Transistors. What is the difference between NPN and PNP transistors

Experienced electricians and electronics engineers know that there are special probes to fully test transistors.

Using them, you can not only check the serviceability of the latter, but also its gain - h21e.

The need for a probe

A probe is a really necessary device, but if you just need to check the transistor for serviceability, it is quite suitable.

Transistor device

Before you start testing, you need to understand what a transistor is.

It has three terminals that form diodes (semiconductors) between them.

Each pin has its own name: collector, emitter and base. First two conclusions p-n transitions are connected in the base.

One p-n junction between the base and the collector forms one diode, the second p-n junction between the base and the emitter forms the second diode.

Both diodes are connected in a circuit back-to-back through the base, and this entire circuit is a transistor.

We are looking for the base, emitter and collector of the transistor

How to find a collector immediately.

To immediately find the collector, you need to find out what power the transistor is in front of you, and they come in medium power, low power and high power.

Medium power and powerful transistors get very hot, so heat needs to be removed from them.

This is done using a special cooling radiator, and heat is removed through the collector terminal, which in these types of transistors is located in the middle and connected directly to the case.

The result is the following heat transfer scheme: collector output – housing – cooling radiator.

If the collector is identified, then determining other conclusions will not be difficult.

There are cases that greatly simplify the search, this is when the device already has the necessary symbols, as shown below.

We make the necessary measurements of forward and reverse resistance.

However, all the same, the three legs sticking out in the transistor can throw many novice electronics engineers into a stupor.

How can you find the base, emitter and collector?

You can’t do this without a multimeter or just an ohmmeter.

So, let's start the search. First we need to find a base.

We take the device and make the necessary measurements of the resistance on the legs of the transistor.

We take the positive probe and connect it to the right terminal. We alternately bring the negative probe to the middle and then to the left terminals.

Between right and middle, for example, we showed 1 (infinity), and between right and left 816 Ohm.

These readings don't tell us anything yet. Let's take further measurements.

Now we move to the left, bring the positive probe to the middle terminal, and successively touch the negative probe to the left and right terminals.

Again the middle one - the right one shows infinity (1), and the middle left one 807 Ohm.

This also doesn’t tell us anything. Let's measure further.

Now we move even more to the left, we bring the positive probe to the leftmost terminal, and the negative one sequentially to the right and middle.

If in both cases the resistance shows infinity (1), then this means that the base is the left terminal.

But where the emitter and collector (middle and right terminals) will still need to be found.

Now you need to measure direct resistance. To do this, now we do everything in reverse, the negative probe to the base (left terminal), and alternately connect the positive one to the right and middle terminals.

Remember one important point: the resistance of the base-emitter p-n junction is always greater than the base-collector p-n junction.

As a result of measurements, it was found that the resistance of the base (left terminal) - right terminal is equal 816 Ohm, and the base resistance is the middle terminal 807 Ohm.

This means that the right pin is the emitter, and the middle pin is the collector.

So, the search for base, emitter and collector is completed.

How to check a transistor for serviceability

To check the transistor with a multimeter for serviceability, it will be enough to measure the reverse and forward resistance of two semiconductors (diodes), which is what we will do now.

There are usually two junction structures in a transistor p-n-p And n-p-n.

P-n-p– this is an emitter junction, you can determine this by the arrow that points to the base.

The arrow that goes from the base indicates that this is an n-p-n junction.

The PnP junction can be opened using a negative voltage applied to the base.

We set the multimeter operating mode switch to the resistance measurement position at the “ 200 ».

We connect the black negative wire to the base terminal, and connect the red positive wire in turn to the emitter and collector terminals.

Those. We check the functionality of the emitter and collector junctions.

Multimeter readings range from 0,5 to 1.2 kOhm They will tell you that the diodes are intact.

Now we swap the contacts, connect the positive wire to the base, and alternately connect the negative wire to the emitter and collector terminals.

There is no need to change the multimeter settings.

The last reading should be much higher than the previous one. If everything is normal, then you will see the number “1” on the device display.

This suggests that the resistance is very high, the device cannot display data above 2000 Ohms, and the diode junctions are intact.

The advantage of this method is that the transistor can be tested directly on the device without unsoldering it from there.

Although there are still transistors where low-resistance resistors are soldered into the p-n junctions, the presence of which may not allow for correct resistance measurements; it can be small, both at the emitter and collector junctions.

In this case, the leads will need to be unsoldered and measurements taken again.

Signs of a transistor malfunction

As noted above, if the measurements of direct resistance (black minus on the base, and plus alternately on the collector and emitter) and reverse (red plus on the base, and black minus alternately on the collector and emitter) do not correspond to the above indicators, then the transistor has failed .

