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    Bipolar junction transistor (BJT)


    1. Overview

    A bipolar transistor consists of three alternating zones - PNP or NPN - made on the same single crystal. The middle area is very thin compared to the others and bears the basic name (B). Extreme areas are named based on external polarization, emitter (E) and collector (C). The three regions have ohmic contacts that are pulled out of the transistor capsule and are called terminals.


    Depending on the type of zones (N or P) that are alternated, there are two categories of transistors: NPN and PNP (fig. 1). Due to the embodiment, two PN junctions appear: the emitter-base (EB) junction and the base-collector (BC) junction which can be assimilated with two semiconductor diodes. In practice, if we do not have a catalog, we may find a transistor whose structure is not known (NPN or PNP). In this situation, a method of identifying the structure of the bipolar transistor is used according to the indications in fig.2. This method assumes the existence of a measuring device, more precisely a digital multimeter, with the switch set to the "diode test" position.


    The donor or acceptor atoms of the emitter and the collector are much larger than the base (about 100 times). For the transistor to operate, the EB junction is polarized directly and the BC junction in the opposite direction with a voltage much higher than the EB junction.

    The following will explain the operation of a commonly used NPN transistor (Fig. 3).

    The concentration of carriers in the emitter (electrons) is much higher than in the base and because the EB junction is directly polarized from an external source U EB (fig. 3), there is a massive injection of electrons from the emitter (represented by an arrow) in the region of the base where it finds a much smaller number of holes. These gaps recombine with a small part of the injected electrons. Due to the fact that the base is very thin, most electrons pass through this region and enter the collector area. The BC junction being polarized in the opposite direction (the positive Ucb voltage is applied to the base), an electric field appears which accelerates, the electrons coming from the base to the collector. In the region of the collector the electrons coming from the base are recombined with the gaps coming from the power supply. It is thus noted that although the BC junction is polarized in reverse, it passes through a large current, almost equal to the direct current of the EB junction.

    This is the main property of the transistor effect which can be stated as follows: a high current can be passed through a polarized junction if the polarity junction is directly in the immediate vicinity (the base is very small). If the thickness of the base is large (greater than the diffusion length of the carriers from the base emitter) then the transistor effect is non-existent and the two series junctions are independent.


    Figure 3 shows the flows of load carriers through the transistor. The emitter current is composed of two components:


    The current I EN  is due to the majority electrons and the current I EP is the inverse current (due to the gaps) of the BE junction which is very small and can be neglected.

    The collector current is formed by a fraction "α" of the electron current of the emitter and the inverse current of holes of the junction BC noted I CB0 :


    The factor α has usual values of the order of 0.900 ... 0.999. The current I CB0  is desirable to be as small as possible. It is thus called the "quality factor" of a transistor. In most applications this current can be neglected for current transistors.

    The base current is determined by the inverse current of the BE (I EP ) junction , the recombination current of the electrons with the base gaps. (I RB ) and current I CB0:


    Based on the above considerations we can write the fundamental relation of the transistor operation:


    For PNP transistors, the operation is identical, with the observation that the external polarizations are of opposite direction and the majority carrier flow is formed by gaps. The current directions as well as the polarizations for the two types of transistors are shown in fig. 1.
    If neglected I CB0  can define a coefficient that shows how many times the collector current is higher than the base current.


    This factor expresses the transistor's DC amplification and shows how a small base current leads to a much larger collector current.

    For current transistors, having a value very close to 1, it results for β large values generally between 10 and 1000. The DC amplification factor depends on the temperature and the size of the collector current. It increases with increasing temperature and decreases at high collector currents.

    Thus, the transistor effect consisted in modifying the current of voids (starting from the emitter and reaching the collector) by modifying the polarization voltage of a directly polarized junction, namely the polarization voltage of the emitter - base junction.

    2. Static characteristics of bipolar transistors

    The transistor manufacturer indicates in the catalog sheets the graphical connection between the transistor currents and the continuous voltages applied between the terminals. This represents the static characteristics. Any of the transistor contacts can be selected as a voltage reference point. In practice, most often the voltages refer to the emitter or in other words the most common mode of connection in transistor schemes, is the common-emitter (EC) connection, which will be described later.


    Fig. 4 presents for a silicon NPN transistor these characteristics and the mounting with which they can be determined. The sizes l B  and U BE  are input sizes, and I C  and U CE are output sizes.

