or rather bipolar transistors.

With rare exceptions, triodes are used in equipment for music lovers.

Bipolar transistors may look like this.

As you can see, bipolar transistors have three terminals and structurally they can look completely different. But on electrical diagrams they look simple and always the same. And all this graphic splendor looks something like this.

This image of transistors is also called UGO (Conventional graphic symbol).

Moreover, bipolar transistors can have different types of conductivity. Eat NPN transistors type and PNP type.

The difference between an n-p-n transistor and a p-n-p transistor is only that it is a “carrier” electric charge(electrons or “holes”). Those. For a pnp transistor, electrons move from the emitter to the collector and are driven by the base. For an n-p-n transistor, electrons go from the collector to the emitter and are controlled by the base. As a result, we come to the conclusion that in order to replace a transistor of one conductivity type with another in a circuit, it is enough to change the polarity of the applied voltage. Or stupidly change the polarity of the power source.

Bipolar transistors have three terminals: collector, emitter and base. I think that it will be difficult to get confused with the UGO, but in a real transistor it’s easier than ever to get confused.

Usually where which output is determined is from the reference book, but you can simply. The terminals of the transistor sound like two diodes connected at a common point (in the area of ​​the base of the transistor).

On the left is a picture for a p-n-p type transistor; when testing, you get the feeling (through multimeter readings) that in front of you are two diodes that are connected at one point by their cathodes. For transistor n-p-n type The diodes at the base point are connected by their anodes. I think after experimenting with a multimeter it will be more clear.

The principle of operation of a bipolar transistor

Now we will try to figure out how a transistor works. I won't go into details internal structure transistors since this information only confuses. Better take a look at this drawing.

This image best explains the working principle of a transistor. In this image, a person controls the collector current using a rheostat. He looks at the base current, if the base current increases, then the person also increases the collector current, taking into account the gain of the transistor h21E. If the base current drops, then the collector current will also decrease - the person will correct it using a rheostat.

This analogy has nothing to do with real work transistor, but it makes it easier to understand the principles of its operation.

For transistors, rules can be noted to help make things easier to understand. (These rules are taken from the book).

1. The collector has a more positive potential than the emitter
2. As I already said, the base-collector and base-emitter circuits work like diodes
3. Each transistor is characterized by limiting values ​​such as collector current, base current and collector-emitter voltage.
4. If rules 1-3 are followed, then the collector current Ik is directly proportional to the base current Ib. This relationship can be written as a formula.

From this formula we can express the main property of a transistor - a small base current controls a large collector current.

Current gain.

It is also denoted as

Based on the above, the transistor can operate in four modes:

1. Transistor cut-off mode— in this mode the base-emitter junction is closed, this can happen when the base-emitter voltage is insufficient. As a result, there is no base current and therefore there will be no collector current either.
2. Transistor active mode- this is the normal mode of operation of the transistor. In this mode, the base-emitter voltage is sufficient to cause the base-emitter junction to open. The base current is sufficient and the collector current is also available. The collector current is equal to the base current multiplied by the gain.
3. Transistor saturation mode - The transistor switches to this mode when the base current becomes so large that the power of the power source is simply not enough to further increase the collector current. In this mode, the collector current cannot increase following an increase in the base current.
4. Inverse transistor mode— this mode is used extremely rarely. In this mode, the collector and emitter of the transistor are swapped. As a result of such manipulations, the gain of the transistor suffers greatly. The transistor was not originally designed to operate in such a special mode.

To understand how a transistor works, you need to look at specific circuit examples, so let's look at some of them.

Transistor in switch mode

A transistor in switching mode is one of the cases transistor circuits with a common emitter. The transistor circuit in switching mode is used very often. This transistor circuit is used, for example, when it is necessary to control powerful load via microcontroller. The controller leg is not capable of pulling a powerful load, but the transistor can. It turns out that the controller controls the transistor, and the transistor controls a powerful load. Well, first things first.