Another sign of a malfunction is when the resistance of the pn junctions in at least one measurement is equal to or close to zero.

This indicates that the diode is broken and the transistor itself is faulty. Using the recommendations given above, you can easily check the transistor with a multimeter for serviceability.

Bipolar transistor is a semiconductor device with two interacting p-n-transitions and with three terminals (Fig. 1.15). Depending on the alternation of doped areas, transistors are distinguished n-p-n-type (Fig. 1.15, A) And p-n-r-type (Fig. 1.15, b).

In Fig. 1.15, V, G symbols of transistors are given p-p-p- And p-n-r- types, respectively. The transistor terminals are designated: E– emitter, B– base, TO– collector.

The emitter and collector regions differ in that the concentration of impurities in the emitter region is much greater than in the collector region. The transition that occurs between the emitter and the base is called emitter junction , and the transition that occurs between the collector and the base is collector .

In Fig. Figure 1.16 shows a circuit diagram for connecting a transistor with connected constant voltage sources and a collector resistor. In this circuit, the base terminal of the transistor is connected to the housing. Therefore this scheme is called circuit for connecting a transistor with a common base (CB).

Distinguish four operating modes of the bipolar transistor :

1) active mode – the emitter junction is open and the collector junction is closed (Fig. 1.16);

2) cut-off mode - both p-n- the junctions are closed, and there is no significant current through the transistor.

To obtain this mode, it is necessary to change the polarity of the source in the circuit (see Fig. 1.16) E E to the opposite;

1) saturation mode - two p-n-transistor junctions are open and direct currents flow through them. To obtain this mode, it is necessary to change the polarity of the source in the circuit (see Fig. 1.16) E K to the opposite;

2) inverse mode – the collector junction is open and the emitter junction is closed. To obtain this mode, it is necessary to change the source polarities to opposite polarities in the circuit (see Fig. 1.16). E K And E E.

The active mode of operation is mainly used to amplify and convert signals. The operation of a bipolar transistor in active mode is based on the phenomenon of diffusion, as well as the effect of charge carrier drift in an electric field.

Transistor operation in active mode

Let's consider the operation of a transistor in active mode using the example of a pnp-type transistor (Fig. 1.16). In this mode, the emitter junction of the transistor is open. The opening voltage is E E= 0.4…0.7 V.

Current flows through the open emitter junction i E (i E= 0.1…10 mA for a low-power transistor). As a rule, in the emitter region of the transistor the concentration of acceptor impurities is many times greater than the concentration of donor impurities in the base region. n- transistor area. Therefore, the concentration of holes in the emitter region is much greater than the concentration of electrons in the base region, and almost the entire emitter current is a hole current.

In singles p-n-transition during hole diffusion into n-region, complete recombination of injected holes with electrons occurs n-regions The same process occurs in the emitter junction of the transistor. Thanks to this process, a base current arises i B(see Fig. 1.16). However, more complex processes occur in the transistor.

The main feature of the transistor design is the relative thin base area b. Base width ( W) in a transistor is much less than the free path of holes ( L). In modern silicon transistors W» 1 µm, and the diffusion length L= 5…10 µm. Consequently, the vast majority of holes reach the collector junction without having time to recombine with base electrons. Once in the reverse-biased collector junction, the holes drift (and accelerate) in the existing junction field.

Having passed through the collector junction, the holes recombine with electrons flowing to the collector from the power source ( E K). Note that this hole current is many times greater than the intrinsic reverse current of the closed collector junction and almost completely determines the collector current ( i K) transistor.

From the analysis of the active mode (Fig. 1.16), the equation for transistor currents follows:

In this equation, the base current is much less than the emitter current and collector current, and
The collector current is almost equal to the emitter current of the transistor.

The relationship between the currents in the transistor is characterized by two parameters:

emitter current transfer coefficient

And base current transfer coefficient

Using formula (1.2), we obtain the formula relationship between transmission coefficients :

Coefficient values α And β depend on the design of the transistor. For most low-power transistors used in communications devices and computers, the coefficient b= 20...200, and the coefficient a = 0,95…0,995.

Transistor amplification properties

Let's consider the amplifying properties of the transistor. Let there be a voltage at the input of the transistor E E= 0.5 V. And let this voltage create a current i E= 5 mA. The power consumed to control the transistor is equal to:

R VX= E Ei E= 0.5 × 5 ×10 -3 = 2.5 mW.

Let the payload resistance in the collector circuit of the transistor (Fig. 1.17) be equal to R K= 1 kOhm. A collector current flows through the load resistor, approximately equal to the emitter current of the transistor: iK» i E. The output power released at the load is equal to:

R N =i K 2R K = 25 mW .