    • Input feature  - represents the variation of the base current depending on the voltage U BE . It is determined by maintaining from P 2  a constant UCE voltage and varies with P1 the polarization voltage of the base U BE . I B  and U BE are measured. This feature is similar to a directly polarized PN junction (Fig. 4c). Occurrence of current I B  and implicitly of current I C,  occurs only when a voltage threshold U D  called open voltage isexceeded. This voltage depends on the semiconductor material. Thus for transistors with AND the opening occurs for U BE voltages between about 0.5 V and 0.65 V and for those with Ge between 0.1 V and 0.2 V.
    • The output characteristic  - expresses the variation of the collector current I C  according to the U CE  for different values of base current I B . For its determination, a certain value of the basic current I B is set with P 1,  which is kept constant. Then with P 2 the  voltage U CE C is changed accordingly. The study of this characteristic shows that at a constant base current, the collector current increases very little with U CE  and in practice it is often considered independent of this voltage. I  depends essentially on I  and therefore on UBE  (fig.4b).
    • The transfer feature  - shows the dependence of the collector current on the base current. It is determined by simultaneously adjusting the P1 and P2 to keep the U CE  At the same time measure the variation of I C  according to I B . This feature is a straight line whose inclination depends on the DC amplification factor (fig. 4d).

    3. The operating modes of the transistors

    From the point of view of the polarization mode of the three junctions there are three operating modes:

    • Normal active regime (zone II of fig. 4b). The transistor has the directly polarized BE junction and the BC junction in the opposite direction. The limits of this regime are determined by the condition of canceling one of the polarization voltages. The collector current of the transistor is controlled by the base circuit.
    • Locking or cutting mode (zone III of fig.4b). The junctions BE and BC are polarized in the opposite direction. The current passing through the transistor is very small (in the order of nanoamperes) and is due to the thermally generated minority carriers. The maximum inverse voltage that can be applied to the BE junction in locked mode depends on the type of transistor and is specified in the catalog. Thus for high frequency transistors with Ge it is about 0.3 V, for transistors with Si of 3-7 V and for transistors with Ge allied of 10-20 V. In case of exceeding this voltage, the BE junction behaves like a diode Zener with a very steep characteristic, an important reverse current appears and if there is no limiting resistance, the transistor is destroyed by thermal packing.
    • The saturation regime (zone I of fig. 4b). The junctions BE and BC are directly polarized. The currents flowing through the transistor are mainly limited by the external circuit. This regime may also occur on a transistor to which the polarization sources are applied for operation in the normal active region. Thus, if in Fig. 4a, the potentiometer P 2  is replaced by the resistance R  connected between the collector and + E C , it can be seen that by increasing the voltage U BE  it can be reached that at a certain moment the current Ic increases to a value that all supply voltage to fall on Rc. There will thus be a limitation of the collector current to the value:


    where the voltage U CE  is very close to zero and marks the boundary between the normal active regime and the saturation regime. Until the Ics value is reached through an IBS current, a normal active regime can be considered practical for a transistor:


    Increasing the base current above the I BS value  will no longer produce a proportional increase of the collector current, remaining at the I CS value  which it cannot be exceeded by being limited by the external circuit. However, the emitter current will continue to increase with the difference between the existing base current and the I BS value .

    In other words, a saturation current I CS  through a transistor whose value depends on the external sizes RC  and E C  can be obtained if a minimum I BS current is injected into the base . The collector-emitter voltage obtained is called saturation voltage - U CEsat .

    4. Dynamic regime of bipolar transistors The

    transistor as a circuit element can be considered as an active quadripole (fig. 5). Since it has only three electrodes, one will be common to the input and output. This electrode or terminal will serve as a voltage reference point and is considered at zero potential (ground).
    When operating in dynamic mode the currents and voltages on the transistor contacts are variable in time.


    Depending on the common terminal chosen, there are three fundamental modes of connection: with the common base (BC), with common emitter (EC) and with common collector (CC) (fig.6).


    a). The common base circuit (BC)  is characterized in that the signal is applied between the base and the emitter and the load resistance R  is mounted between the collector and the base (in terms of which the sources E B  and E 0  are presented in sc).

    Due to the high value of the input current which is the emitter current, the amplification in the current is close to the unit and the input impedance is reduced to tens or hundreds of ohms. This is a disadvantage in the case of multi-stage amplifiers where the low input impedance dampens the output impedance of the previous floor, which requires the use of complicated adaptive circuits. However, the BC mount is widely used in high frequency amplifiers, being preferred to the EC mount where the transistor-collector-specific reaction capacity can produce the floor auto-oscillation. In BC connection, this capacity only appears in the output circuit.