The main idea of ​​this mode is that the base current controls the collector current. Moreover, the collector current is much greater than the base current. Here you can see with the naked eye that the current signal is amplified. This amplification is carried out using the energy of the power source.

The figure shows a diagram of the operation of a transistor in switching mode.

For transistor circuits, voltages do not play a big role, only currents matter. Therefore, if the ratio of the collector current to the base current is less than the gain of the transistor, then everything is okay.

In this case, even if we have a voltage of 5 volts applied to the base and 500 volts in the collector circuit, then nothing bad will happen, the transistor will obediently switch the high-voltage load.

The main thing is that these voltages do not exceed the limit values ​​for a specific transistor (set in the transistor characteristics).

As far as we know, the current value is a characteristic of the load.

We don't know the resistance of the light bulb, but we know the operating current of the light bulb is 100 mA. In order for the transistor to open and allow such current to flow, you need to select the appropriate base current. We can adjust the base current by changing the value of the base resistor.

Since the minimum value of the transistor gain is 10, then for the transistor to open, the base current must become 10 mA.

The current we need is known. The voltage across the base resistor will be This voltage value across the resistor is due to the fact that 0.6V-0.7V is dropped at the base-emitter junction and we must not forget to take this into account.

As a result, we can easily find the resistance of the resistor

All that remains is to choose a specific value from a number of resistors and it’s in the bag.

Now you probably think that the transistor switch will work as it should? That when the base resistor is connected to +5 V the light bulb lights up, when it is turned off the light bulb goes out? The answer may or may not be yes.

The thing is that there is a small nuance here.

The light bulb will go out when the resistor potential is equal to the ground potential. If the resistor is simply disconnected from the voltage source, then everything is not so simple. The voltage on the base resistor can miraculously arise as a result of interference or some other otherworldly evil spirits :)

To prevent this effect from happening, do the following. Another resistor Rbe is connected between the base and emitter. This resistor is chosen with a value at least 10 times larger than the base resistor Rb (In our case, we took a 4.3 kOhm resistor).

When the base is connected to any voltage, the transistor works as it should, the resistor Rbe does not interfere with it. This resistor consumes only a small portion of the base current.

In the case when voltage is not applied to the base, the base is pulled up to the ground potential, which saves us from all kinds of interference.

So, in principle, we have figured out the operation of the transistor in the key mode, and as you can see, the key mode of operation is a kind of voltage amplification of the signal. After all, we controlled a voltage of 12 V using a low voltage of 5V.

Emitter follower

An emitter follower is a special case of common-collector transistor circuits.

A distinctive feature of a circuit with a common collector from a circuit with a common emitter (option with a transistor switch) is that this circuit does not amplify the voltage signal. What went in through the base came out through the emitter, with the same voltage.

Indeed, let’s say we applied 10 volts to the base, while we know that at the base-emitter junction somewhere around 0.6-0.7V is dropped. It turns out that at the output (at the emitter, at the load Rн) there will be a base voltage of minus 0.6V.

It turned out 9.4V, in a word, almost as much as went in and out. We made sure that this circuit will not increase the signal for us in terms of voltage.

“What is the point then of turning on the transistor like this?” you ask. But it turns out that this scheme has another very important property. The circuit for connecting a transistor with a common collector amplifies the signal in terms of power. Power is the product of current and voltage, but since the voltage does not change, then power increases only due to current! The load current is the sum of the base current plus the collector current. But if you compare the base current and the collector current, the base current is very small compared to the collector current. It turns out that the load current is equal to the collector current. And the result is this formula.

Now I think it’s clear what the essence of the emitter follower circuit is, but that’s not all.

The emitter follower has another very valuable quality - high input impedance. This means that this transistor circuit draws almost no current input signal and does not create a load for the signal source circuit.

To understand the principle of operation of a transistor, these two transistor circuits will be quite sufficient. And if you experiment with a soldering iron in your hands, then the epiphany simply won’t keep you waiting, because theory is theory and practice is personal experience hundreds of times more valuable!

Where can I buy transistors?