Consequently, the circuit (see Fig. 1.17) provides tenfold power amplification. Note that to provide such amplification, it is required that a large blocking voltage be applied to the collector junction:

E K >U K,

Where U K = i K RK– voltage drop across the load resistance in the collector circuit.

The increased output signal energy is provided by the power supply in the collector circuit.

Let's consider other modes of operation of the transistor:

· in mode saturation a forward current of the collector junction arises. Its direction is opposite to the direction of the diffusion current of holes. The resulting collector current decreases sharply, and the amplification properties of the transistor sharply deteriorate;

Rarely used transistor inverse mode, since the injection properties of the collector are much worse than the injection properties of the emitter;

· V mode cutoffs all currents through the transistor are practically equal to zero - both junctions of the transistor are closed, and the amplifying properties of the transistor do not appear.

In addition to the considered circuit for connecting a transistor with a common base, two other circuits are used:

1) when connected to the transistor emitter body, we get common emitter (CE) circuit (Fig. 1.17). The OE scheme is most often found in practice;

2) when connected to the transistor collector housing we get circuit with a common collector (OK) . In these circuits, the control voltage is applied to the base terminal of the transistor.

The dependence of the currents through the terminals of the transistor on the voltages applied to the transistor is called current-voltage characteristics (volt-ampere characteristics) transistor.

For a circuit with a common emitter (Fig. 1.17), the current-voltage characteristics of the transistor look like (Fig. 1.18, 1.19). Similar graphs can be obtained for a scheme with a common base. Curves (see Fig. 1.18) are called input characteristics of the transistor , since they show the dependence of the input current on the control input voltage supplied between the base and emitter of the transistor. The input characteristics of the transistor are close to the characteristics p-n-transition.

The dependence of the input characteristics on the collector voltage is explained by an increase in the width of the collector junction and, consequently, a decrease in the base thickness with an increase in the reverse voltage at the transistor collector (Early effect).

Curves (see Fig. 1.19) are called output characteristics of the transistor . They are used to determine the collector current of the transistor. An increase in the collector current corresponds to an increase in the control voltage at the base of the transistor:

u BE4 > u BE3 > u BE2 > u BE1..

At u FE£ U US(see Fig. 1.19) the voltage at the collector of the transistor becomes less than the voltage at the base. In this case, the collector junction of the transistor opens, and the saturated mode occurs
iation, in which the collector current sharply decreases.

At a high voltage on the collector, the collector current begins to increase, as a process of avalanche (or thermal) breakdown of the collector junction of the transistor occurs.

From the analysis of the current-voltage characteristics of the transistor it follows that the transistor, like the diode, belongs to nonlinear elements. However, in active mode with u FE> U US The collector current of the transistor changes approximately in direct proportion to the increments of the input control voltage at the base of the transistor, i.e. The output circuit of the transistor is close in properties to an ideal controlled current source. The collector current in active mode is practically independent of the load connected to the transistor collector.

In Fig. 1.20 shows the simplest linear equivalent transistor circuit , obtained for the active operating mode when applying small amplitude alternating signals to the transistor ( U m < 0,1 В). Основным элементом этой схемы является источник тока, управляемый входным напряжением:

I K =SU BE,

Where S– transistor transconductance, equal to 10...100 mA/V for low-power transistors.

Resistance r CE characterizes energy losses in the collector circuit. Its value for low-power transistors is tens and hundreds of kilo-ohms. Emitter junction resistance ( r BE) is equal to hundreds of ohms or units of kilo-ohms. This resistance characterizes the energy lost to control the transistor. The values ​​of the parameters of the equivalent circuit can be found by indicating the operating points at the input and output I-V characteristics of the transistor and determining the corresponding derivatives at these operating points (or specifying the increments of the corresponding currents and voltages at the operating points).

Device and principle of operation

The first transistors were made from germanium. Currently, they are made primarily from silicon and gallium arsenide. The latter transistors are used in high-frequency amplifier circuits. A bipolar transistor consists of three differently doped semiconductor regions: the emitter E, bases B and collector C. Depending on the type of conductivity of these zones, NPN (emitter - n-semiconductor, base - p-semiconductor, collector - n-semiconductor) and PNP transistors are distinguished. Conductive contacts are connected to each of the zones. The base is located between the emitter and collector and is made of a lightly doped semiconductor with high resistance. The total base-emitter contact area is significantly smaller than the collector-base contact area (this is done for two reasons - the large area of ​​the collector-base junction increases the likelihood of minority charge carriers being extracted into the collector, and since in operating mode the collector-base junction is usually switched on in reverse bias, which increases heat generation and promotes heat removal from the collector), therefore a general bipolar transistor is an asymmetrical device (it is impossible to swap the emitter and collector by changing the connection polarity and resulting in a bipolar transistor absolutely similar to the original one).