    The output impedance is high, in the order of hundreds of kOhmi or MOhmi. The gain of the voltage is also large, and in the particular case when there is the collector and emitter resistors R  and R is  equal (for low frequencies) about their ratio R C / R E .

    The phase of the output signal is identical to that of the input signal. This can be explained in the simplest way: if the input voltage tends to increase, then the potential of the emitter will increase, which will decrease the collector current and thus increase the collector voltage (ie the output voltage).

    b). Circuit with common collector (CC) it is characterized by the fact that the input signal is applied between the base and the collector and the load resistance R E  is connected between the emitter and the collector (from the point of view of E. E B  and E C  are in sc). If the diagram in Fig. 6c is redrawn as in Fig. 6d, we notice that only a fraction of the input voltage U1 is applied between the base and the emitter (U BE ). This will produce a variation in the current I B , I E  and I C . The emitter current produces on the load resistance R E  an output voltage U 2  lower than the input voltage (U 1 = U BE  + U 2). Therefore the voltage amplification is subunit (0.09 - 0.95).

    Due to the low value of the input current (the base current), the amplification of the current and the input impedance are high.

    The output impedance is very low. As an actual value, the input impedance is in the order of tens of kilos Ohms, and the output impedance is in the order of tens of ohms. Both impedances are β, I C  and R E dependent . Due to this particular feature of the two impedances, the DC connection is used in practice especially for adaptation.

    Since the voltage amplification is almost unitary, the floor is also called a repeater on the transmitter, which practically reproduces the input signal as amplitude and phase.

    c). Circuit with common emitter (EC).  The input signal is applied between the base and the emitter and the load resistance is connected between the collector and the emitter. Since the input current, which is the base current, has a low value, compared to I E , the input impedance is higher than at the BC connection, which allows the production of multi-stage amplifiers without special adaptation measures. Also the output impedance is relatively high being in the order of tens or hundreds of kilohms.

    The voltage gain, if a resistor R E is considered in the emitter circuit, is given approximately, for low frequencies, by the ratio R C / R E, and the amplification in current is the factor ß. It is the assembly most often used in practice as a result of the above.

    As an important observation it should be noted that the amplified signal in voltage at the output is in phase with that of the input. Thus, if a variation of the input voltage (U BE ) is assumed in an upward direction, this causes an increase of the current I B  and therefore I C , which leads to an increase of the voltage drop on R C  and ultimately to a decrease of the output voltage (U CE ).

    In the table below the characteristics of the basic schemes of fig. 6 are concentrated.


    Figure 7 shows an amplifier stage in EC connection with an NPN transistor.

    Considering that the voltage of the US signal source is zero, we observe that the EC power supply is divided on the Rc and the transistor between the collector and emitter according to the relation:

    This equation can be represented in the plane of the static output characteristics (fig.4b and fig.7b) by a straight line AB and also called a straight line whose ends are characterized by:

    By choosing a polarization of the base (EB) a current I BO can be established  whose characteristic intersects the right of charge at point P. This point is called a point of It also corresponds to a collector current I CO  and a voltage U CE corresponding to the output characteristics .

    However, if an alternating voltage component U S is superimposed on the base polarization, the base current varies: I B  = I B2  - I B1 . This will determine in the collector circuit variations of the collector current (A / c) and the collector-emitter voltage (U CE ) around the static value I C0 , respectively U CE0 .

    In other words, if an AC signal is applied to the base circuit, the same signal is amplified in the collector circuit, but also amplified in antiphase. The amplification transistor depends on the external sizes and R B , R C .

    The amplification factor defined above is a parameter that expresses the ratio I C / I B  in cc or at low frequencies (about 1 kHz). As the working frequency increases, the ratio of the value of the collector AC to the value of the base AC becomes smaller than β and in this situation a new parameter "h 21e " is defined and also called the direct current transfer ratio. It decreases as the frequency increases. The rate at which it becomes equal to 1 and is called the cutoff frequency is denoted by f t .

    As an observation to this parameter, it should be noted that the cutoff frequency f T in the EC connection it is lower than the BC connection, where it is β times higher. This justifies the use of BC mounting in high frequency amplifiers.

    The voltage amplification of this floor can be expressed as in the case of a pentode by:


    where R C  is the load impedance and S is the slope of the transistor.