Like all other radio components, transistors can be purchased at any nearby radio parts store. If you live somewhere on the outskirts and have not heard of such stores (like I did before), then the last option remains - order transistors from an online store. I myself often order radio components through online stores, because something may simply not be available in a regular offline store.

However, if you are assembling a device purely for yourself, then you can not worry about it, but extract it from the old one, and, so to speak, breathe new life into the old radio component.

Well friends, that’s all for me. I told you everything that I planned today. If you have any questions, then ask them in the comments, if you have no questions, then write comments anyway, your opinion is always important to me. By the way, don’t forget that everyone who leaves a comment for the first time will receive a gift.

Also, be sure to subscribe to new articles, because a lot of interesting and useful things await you.

I wish you good luck, success and a sunny mood!

From n/a Vladimir Vasiliev

P.S. Friends, be sure to subscribe to updates! By subscribing, you will receive new materials directly to your email! And by the way, everyone who signs up will receive a useful gift!

So, the third and final part of the story about bipolar transistors on our website =) Today we will talk about the use of these wonderful devices as amplifiers, we will consider possible connection diagrams bipolar transistor and their main advantages and disadvantages. Let's get started!

This circuit is very good when using high frequency signals. In principle, this is why the transistor is turned on in the first place. Very big disadvantages are the low input resistance and, of course, the lack of current amplification. See for yourself, at the input we have the emitter current, at the output.

That is, the emitter current is greater than the collector current by a small amount of the base current. This means that there is not just no current gain, moreover, the output current is slightly less than the input current. Although, on the other hand, this circuit has a fairly large voltage transfer coefficient) These are the advantages and disadvantages, let’s continue….

Connection diagram for a bipolar transistor with a common collector

This is what the wiring diagram for a bipolar transistor with a common collector looks like. Does it remind you of anything?) If we look at the circuit from a slightly different angle, we recognize our old friend here - the emitter follower. There was almost a whole article about it (), so we have already covered everything related to this scheme. Meanwhile, we are waiting for the most commonly used circuit - with a common emitter.

Connection circuit for a bipolar transistor with a common emitter.

This circuit has earned popularity for its amplifying properties. Of all the circuits, it gives the greatest gain in current and voltage; accordingly, the increase in signal power is also large. The disadvantage of the circuit is that the amplification properties are strongly influenced by increasing temperature and signal frequency.

We got acquainted with all the circuits, now let’s take a closer look at the last (but not the least important) amplifier circuit based on a bipolar transistor (with a common emitter). First, let's depict it a little differently:

There is one minus here - the grounded emitter. When the transistor is turned on in this way, the output contains nonlinear distortion, which, of course, need to be fought. Nonlinearity occurs due to the influence of the input voltage on the emitter-base junction voltage. Indeed, there is nothing “extra” in the emitter circuit; the entire input voltage turns out to be applied precisely to the base-emitter junction. To cope with this phenomenon, we add a resistor to the emitter circuit. So we get negative feedback.

What is this?

To put it briefly, then negative inverse principle th communications lies in the fact that some part of the output voltage is transferred to the input and subtracted from the input signal. Naturally, this leads to a decrease in the gain, since the input of the transistor, due to the influence of feedback, will receive a lower voltage value than in the absence of feedback.

Nevertheless, negative feedback is very useful for us. Let's see how it will help reduce the influence of the input voltage on the voltage between the base and emitter.

So, even if there is no feedback, an increase in the input signal by 0.5 V leads to the same increase. Everything is clear here 😉 And now let’s add feedback! And in the same way, we increase the input voltage by 0.5 V. Following this, , increases, which leads to an increase in the emitter current. And an increase leads to an increase in the voltage across the feedback resistor. It would seem, what's wrong with this? But this voltage is subtracted from the input! Look what happened:

The input voltage has increased - the emitter current has increased - the voltage across the negative feedback resistor has increased - the input voltage has decreased (due to subtraction) - the voltage has decreased.

That is, negative feedback prevents the base-emitter voltage from changing when the input signal changes.