In the active operating mode, the transistor is turned on so that its emitter junction is biased in the forward direction (open), and the collector junction is biased in the opposite direction (closed). For definiteness, let's consider npn transistor, all reasoning is repeated absolutely similarly for the case pnp transistor, replacing the word “electrons” with “holes”, and vice versa, as well as replacing all voltages with opposite signs. IN npn In a transistor, electrons, the main current carriers in the emitter, pass through the open emitter-base junction (injected) into the base region. Some of these electrons recombine with the majority charge carriers in the base (holes). However, because the base is made very thin and relatively lightly doped, most of the electrons injected from the emitter diffuse into the collector region. The strong electric field of the reverse-biased collector junction captures electrons and carries them into the collector. The collector current is thus practically equal to the emitter current, with the exception of a small recombination loss in the base, which forms the base current (I e = I b + I k). The coefficient α connecting the emitter current and the collector current (I k = α I e) is called the emitter current transfer coefficient. The numerical value of the coefficient α is 0.9 - 0.999. The higher the coefficient, the more efficiently the transistor transmits current. This coefficient depends little on the collector-base and base-emitter voltages. Therefore, over a wide range of operating voltages, the collector current is proportional to the base current, the proportionality coefficient is equal to β = α / (1 − α) = (10..1000). Thus, by varying a small base current, a much larger collector current can be controlled.

Operating modes of a bipolar transistor

Normal active mode

The emitter-base junction is connected in the forward direction (open), and the collector-base junction is in the reverse direction (closed)
U EB >0;U KB<0 (для транзистора p-n-p типа, для транзистора n-p-n типа условие будет иметь вид U ЭБ <0;U КБ >0);

Inverse active mode

The emitter junction has a reverse connection, and the collector junction has a direct connection.

Saturation mode

Both pn junctions are forward biased (both open). If the emitter and collector pn junctions are connected to external sources in the forward direction, the transistor will be in saturation mode. The diffusion electric field of the emitter and collector junctions will be partially weakened by the electric field created by external sources Ueb and Ukb. As a result, the potential barrier that limited the diffusion of the main charge carriers will decrease, and the penetration (injection) of holes from the emitter and collector into the base will begin, that is, currents called saturation currents of the emitter (IE.sat) and collector (IK) will flow through the emitter and collector of the transistor. us).

Cut-off mode

In this mode, both p-n junctions of the device are biased in the opposite direction (both are closed). The cutoff mode of the transistor is obtained when the emitter and collector p-n junctions are connected to external sources in the opposite direction. In this case, very small reverse currents of the emitter (IEBO) and collector (ICBO) flow through both p-n junctions. The base current is equal to the sum of these currents and, depending on the type of transistor, ranges from units of microamps - µA (for silicon transistors) to units of milliamps - mA (for germanium transistors).

Barrier mode

In this mode base transistor for direct current is connected short-circuited or through a small resistor with its collector, and in collector or in emitter The transistor circuit is turned on by a resistor that sets the current through the transistor. In this connection, the transistor is a kind of diode connected in series with a current-setting resistor. Such cascade circuits are distinguished by a small number of components, good high-frequency isolation, a large operating temperature range, and insensitivity to transistor parameters.

Connection schemes

Any transistor connection circuit is characterized by two main indicators:

• Current gain I out / I in.
• Input resistance Rin =Uin /Iin

Connection diagram with a common base

Common base amplifier.

• Among all three configurations, it has the lowest input and highest output impedance. It has a current gain close to unity and a large voltage gain. The signal phase is not inverted.
• Current gain: I out /I in =I to /I e =α [α<1]
• Input resistance R in =U in /I in =U be /I e.

The input resistance for a circuit with a common base is small and does not exceed 100 Ohms for low-power transistors, since the input circuit of the transistor is an open emitter junction of the transistor.

Advantages:

• Good temperature and frequency properties.
• High permissible voltage

Disadvantages of a common base scheme:

• Low current gain because α< 1
• Low input impedance
• Two different voltage sources for power.

Connection circuit with common emitter

• Current gain: I out /I in =I to /I b =I to /(I e -I to) = α/(1-α) = β [β>>1]
• Input resistance: R in =U in /I in =U be /I b

Advantages:

• High current gain
• High voltage gain
• Highest power gain
• You can get by with one power source
• The output AC voltage is inverted relative to the input.

Flaws:

• Worse temperature and frequency properties compared to a common base circuit

Common collector circuit

• Current gain: I out /I in =I e /I b =I e /(I e -I k) = 1/(1-α) = β [β>>1]
• Input resistance: R in = U in / I in = (U b e + U k e) / I b

Advantages:

• High input impedance
• Low output impedance

Flaws:

• The voltage gain is less than 1.