    The slope is a parameter that shows how much the output current (collector) varies in mA for 1 V input voltage variation (U BE ). It is expressed in mA / V. A common feature of bipolar transistors is that the slope increases almost linearly with the current, ie approx. 35 mA / V for each mA of the collector current. For example, if a transistor has I C = 5mA then S = 35 * 5 = 175mA / V and if R C  = 1K we have a voltage amplification A u  = 175 * 1 = 175.


    At high currents, close to the maximum permissible collector current, the slope is smaller and no longer increases linearly with I C . Also the slope depends on the working frequency. The linear law is generally valid at low frequencies. At medium and high frequencies it decreases with the frequency reaching approx. 25-30% T at frequencies close to f * T . This is mainly due to the time required to travel through the base thickness, at frequencies of the order of (0.1-0.2) * f T it becomes comparable with the frequency period and the collector current ceases to immediately track the instantaneous variations of the base current. As a result there is a reduction of the amplification and a phase difference between the output current and the input current.

    Positioning the static operating point "P" (fig.7b) on the load line is particularly important. They depend on the operation of the transistor in linear or non-linear regime as well as the time it drives from the total of a period. Fig. 8 presents some particular situations of static point positioning used in practice.

    If a linear operation is desired, the static point will be in the middle of the load line in M 2. In this case the transistor will conduct the entire signal period (360 ° or 2π). Alternans collector voltage are symmetrical and may be almost equal to half the maximum amplitude of E C The transistor is said to be in Class A operation .

    If the static point is close to the saturation zone "M 1 " then one of the alternations will be limited and nonlinear operation will occur. By choosing the operating point "M 4 " at the intersection of the load line with the EC axis  , a half-period (180 ° or π) transistor conduction is obtained. The signal in the collector will have only one alternation and the  transistor will operate in class B. The direct current consumed from the source is null in the absence of the signal and it increases as it increases. This regime allows high energy efficiency (up to 80%).

    However, if the operating point is M 3,  which is close to the cut-off point of the transistor, then it is possible that from a certain level of the input signal, the transistor will lead for less than one period. The transistor will operate in class AB  and the power consumption from the source when no input signal is applied is small. This mode of operation is used in high-performance audio amplifiers to reduce the connection distortion.

    From a practical point of view, it sometimes appears that the transistor needs to run for less than half a period. In this case the operating point will be M 5  and the transistor will operate in class C.  For this, the input circuit will have a polarization of the base which will allow only from a certain level of the positive alternation of the input signal the transistor to be open. This regime has the highest energy efficiency and is used in RF amplification or frequency multiplication (the percentage of harmonics is high due to the shape of the collector current pulse) of the transmitters.

    One last class of operation  is the D class. In this case, the transistor works in switching mode, blocking saturation. The power dissipated by the transistor in the two states is minimal and very large amplifications can be obtained with very high efficiency. The disadvantage is that the input part is complicated.

    5. biasing the transistors

    in the above was found that EC and BC junctions were two separate sources is polarized with B  and E C . This practically creates many difficulties. Therefore, the most widespread polarization mode is that which uses a common power source - as shown in Fig. 9 for an NPN transistor in EC connection and class A amplification regime.


    Source E C it provides both the collector current and the basic current necessary to position the static operating point on the load line in the desired area (fig. 9a). The major disadvantage of this scheme is that due to the large dispersion of the transistor parameters (IB, β) on the one hand, and on the other hand due to their variation with temperature, the static operating point cannot be controlled in practice.

    The diagram in figure 9b removes the dispersion of the base current and thus of ß, by mounting a polarization divider RB1, RB2 by which a current of about (10-20) is established * I B. Thus, the voltage of the base is practically stabilized. Without having the claim that there are no exceptions, for the circuit in Fig. 9b (which is the most common), the following domains for the circuit resistors can be considered as usual, if the transistor is low power:

    The emitter resistor R E  which introduces a reaction negative, it has a great role in improving the parameters of the scheme in terms of temperature. Thus, due to the increase in temperature, currents I C  and I E  tend to increase. It will also increase the potential of the emitter to the mass and how the base is maintained at a constant voltage due to the divisor R B1  and R B2, a decrease of UBE voltage and therefore of IC, IE currents will result. The disadvantage of this scheme is that due to the relatively low values of resistors R B1  and R B2  a process of reducing the input impedance of the floor takes place, which is especially annoying in multi-stage amplifiers. To eliminate this disadvantage, a boostrap connection can be used to polarize the base, which also preserves the advantages of the previous scheme due to the existence of the base divider (fig. 9c).