As a result, our amplifier circuit with a common emitter was supplemented with a resistor in the emitter circuit:

There is another problem with our amplifier. If a negative voltage value appears at the input, the transistor will immediately close (the base voltage will become less than the emitter voltage and the base-emitter diode will close), and nothing will happen at the output. This is somehow not very good) Therefore, it is necessary to create bias. This can be done using a divisor as follows:

We got such a beauty 😉 If the resistors are equal, then the voltage on each of them will be equal to 6V (12V / 2). Thus, in the absence of a signal at the input, the base potential will be +6V. If a negative value, for example -4V, comes to the input, then the base potential will be equal to +2V, that is, the value is positive and does not interfere with the normal operation of the transistor. This is how useful it is to create an offset in the base circuit)

How else could we improve our scheme...

Let us know what signal we will amplify, that is, we know its parameters, in particular the frequency. It would be great if there was nothing at the input except the useful amplified signal. How to ensure this? Of course, using a high-pass filter) Let's add a capacitor, which, in combination with a bias resistor, forms a high-pass filter:

This is how the circuit, in which there was almost nothing except the transistor itself, was overgrown with additional elements 😉 Perhaps we’ll stop there; soon there will be an article devoted to the practical calculation of an amplifier based on a bipolar transistor. In it we will not only compose schematic diagram amplifier, but we will also calculate the ratings of all elements, and at the same time select a transistor suitable for our purposes. See you soon! =)

Amplifiers contain transistors, as well as elements such as resistors, capacitors and inductors. The parameters of the elements used (their ratings and voltages) depend on the requirements for the amplifier, as well as on the type of transistors used. With the advent of transistors various types New configurations of amplifier circuits became possible. In biopolar r -n- p- or n - r -n- the transistor creates regions alternating in a certain order with various types conductivities forming the base, emitter and collector. The transistor is called bipolar, since charge transfer in it is carried out by both electrons and holes. IN field or (unipolar) In transistors, charges are carried by carriers of one type: either electrons or holes. Field-effect transistors (FETs) have three regions called gate, source and drain. Depending on the type of media used, there are two types of field-effect transistors: p- and I-channel. Correspond to different types of transistors various characteristics, described in more detail in this section.

The most common circuit for constructing an amplifier based on a bipolar transistor is a circuit with a common (grounded) emitter (CE); Variants of such schemes are shown in Fig. 11.1. The term "common emitter" indicates that in a suitable circuit the resistance between the emitter terminal and ground for the signal is low, but it does not follow that it is low in all cases and for DC. So, for example, in the diagrams shown in Fig. 1.1, A And b, the emitters are directly grounded, and in the circuit in Fig. 1.1, a resistance is connected between the emitter and the ground, shunted by a capacitor. Therefore, if the reactance of this capacitor for the signal is small, we can assume that the emitter is practically grounded for the signal.

To operate in class A (Section 1.4), the bias voltage between the base and emitter must be forward (unlocking), and between the collector and emitter - reverse (blocking). To achieve this bias, the polarity of the power supplies is selected depending on the type of transistor used. For transistor r -n - p-type (Fig. 11 L, a) the plus of the bias source must be connected to the p-type emitter, and the minus to the i-type base. Thus, forward bias is obtained at a negative base potential relative to the emitter. To reverse bias a p-type collector, its potential must be negative. To do this, the power source is connected with the positive pole to the emitter, and the negative pole to the collector.

The input signal creates a resistor R 1 voltage drop, which is algebraically added to the constant bias voltage. As a result, the total base potential changes in accordance with the signal. As the base potential changes, the collector current changes, and therefore the voltage across the resistor R2. With a positive half-wave of the input voltage, the forward bias decreases and the current through R 2 decreases accordingly. Voltage drop per R 2 also decreases, resulting in a 180° phase shift between the input and output signals.

If transistor n is used - r- n-type (Fig. 1.1.6), then the polarity of both power sources is reversed. In this case, the base junction also turns out to be biased in the forward direction, and the collector junction in the opposite direction. As in the previous case, a phase shift of 180° is formed between the input and output signals.