A circuit with this connection is called an “emitter follower”

Basic parameters

• Current transfer coefficient
• Input impedance
• Output conductivity
• Reverse current collector-emitter
• On time
• Limit frequency of base current transfer coefficient
• Reverse collector current
• Maximum permissible current
• Cutoff frequency of current transfer coefficient in a circuit with a common emitter

Transistor parameters are divided into intrinsic (primary) and secondary. Intrinsic parameters characterize the properties of the transistor, regardless of its connection circuit. The following are taken as the main own parameters:

• current gain α;
• resistance of the emitter, collector and base to alternating current r e, r k, r b, which are:
  • r e - the sum of the resistances of the emitter region and the emitter junction;
  • r k - the sum of the resistances of the collector area and the collector junction;
  • r b - transverse resistance of the base.

Equivalent circuit of a bipolar transistor using h-parameters

Secondary parameters are different for different transistor switching circuits and, due to its nonlinearity, are valid only for low frequencies and small signal amplitudes. For secondary parameters, several parameter systems and their corresponding equivalent circuits have been proposed. The main ones are mixed (hybrid) parameters, denoted by the letter “h”.

Input impedance- transistor resistance to input alternating current during a short circuit at the output. The change in input current is the result of a change in the input voltage, without the influence of feedback from the output voltage.

H 11 = U m1 /I m1 at U m2 = 0.

Voltage feedback factor shows what proportion of the output alternating voltage is transferred to the input of the transistor due to feedback in it. There is no alternating current in the input circuit of the transistor, and a change in the input voltage occurs only as a result of a change in the output voltage.

H 12 = U m1 /U m2 at I m1 = 0.

Current transfer coefficient(current gain) shows the gain of AC current at zero load resistance. The output current depends only on the input current without being influenced by the output voltage.

H 21 = I m2 /I m1 at U m2 = 0.

Output conductivity- internal conductivity for alternating current between output terminals. The output current changes under the influence of the output voltage.

H 22 = I m2 /U m2 at I m1 = 0.

The relationship between alternating currents and transistor voltages is expressed by the equations:

U m1 = h 11 I m1 + h 12 U m2 ;
I m2 = h 21 I m1 + h 22 U m2.

Depending on the transistor connection circuit, letters are added to the digital indices of the h-parameters: “e” - for the OE circuit, “b” - for the OB circuit, “k” - for the OK circuit.

For the OE circuit: I m1 = I mb, I m2 = I mk, U m1 = U mb-e, U m2 = U mk-e. For example, for this scheme:

H 21e = I mк /I mb = β.

For the OB circuit: I m1 = I mе, I m2 = I mк, U m1 = U mе-b, U m2 = U mк-b.

The transistor's own parameters are related to the h-parameters, for example for an OE circuit:

With increasing frequency, the collector junction capacitance C k begins to have a harmful effect on the operation of the transistor. The resistance of the capacitance decreases, the current through the load resistance and, consequently, the gain factors α and β decreases. The resistance of the emitter junction capacitance C e also decreases, however, it is shunted by a small junction resistance r e and in most cases may not be taken into account. In addition, with increasing frequency, an additional decrease in the coefficient β occurs as a result of a lag in the phase of the collector current from the phase of the emitter current, which is caused by the inertia of the process of moving carriers through the base from the emitter junction to the collector and the inertia of the processes of accumulation and resorption of charge in the base. Frequencies at which the coefficients α and β decrease by 3 dB are called limiting frequencies of the current transfer coefficient for the OB and OE schemes, respectively.

In the pulse mode, the collector current pulse begins with a delay of a delay time τ з relative to the input current pulse, which is caused by the finite travel time of the carriers through the base. As carriers accumulate in the base, the collector current increases during the rise time τ f. On time transistor is called τ on = τ h + τ f.

Transistor manufacturing technology

• Epitaxial-planar
• Splavnaya
  • Diffusion
  • Diffusion-alloy

Application of transistors

• Demodulator (Detector)
• Inverter (logic element)
• Microcircuits based on transistor logic (see transistor-transistor logic, diode-transistor logic, resistor-transistor logic)

See also

Literature

Notes

The bipolar transistor is one of the oldest but most famous type of transistor, and is still used in modern electronics. A transistor is indispensable when you need to control a fairly powerful load for which the control device cannot provide sufficient current. They come in different types and capacities, depending on the tasks performed. Basic knowledge and formulas about transistors can be found in this article.

Introduction

Before starting the lesson, let's agree that we are discussing only one type of way to turn on a transistor. A transistor can be used in an amplifier or receiver, and typically each transistor model is manufactured with certain characteristics to make it more specialized to perform better in a particular application.