    Since in the EC connection the signal on the emitter is in phase with the base one, a positive reaction at the input (ie an increase of the signal in the base) is applied through capacitor C, which translates into increasing the impedance in the input.

    6. The switching transistor The

    The operating modes in the blocking and saturation state have been described previously (point 3). The switching mode of a transistor means a dynamic mode in which the transistor operates alternately, saturated-blocked (see fig10).


    Figure 10 shows the diagram of a switch in CE connection with an NPN transistor, as well as the current pulses with the corresponding times. Right lock is characterized transistor load operating point A where I C  is zero and U EC  = E C . Also, the saturation of the transistor corresponds to an operating point B, which is obtained by injecting a minimum basic current I BSmin .

    A current I CS = β * I BSmin is obtained in the collector . In practice, however, a current I BS  > I BSmin is applied to guarantee transistor saturation . The collector current can no longer increase and then I CS  <β * I BS .

    The collector voltage in this case will be very low: U CEsat = (0,1..0,5) V. We consider that until the moment t 0 the  transistor is blocked by the negative value U 1  of the input signal (U i ) applied on the base. At this time, there is a jump in the input voltage from the value U 1  to the positive value U 2 , soon followed by the jump of the base current from zero to I B1 > I BSmin .

    [adv_2] Due to the fact that the charge carriers (electrons) injected quickly by the emitter in the base need a time to reach the collector, the current I  will hold at zero a delay time t t   after which it begins to increase to the stationary value. The CS .

    The time when the current increases from zero to 0.9 from its final value is called the rise or rise time - t r.  It follows that from the time t 0  when the saturation switch command was applied, until the moment the current of the collector reached 0.9 of the maximum value passed a time called "direct switching time":


    If at time t 3, the input signal decreases sharply from the value U 2  positive to U 1  negative, the base current will also tend to decrease sharply from I B1  to I B2  changing its meaning due to the fact that in the base region there are load carriers in big number. The resistance R B  has the role of limiting the base current to the value I B2  and thus protecting the BE junction. The surplus of electrical charge in the base area will cause the I CS  to maintain a "t S  " time after which it begins to decrease. During this time the load stored on the basis of the wave and the name of the storage time take place.

    At time t 4 saturation occurs in the transistor output and the operating point will shift from B to A in a time T C . During this time the basic task continues to be evacuated until it is almost completely canceled. The time t C  is called the fall time and is defined as the time when the collector current drops from the value I CS  to 0.1 * I CS .

    "Reverse switching time" means the time interval from the moment the locking command is applied to the moment when the collector current drops to 0.1 from its maximum value and is:


    Note:  In the catalogs of the various manufacturers, they are still meeting for some time. above and the following notations with meaning: t d = t i ; t f  = t c .

    From a practical point of view, it is desirable that U CEsat be  as small as the high value of the current I CS  passing through the transistor will produce a high power dissipation. For this the current I  will have to be considerably increased compared to I BSmin . However, the increase will not have to be exaggerated, as the storage time will increase and thus the switching properties of the transistor worsen.

    The typical case of operation in this regime is the final floor of the BO on the TVs.

    7. Limit parameters of transistors

    In the catalog sheets made available to the beneficiaries of the transistor manufacturers, a series of parameters must be specified which should not be exceeded, as follows:

    • Maximum junction temperature.  It depends on the nature of the semiconductor material. For Si transistors we have T jmax  = 125-175 ° C, and for those with Ge, T jmax = 80-100 ° C. This is usually ensured by the choice of current regimes and judicious voltages and where appropriate by special cooling measures. It should be noted that the lower operating limit for all types of transistors is T jmin  = 65 ° C.
    • The maximum collector current  of a transistor type depends on a number of factors from which we mention the value of the power dissipated under saturation regime, the threshold to which the decrease of the amplification factor in the current is allowed. This current corresponds to the permanent and noted in the catalog I Q . There is also defined a peak current "I CM " which can only be reached in pulses with a maximum duration well established. This is limited by the existence of irregularities of the flat forms of the junctions where in certain areas high densities of current can occur which produce a dangerous heating and therefore the destruction of the transistor.
    • The maximum base current allowed  in permanent mode is noted with I B , and in pulse regime I BM .
    • The emitter-base inverse voltage represents the maximum allowable voltage that can be applied to the EB junction in the blocking direction. The importance of this parameter has been explained in detail, at point 3. in the catalogs it is denoted with U EB0 . The index 0 shows that it is determined for the situation with the collector empty (I C  = 0).
    • The maximum collector voltage  depends on the transistor connection mode. Thus in the catalogs the following voltages are specified:
    • U CB0  is the reverse voltage applied to the collector-base junction when the emitter is open or empty (I E  = 0). Since the emitter-base junction is inert, the transistor behaves like a reverse polarized diode. It is the highest voltage the transistor can withstand.
    • U CEX  represents the collector-emitter voltage with the EB junction blocked by a reverse voltage to the normal situation when it is open. It is smaller than U CB0 .
    • U CES  is the collector-emitter voltage when the EB junction is shorted from the outside. As this junction will be slightly activated, the U CES voltage  is slightly lower than the U CEX ;
    • U CER  represents the collector-emitter voltage when a resistor is connected between the emitter and the base. It is even smaller than U CES  and U CEX .
    • The CEO  represents the collector-emitter voltage with the base empty (I B  = 0). It is usually the lowest voltage.

    It should be noted that these voltages cause a reverse polarization of the collector-base junction, and the base-emitter junction may be in the mentioned situations. The inverse voltages presented correspond to the inverse (residual) currents: I CB0 , I CE0 , I CER , I CES  and I CEX . The largest residual current is I CE0 = 3 * I CB0 . This doubles as the temperature rises by approx. 70 0 C. These currents are the limit inverse currents which must not be exceeded, in Fig. 11 the behavior of the transistors at high voltages is shown.


    The maximum reverse voltages that a transistor can withstand are in the area where an avalanche process called the first piercing begins. This regime does not become dangerous as long as these voltages together with the corresponding residual currents remain inside the parabola which represents the maximum dissipated power (fig. 11a). The maximum reverse currents are the lower the voltages increase. In case of exceeding these voltages due to reaching the Avalanche threshold Up, the multiplication process can no longer be controlled and the transistor enters the secondary throughput manifested by the decrease of the voltage drop between the collector and emitter and the current increase (fig. 11b).


    The maximum dissipated power theoretically represents the power dissipated on the two junctions:


    since in the normal active regime UBE << UCB, we can practically consider:


    in the plane of the output characteristics, equation (14) represents a parabola which together with the maximum collector current and the collector voltage- maximum emitter delimits the allowed area of operation (fig.12).

    Maximum power dissipation is also noted in catalogs and P M . In the case of power transistors, the dissipation is ensured by the installation of radiators whose calculation takes into account the temperature of the junction of the capsule and the environment as well as the thermal resistance that intervenes between the junction and the environment.

    8. Application

    The circuit of Fig. 13, supplied with E C, is given= 18Vcc and using a BC109B type transistor having β = 300 and I CB0  negligible. Knowing that the transistor must work at the static point I C = 3mA, U CE = 8V, U BE = 0.6V, the values of resistances R 1 , R 2 , R 3  and R 4 are required .



    We write the following characteristic equations resulting from the application of Kirchhoff's theorem in fig.13:


    Since β = 300 we can write:


    Because α≈1 we can write:


    At the same time,


    System (15) being two equations with four unknowns, we choose: R 4 = 1 kOhmi and R 2 = 10 kOhmi, according to the domains indicated in table 2.

    From the first equation of the system (15) it results:


    Which is rounded to the nearest standard, from the 10% tolerance class, that is R 3 = 2.2kΩ .
    From the second equation of the system (15) it is written:


    A standardized value of R 3 = 39kΩ will be adopted .

    OBS.  Having known the resistances R1  and R 2 , the voltage E B can be calculated  using the voltage divider relation: E B = E C (R 2 / (R 1 + R 2 )).

    9. Setting the static point to the desired value

    If the circuit calculated in the previous application is performed experimentally and the static operating point is measured, deviations from the values initially imposed will be found. These are due to the following causes:

    • the circuit cannot be made with the calculated values of the resistors, but with their standardized values, which differ slightly from the first ones. Moreover, the resistances belonging to the tolerance class 10%, for example, have actual values different from the standard value written on them by ± 10%;
    • parameter ß can have a dispersion around the nominal value of -50% to + 100%, for the same type of transistor, a very common case in practice;
    • the static operating point varies with the temperature and it is very likely that the temperature of the transistor will differ from that given in the catalog as a reference.