In Fig. 1.1, a and b show the main elements of the amplifier, and the amplifier circuit used in practice is shown in Fig. 1.1.6. Here, capacitor C 1 does not allow the constant component of the input signal to pass through, but has a low reactance for its variable component, which is thus supplied to the resistor R 2 . (This is the so-called R.C.-connection; it is described in more detail in section. 1.5). Base forward bias voltage comes from voltage divider Ri- R2, which is connected to the power source. The required forward bias value of the transistor base is obtained by properly choosing the ratio of the resistance values R 1 And R 2 . Moreover, in the transistor n - r- n-type base potential is set more positive than the emitter. The collector resistor on which the output signal is generated is usually called a load resistor and is designated R n. Through the isolation capacitor C 3, the signal is sent to the next stage. Input and output circuits must have a common grounded point (Fig. 1.1, A).

The base current amplification factor for a circuit with OE is given by the following relation:

where p is the base current amplification factor;

DI b - base current increment; DI k - the corresponding increment in the collector current at -

Rice. 1.1. Common emitter circuits.

Thus, p is equal to the ratio of the increment of the collector current to the corresponding increment of the base current at a constant collector voltage. The signal current gain is also called direct current transfer coefficient [ With a sufficiently large resistance value R 2 the alternating component of the signal current is practically equal to the alternating component of the base current. - Note ed.]

Resistor R 3 (Fig. 1.1.5) has a stabilizing effect on the transistor current when the temperature changes. The voltage drop across R 3 creates a reverse (turn-off) bias at the emitter junction of the transistor, as it increases the emitter potential. Therefore, it reduces the positive forward base bias by the amount of this voltage drop. The presence of an alternating voltage component on Rz would cause a decrease in the output signal and, consequently, the gain of the amplifier (see Section 1.8). To eliminate this effect, resistor R3 is shunted with capacitor C2.

When the transistor heats up, the DC component of the collector current increases. Accordingly, the voltage drop across R z, which results in a reduction in forward base bias as well as collector current. As a result, partial compensation of the temperature drift of the current is carried out.

Rice. 1.2. Common Source Circuits

In Fig. Figure 1.2 shows a field-effect transistor amplifier circuit equivalent to an OE circuit, which is called a common-source circuit. In this circuit, the gate corresponds to the base of the bipolar transistor, the source to the emitter, and the drain to the collector. In diagram 1.2, A FET with n-type channel is shown. For a transistor with a p-type channel, the arrow on the gate will be directed in the opposite direction. In Fig. 1.2, b also shows a transistor with a d-type channel, and in Fig. 1.2, V- with a p-type channel.

FET bias circuits differ from bipolar transistor bias circuits due to significant differences in the characteristics of these devices. Bipolar transistors are amplifiers signal current and reproduce at the output the amplified input signal current, while in field-effect transistors the output signal current is controlled by the one applied to the input signal voltage.

There are two types of PT: with control r- n-junction and metal-oxide-semiconductor (MOS). (MOS transistors are also called insulated gate field effect transistors.) Both types of field effect transistors are made with n – and p-channels.

In the diagram in Fig. 1.2, and a PT with a control is used r- I-transition, and in the diagram in Fig. 1.2, b - MOS transistor operating in enrichment mode. In Fig. 1.2, V shows a MOSFET operating in depletion mode. In MOS transistors, the gate is depicted as a capacitor plate, which symbolizes the capacitance resulting from the formation of a very thin oxide layer that insulates the metal contact of the gate terminal from the channel. (The term "MOS transistor" comes from this production method.)

Since FETs are driven by input voltage rather than current like bipolar transistors, the signal current gain parameter is replaced by conductance g m. Transfer conductance is a measure of the quality of a field effect transistor and characterizes the ability of the gate voltage to drive drain current. The expression for transfer conductivity is as follows:

Unit of measurement g m, called siemens, is the reciprocal of the unit of resistance (1 cm = 1/ohm). As follows from expression (1.2), the parameter g m for a FET, it is the ratio of the drain current increment to the gate voltage increment at a constant voltage between source and drain.