The transistor has 3 terminals: base, collector and emitter. It is impossible to say unambiguously which of them is the input and which is the output, since they are all connected and influence each other in one way or another. When a transistor is turned on in switch mode (load control), it acts like this: the base current controls the current from the collector to the emitter or vice versa, depending on the type of transistor.

There are two main types of transistors: NPN and PNP. To understand this, we can say that the main difference between these two types is the direction of the electric current. This can be seen in Figure 1.A, where the direction of the current is indicated. In an NPN transistor, one current flows from the base into the transistor and the other current flows from the collector to the emitter, but in a PNP transistor the opposite is true. From a functional point of view, the difference between these two types of transistors is the voltage across the load. As you can see in the picture, the NPN transistor provides 0V when it is turned on, and the PNP provides 12V. You'll understand later why this affects transistor selection.

For simplicity, we will only study NPN transistors, but all this applies to PNP, taking into account that all currents are reversed.

The figure below shows the analogy between a switch (S1) and a transistor switch, where it can be seen that the base current closes or opens the path for current from the collector to the emitter:

By knowing exactly the characteristics of a transistor, you can get the most out of it. The main parameter is the DC gain of the transistor, which is usually denoted Hfe or β. It is also important to know the maximum current, power and voltage of the transistor. These parameters can be found in the documentation for the transistor, and they will help us determine the value of the base resistor, which is described below.

Using an NPN transistor as a switch

The figure shows the inclusion of an NPN transistor as a switch. You will encounter this inclusion very often when analyzing various electronic circuits. We will study how to run a transistor in the selected mode, calculate the base resistor, transistor current gain and load resistance. I propose the simplest and most accurate way to do this.

1. Assume that the transistor is in saturation mode: In this case, the mathematical model of the transistor becomes very simple, and we know the voltage at point V c. We will find the value of the base resistor at which everything will be correct.

2. Determination of collector saturation current: The voltage between collector and emitter (V ce) is taken from the transistor documentation. The emitter is connected to GND, respectively V ce = V c - 0 = V c. Once we know this value, we can calculate the collector saturation current using the formula:

Sometimes, the load resistance R L is unknown or cannot be as precise as the relay coil resistance; In this case, it is enough to know the current required to start the relay.
Make sure that the load current does not exceed the maximum collector current of the transistor.

3. Calculation of the required base current: Knowing the collector current, you can calculate the minimum required base current to achieve that collector current using the following formula:

It follows from it that:

4. Exceeding permissible values: After you have calculated the base current, and if it turns out to be lower than that specified in the documentation, then you can overload the transistor by multiplying the calculated base current, for example, by 10 times. Thus, the transistor switch will be much more stable. In other words, the transistor's performance will decrease if the load increases. Be careful not to exceed the maximum base current stated in the documentation.

5. Calculation of the required value of R b: Considering an overload of 10 times, the resistance R b can be calculated using the following formula:

where V 1 is the transistor control voltage (see Fig. 2.a)

But if the emitter is connected to ground, and the base-emitter voltage is known (about 0.7V for most transistors), and assuming that V 1 = 5V, the formula can be simplified to the following:

It can be seen that the base current is multiplied by 10 taking into account the overload.
When the value of R b is known, the transistor is "set" to operate as a switch, also called "saturation and cutoff mode", where "saturation" is when the transistor is fully open and conducting current, and "cutting" is when it is closed and not conducting current .

Note: When we say , we are not saying that the collector current must be equal to . This simply means that the transistor's collector current can rise to this level. The current will follow Ohm's laws, just like any electrical current.

Load calculation

When we considered that the transistor was in saturation mode, we assumed that some of its parameters did not change. This is not entirely true. In fact, these parameters were changed mainly by increasing the collector current, and therefore it is safer for overload. The documentation indicates a change in transistor parameters during overload. For example, the table in Figure 2.B shows two parameters that change significantly:

H FE (β) varies with collector current and voltage V CEsat. But V CEsat itself changes depending on the collector and base current, as shown in the table below.

The calculation can be very complex, since all the parameters are closely and complexly interrelated, so it is better to take the worst values. Those. the smallest H FE, the largest V CEsat and V CEsat.

Typical application of a transistor switch

In modern electronics, a transistor switch is used to control electromagnetic relays, which consume up to 200 mA. If you want to control a relay with a logic chip or microcontroller, then a transistor is indispensable. In Figure 3.A, the resistance of the base resistor is calculated depending on the current required by the relay. Diode D1 protects the transistor from the pulses that the coil generates when turned off.