In a field-effect transistor with a control r- n-junction and n-type channel (Fig. 1.2, a) when a negative voltage is applied to the gate, the channel is depleted of charge carriers and the channel conductivity decreases. (For a p-channel FET, the conductivity decreases when a positive voltage is applied to the gate.) Since a unijunction FET has only two zones with different types conductance (the source and drain terminals are connected to one zone and the gate terminal to the other), the conductivity between the source and drain is the same type as the channel conductance. Therefore, unlike a bipolar transistor, which has U Q 3 = 0 collector current is 0, channel current can flow even at zero gate-source voltage. Since the channel current is a function of the voltage Uzi, the field-effect transistor channel with the control r- an n-junction can conduct current in both directions: from source to drain and in the opposite direction (with a bipolar transistor, the collector current in operating mode always has one direction). In this case, the operating point (for example, for class A circuits) for such transistors is set by applying voltage reverse bias gate, in contrast to forward biasing of the base junction in bipolar transistors [In a transistor with a control r- an n-junction usually applies a blocking voltage U 8i to the junction (negative for the n-channel) and the maximum current in the channel is obtained at U 3 i = 0. The direction of the current in the channel depends on the polarity of the power source connected to the channel; When the polarity of the power supply is reversed, the terminal that was the drain becomes the source and vice versa. - Note ed.].

As noted above, the gate in MOS transistors is isolated from the channel by a dielectric, such as silicon dioxide (SiO 2). In this case, the gate has a very high input resistance and can be supplied with both forward bias to enrich the channel with carriers (which will increase the passing current) and reverse bias to deplete the channel with carriers (which will reduce the current channel a). Therefore, it is possible to manufacture two different types of MOS transistors: for operation in enriched and depleted modes (here we mean MOS transistors with a built-in channel).

A depletion MOSFET has a drain current at zero input bias. Using reverse bias voltage, the drain current is reduced to a certain value depending on the required dynamic range input signal. As shown in Fig. 1.2.6, for depletion type transistors, the line representing the channel is continuous, which means the presence of a closed circuit and the flow of current in the channel (drain current) at zero gate bias.

In enriched type MOSFETs, the drain current at zero bias is small. The bias voltage increases the drain current to a certain value depending on the dynamic range of the input signal. In enriched type MOS transistors, the line representing the channel is intermittent, which symbolizes a circuit break at zero bias. In order to increase the current to the amount necessary for the normal operation of a circuit such as an amplifier, an appropriate bias must be used.

Performance characteristics of the circuits shown in Fig. 1.D are similar to the characteristics of the circuits presented in Fig. 1.11. Scheme in Fig. 1.2, in the most suitable for practical use. As in the previously discussed case, there is a phase inversion between the input and output signals. The power supply voltage is usually denoted as Ec. In order to reduce the signal voltage drop across the internal resistance of the power and bias sources, they are shunted with capacitors of the appropriate size (Fig. 11.2, a). The signal currents of the gate and drain circuits are closed through these capacitors.

The necessary explanations have been given, let's get to the point.

Transistors. Definition and history

Transistor- an electronic semiconductor device in which the current in a circuit of two electrodes is controlled by a third electrode. (transistors.ru)

Very quickly, transistors replaced vacuum tubes in various electronic devices. In this regard, the reliability of such devices has increased and their size has decreased significantly. And to this day, no matter how “sophisticated” the microcircuit is, it still contains many transistors (as well as diodes, capacitors, resistors, etc.). Only very small ones.

By the way, initially “transistors” were resistors whose resistance could be changed using the amount of applied voltage. If we ignore the physics of processes, then a modern transistor can also be represented as a resistance that depends on the signal supplied to it.

What is the difference between field-effect and bipolar transistors? The answer lies in their very names. In a bipolar transistor, charge transfer involves And electrons, And holes (“encore” - twice). And in the field (aka unipolar) - or electrons, or holes.

Also, these types of transistors differ in application areas. Bipolar ones are used mainly in analog technology, and field ones - in digital technology.

And finally: the main area of ​​application of any transistors- gain weak signal due to an additional power source.

Bipolar transistor. Operating principle. Main Features

Before considering the physics of how a transistor operates, let us outline the general problem.


It is as follows: a strong current flows between the emitter and collector ( collector current), and between the emitter and base there is a weak control current ( base current). The collector current will change depending on the change in base current. Why?
Let's consider the p-n junctions of the transistor. There are two of them: emitter-base (EB) and base-collector (BC). In the active mode of operation of the transistor, the first of them is connected with forward bias, and the second with reverse bias. What happens at the p-n junctions? For greater certainty, we will consider an n-p-n transistor. For p-n-p everything is similar, only the word “electrons” needs to be replaced with “holes”.

Since the EB junction is open, electrons easily “run across” to the base. There they partially recombine with holes, but O Most of them, due to the small thickness of the base and its low doping, manage to reach the base-collector transition. Which, as we remember, is reverse biased. And since electrons in the base are minority charge carriers, the electric field of the transition helps them overcome it. Thus, the collector current is only slightly less than the emitter current. Now watch your hands. If you increase the base current, the EB junction will open more strongly, and more electrons will be able to slip between the emitter and collector. And since the collector current is initially greater than the base current, this change will be very, very noticeable. Thus, the weak signal received at the base will be amplified. Once again, a large change in collector current is a proportional reflection of a small change in base current.

I remember that the principle of operation of a bipolar transistor was explained to my classmate using the example of a water tap. The water in it is the collector current, and the base control current is how much we turn the knob. A small force (control action) is enough to increase the flow of water from the tap.

In addition to the processes considered, a number of other phenomena can occur at the p-n junctions of the transistor. For example, with a strong increase in voltage at the base-collector junction, avalanche charge multiplication may begin due to impact ionization. And coupled with the tunnel effect, this will first give an electrical breakdown, and then (with increasing current) a thermal breakdown. However, thermal breakdown in a transistor can occur without electrical breakdown (i.e., without increasing the collector voltage to breakdown voltage). One excessive current through the collector will be enough for this.

Another phenomenon is due to the fact that when the voltages on the collector and emitter junctions change, their thickness changes. And if the base is too thin, then a closing effect may occur (the so-called “puncture” of the base) - a connection between the collector junction and the emitter junction. In this case, the base region disappears and the transistor stops working normally.

The collector current of the transistor in the normal active mode of operation of the transistor is greater than the base current by a certain number of times. This number is called current gain and is one of the main parameters of the transistor. It is designated h21. If the transistor is turned on without load on the collector, then when constant voltage collector-emitter ratio of collector current to base current will give static current gain. It can be equal to tens or hundreds of units, but it is worth considering the fact that in real circuits this coefficient is smaller due to the fact that when the load is turned on, the collector current naturally decreases.

The second important parameter is transistor input resistance. According to Ohm's law, it is the ratio of the voltage between the base and emitter to the control current of the base. The larger it is, the lower the base current and the higher the gain.

The third parameter of a bipolar transistor is voltage gain. It is equal to the ratio of the amplitude or effective values ​​of the output (emitter-collector) and input (base-emitter) alternating voltages. Since the first value is usually very large (units and tens of volts), and the second is very small (tenths of volts), this coefficient can reach tens of thousands of units. It is worth noting that each base control signal has its own voltage gain.

Transistors also have frequency response , which characterizes the transistor’s ability to amplify a signal whose frequency approaches the cut-off amplification frequency. The fact is that as the frequency of the input signal increases, the gain decreases. This is due to the fact that the passage time of the main physical processes(the time of movement of carriers from the emitter to the collector, the charge and discharge of barrier capacitive junctions) becomes commensurate with the period of change of the input signal. Those. the transistor simply does not have time to react to changes in the input signal and at some point simply stops amplifying it. The frequency at which this happens is called boundary.