2. Connecting an open collector transistor:

Many devices, such as the 8051 family of microcontrollers, have open-collector ports. The base resistor resistance of the external transistor is calculated as described in this article. Note that the ports can be more complex, and often use FETs instead of bipolar ones and are called open-drain outputs, but everything remains exactly the same as in Figure 3.B

3. Creating a logical element OR-NOT (NOR):

Sometimes you need to use a single gate in a circuit and you don't want to use a 14-pin 4-gate chip either due to cost or board space. It can be replaced with a pair of transistors. Note that the frequency characteristics of such elements depend on the characteristics and type of transistors, but usually below 100 kHz. Reducing the output resistance (Ro) will increase power consumption but increase the output current.
You need to find a compromise between these parameters.

The figure above shows a NOR gate built using 2 2N2222 transistors. This can be done with PNP 2N2907 transistors, with minor modifications. You just have to consider that all the electrical currents then flow in the opposite direction.

Finding errors in transistor circuits

When a problem occurs in circuits containing many transistors, it can be quite difficult to know which one is bad, especially when they are all soldered in. I give you some tips that will help you find the problem in such a scheme quickly:

1. Temperature: If the transistor gets very hot, there is probably a problem somewhere. It is not necessary that the problem is a hot transistor. Usually the defective transistor does not even heat up. This temperature increase may be caused by another transistor connected to it.

2. Measuring V CE of transistors: If they are all the same type and all work, then they should have approximately the same VCE. Finding transistors that have different V CE is a quick way to detect defective transistors.

3. Measuring the voltage across the base resistor: The voltage across the base resistor is quite important (if the transistor is turned on). For a 5V NPN transistor driver, the voltage drop across the resistor should be greater than 3V. If there is no voltage drop across the resistor, then either the transistor or the transistor control device is defective. In both cases, the base current is 0.

A PNP transistor is an electronic device, in a certain sense the inverse of an NPN transistor. In this type of transistor design, its PN junctions are opened by voltages of reverse polarity with respect to the NPN type. In the symbol of the device, the arrow, which also determines the emitter output, this time points inside the transistor symbol.

Device design

The design circuit of a PNP-type transistor consists of two regions of p-type semiconductor material on either side of a region of n-type material, as shown in the figure below.

The arrow identifies the emitter and the generally accepted direction of its current ("inward" for a PNP transistor).

The PNP transistor has very similar characteristics to its NPN bipolar counterpart, except that the directions of currents and voltage polarities in it are reversed for any of the possible three connection schemes: with a common base, with a common emitter, and with a common collector.

The main differences between the two types of bipolar transistors

The main difference between them is that holes are the main current carriers for PNP transistors, NPN transistors have electrons in this capacity. Therefore, the polarities of the voltages supplying the transistor are reversed, and its input current flows from the base. In contrast, with an NPN transistor, the base current flows into it, as shown below in the circuit diagram for connecting both types of devices with a common base and a common emitter.

The operating principle of a PNP-type transistor is based on the use of a small (like the NPN-type) base current and a negative (unlike the NPN-type) base bias voltage to control a much larger emitter-collector current. In other words, for a PNP transistor, the emitter is more positive with respect to the base and also with respect to the collector.

Let's look at the differences between the PNP type in the connection diagram with a common base

Indeed, it can be seen that the collector current IC (in the case of an NPN transistor) flows from the positive terminal of battery B2, passes through the collector terminal, penetrates into it and must then exit through the base terminal to return to the negative terminal of the battery. In the same way, looking at the emitter circuit, you can see how its current from the positive terminal of battery B1 enters the transistor at the base terminal and then penetrates into the emitter.

Thus, both the collector current IC and the emitter current I E pass through the base terminal. Since they circulate along their circuits in opposite directions, the resulting base current is equal to their difference and is very small, since IC is slightly less than I E. But since the latter is still larger, the direction of flow of the difference current (base current) coincides with I E, and therefore a PNP-type bipolar transistor has a current flowing out of the base, and an NPN-type one has an inflowing current.

Differences between PNP type using the example of a connection circuit with a common emitter

In this new circuit, the base-emitter PN junction is biased by battery voltage B1 and the collector-base junction is reverse biased by battery voltage B2. The emitter terminal is thus common to the base and collector circuits.

The total emitter current is given by the sum of two currents I C and I B; passing through the emitter terminal in one direction. Thus, we have I E = I C + I B.

In this circuit, the base current I B simply “branches off” from the emitter current I E, also coinciding with it in direction. In this case, a PNP-type transistor still has a current flowing from the base I B, and an NPN-type transistor has an inflowing current.

In the third of the known transistor switching circuits, with a common collector, the situation is exactly the same. Therefore, we do not present it in order to save space and time for readers.

PNP transistor: connecting voltage sources

The base-to-emitter voltage source (V BE) is connected negative to the base and positive to the emitter because the PNP transistor operates when the base is biased negatively relative to the emitter.

The emitter supply voltage is also positive with respect to the collector (V CE). Thus, with a PNP-type transistor, the emitter terminal is always more positive in relation to both the base and collector.

The voltage sources are connected to the PNP transistor as shown in the figure below.

This time the collector is connected to the supply voltage VCC through a load resistor, R L, which limits the maximum current flowing through the device. A base voltage VB, which biases it negatively relative to the emitter, is applied to it through a resistor RB, which again is used to limit the maximum base current.

Operation of a PNP transistor stage

So, to cause base current to flow in a PNP transistor, the base must be more negative than the emitter (current must leave the base) by about 0.7 volts for a silicon device or 0.3 volts for a germanium device. The formulas used to calculate base resistor, base current or collector current are the same as those used for an equivalent NPN transistor and are presented below.

We see that the fundamental difference between an NPN and a PNP transistor is the correct biasing of the pn junctions, since the directions of the currents and the polarities of the voltages in them are always opposite. Thus, for the above circuit: I C = I E - I B, since the current must flow from the base.

Generally, a PNP transistor can be replaced by an NPN transistor in most electronic circuits, the only difference being the voltage polarity and current direction. Such transistors can also be used as switching devices, and an example of a PNP transistor switch is shown below.

Transistor characteristics

The output characteristics of a PNP transistor are very similar to the corresponding curves of an equivalent NPN transistor, except that they are rotated 180° to allow for reverse polarity of voltages and currents (the base and collector currents of a PNP transistor are negative). Similarly, to find the operating points of a PNP type transistor, its dynamic load line can be depicted in the third quarter of the Cartesian coordinate system.

Typical characteristics of the 2N3906 PNP transistor are shown in the figure below.

Transistor pairs in amplifier stages

You may wonder what is the reason to use PNP transistors when there are many NPN transistors available that can be used as amplifiers or solid state switches? However, having two different types of transistors - NPN and PNP - provides great advantages when designing power amplifier circuits. These amplifiers use “complementary” or “matched” pairs of transistors (representing one PNP transistor and one NPN transistor connected together, as shown in the figure below) in the output stage.

Two corresponding NPN and PNP transistors with similar characteristics, identical to each other, are called complementary. For example, TIP3055 (NPN type) and TIP2955 (PNP type) are a good example of complementary silicon power transistors. They both have a DC gain β=I C /I B matched within 10% and a large collector current of around 15A, making them ideal for motor control or robotic applications.

In addition, class B amplifiers use matched pairs of transistors in their output power stages. In them, the NPN transistor conducts only the positive half-wave of the signal, and the PNP transistor only conducts its negative half.

This allows the amplifier to pass the required power through the speaker in both directions at a given power rating and impedance. As a result, the output current, which is usually on the order of several amperes, is evenly distributed between the two complementary transistors.

Transistor pairs in electric motor control circuits

They are also used in H-bridge control circuits for reversible DC motors, which make it possible to regulate the current through the motor evenly in both directions of its rotation.

The H-bridge circuit above is so called because the basic configuration of its four transistor switches resembles the letter "H" with the motor located on the cross line. The transistor H-bridge is probably one of the most commonly used types of reversible DC motor control circuit. It uses “complementary” pairs of NPN and PNP transistors in each branch to act as switches to control the motor.

Control input A operates the motor in one direction, while input B is used for reverse rotation.

For example, when transistor TR1 is on and TR2 is off, input A is connected to the supply voltage (+Vcc), and if transistor TR3 is off and TR4 is on, then input B is connected to 0 volts (GND). Therefore, the motor will rotate in one direction, corresponding to the positive potential of input A and the negative potential of input B.

If the switch states are changed so that TR1 is off, TR2 is on, TR3 is on, and TR4 is off, the motor current will flow in the opposite direction, causing it to reverse.

By using opposite logic levels "1" or "0" on inputs A and B, you can control the direction of rotation of the motor.

Determining the type of transistors

Any bipolar transistors can be thought of as consisting essentially of two diodes connected together back to back.

We can use this analogy to determine whether a transistor is a PNP or NPN type by testing its resistance between its three terminals. Testing each pair of them in both directions using a multimeter, after six measurements we get the following result:

1. Emitter - Base. These leads should act like a normal diode and only conduct current in one direction.

2.Collector - Base. These leads should also act like a normal diode and only conduct current in one direction.

3. Emitter - Collector. These conclusions should not be drawn in any direction.

Transition resistance values ​​of transistors of both types

Then we can determine the PNP transistor to be healthy and closed. A small output current and a negative voltage at its base (B) relative to its emitter (E) will open it and allow much more emitter-collector current to flow. PNP transistors conduct at a positive emitter potential. In other words, a PNP bipolar transistor will conduct only if the base and collector terminals are negative with respect to the emitter.