Also, the parameters of the bipolar transistor are:

• reverse current collector-emitter
• on time
• reverse collector current
• maximum permissible current

Conditional n-p-n notation and pnp transistors differ only in the direction of the arrow indicating the emitter. It shows how current flows in a given transistor.

Operating modes of a bipolar transistor

1. Inverse active mode. Here the BC transition is open, but on the contrary, the EB is closed. The amplification properties in this mode, of course, are worse than ever, so transistors are used very rarely in this mode.
2. Saturation mode. Both crossings are open. Accordingly, the main charge carriers of the collector and emitter “run” to the base, where they actively recombine with its main carriers. Due to the resulting excess of charge carriers, the resistance of the base and p-n junctions decreases. Therefore, a circuit containing a transistor in saturation mode can be considered short-circuited, and this radio element itself can be represented as an equipotential point.
3. Cut-off mode. Both transitions of the transistor are closed, i.e. the current of the main charge carriers between the emitter and collector stops. Flows of minority charge carriers create only small and uncontrollable thermal transition currents. Due to the poverty of the base and transitions with charge carriers, their resistance increases greatly. Therefore, it is often believed that a transistor operating in cutoff mode represents an open circuit.
4. Barrier mode In this mode, the base is directly or through a low resistance connected to the collector. A resistor is also included in the collector or emitter circuit, which sets the current through the transistor. This creates the equivalent of a diode circuit with a resistor in series. This mode is very useful, as it allows the circuit to operate at almost any frequency, over a wide temperature range and is undemanding to the parameters of the transistors.

Switching circuits for bipolar transistors

Since the transistor has three contacts, in general, power must be supplied to it from two sources, which together produce four outputs. Therefore, one of the transistor contacts has to be supplied with a voltage of the same sign from both sources. And depending on what kind of contact it is, there are three circuits for connecting bipolar transistors: with a common emitter (CE), a common collector (OC) and a common base (CB). Each of them has both advantages and disadvantages. The choice between them is made depending on which parameters are important to us and which can be sacrificed.

Connection circuit with common emitter

But in addition to all the goodies, the OE scheme also has a significant drawback. It lies in the fact that an increase in frequency and temperature leads to a significant deterioration in the amplification properties of the transistor. Thus, if the transistor is to operate at high frequencies, then it is better to use a different switching circuit. For example, with a common base.

Connection diagram with a common base

In a common-base circuit, the signal phase does not invert, and the noise level at high frequencies is reduced. But, as already mentioned, its current gain is always slightly less than unity. True, the voltage gain here is the same as in a circuit with a common emitter. The disadvantages of a common base circuit also include the need to use two power supplies.

Connection diagram with a common collector

Let me remind you that negative feedback is such feedback in which the output signal is fed back to the input, thereby reducing the level of the input signal. Thus it happens automatic adjustment in case of accidental change of input signal parameters

The current gain is almost the same as in the common emitter circuit. But the voltage gain is small (the main drawback of this circuit). It approaches unity, but is always less than it. Thus, the power gain is equal to only a few tens of units.

In a common collector circuit, there is no phase shift between the input and output voltage. Since the voltage gain is close to unity, output voltage the phase and amplitude coincides with the input one, i.e., repeats it. That is why such a circuit is called an emitter follower. Emitter - because the output voltage is removed from the emitter relative to the common wire.

This inclusion is used to coordinate transistor stages or when the input source has a high input impedance (such as a piezoelectric pickup or condenser microphone).

Two words about cascades

There may also be a need for a transistor with good sensitivity and at the same time good gain. In such cases, a cascade of a sensitive but low-power transistor (VT1 in the figure) is used, which controls the power supply of a more powerful fellow (VT2 in the figure).

Other applications of bipolar transistors

Marking

Helpful comments:
http://habrahabr.ru/blogs/easyelectronics/133136/#comment_4419173

Tags:

• transistors
• bipolar transistors
• electronics