    Therefore, in order to bring the static operating point to the desired value, the following adjustments can be made (see circuit in fig. 13):
    a).  For the modification of the U EC voltage only , it is taken into account the explanation of the physical operation of the transistor, which, as shown, can be considered a constant current generator between the emitter and the collector. Therefore, by changing the resistance R 3 , the voltage drop on it is changed, the difference up to the value (E C -R 4 · I E ) being taken by U CE  (see the first equation in the system (15)). So increasing R 3 decreases U CE and vice versa, with the precaution of not increasing too much on R 3 , which would result in the transistor leaving the normal active region of the characteristics. It should be noted that at this setting the collector current remains constant.

    b).  To change the collector current, it can be operated either by changing R 4 or the base divider R 1 , R 2  . Usually the last option is preferred: if the resistance R1 is increased, the positive potential brought by the divisor R 1 , R 2  on the basis of the transistor decreases, the emitter-base junction is weaker polarized and therefore the collector current decreases (to the limit, if R1 → ∞ the base has the potential of mass and I C= 0). Obviously, if R1 decreases, the collector current will increase. Similarly, if R 2  is reduced, the fraction of + E C  that is brought to the base decreases and the collector current will decrease as well (at the limit, if R 2 = 0, the base gets the potential of mass and I C = 0). Conversely, if R 2  increases, the collector current increases.

    It is observed that with the change of the collector current, the voltage drop on the resistors R 3 , R 4 also changes , which in turn results in the change of the collector-emitter voltage, ie the increase of I C  corresponds to the decrease of U CE  and vice versa.

    10. Variation of the static operating point with the temperature

    In any of the polarized circuits (fig.6), at a temperature increase, the collector current tends to increase, which leads to the decrease of the U EC voltage .
    The increase of the collector current is due to the following causes:

    • When the temperature rises, the residual current of the base collector junction increases strongly, thus contributing to the increase of the collector current
    • For a directly polarized junction, crossed by constant current, if the temperature rises, the voltage at its terminals decreases. Therefore, in the case of the transistor, raising the temperature will lead to the decrease of the base-emitter voltage, for constant emitter current. It turns out that if the polarization circuit keeps the UBE constant, there is an increase in the transistor current, so also the collector current.
    • Experimentally, an increase of the amplification factor in current with the temperature is constant. Therefore, if the polarization circuit provides a constant injection of current into the base, an increase in the collector current will increase with increasing temperature.

    In conclusion, the collector current is a function of temperature through I CB0 , U BE  and ß.

    11. Methods for stabilizing static operating point variations

    The problem of stabilizing the static operating point in relation to temperature is one of the critical problems that occur in semiconductor devices. The variation of the temperature should not only mean the variation of the ambient temperature (although this is important), but also the heating of the transistor when it is crossed by electric current (by simply turning on the circuit). Therefore, the variation of the temperature means the variation of the temperature of the junctions, which can have multiple causes.

    The methods used for stabilization are of two types :

    • Linear stabilization methods. These methods consist of introducing suitable resistors (in size and position) in the polarization circuit (Fig. 14a). Thus, it can be found experimentally (and verified by calculation) that in general, the introduction of a resistor in the transistor emitter has a favorable effect, the more pronounced the higher the resistance value.
    • Nonlinear stabilization methods. These methods consist of introducing nonlinear elements (eg diode, thermistor, Zener diode) into the polarization circuit (fig. 14b and c). It is intended that, by temperature variation of a parameter characteristic of the nonlinear element, to compensate the tendency of variation of the collector current.

    For example, let's look at the circuit in figure 14b, where the diode D is made of the same material as the transistor and has residual current I0. The mechanism of compensation is as follows: at an increase in ambient temperature, the initial tendency of the collector current is to increase. At the same time, the residual current of the diode is increased, so the voltage drop on the resistor R 1 increases , which causes the potential of the base to decrease, and the collector current to have a decreasing tendency. This compensates for the initial growth trend of I C .
    To choose the correct diode we will have to write the equation:


    from which the value of I0 is derived.

    Another common circuit is the one drawn in Fig. 14c, in which the resistance R 2 is a thermistor. The circuit acts as follows: a rise in ambient temperature is assumed. It entails the increase of the collector current; at the same time the value of the resistance R 2  decreases, so the continuous potential brought on the base of the transistor by the divider R 1  and R 2 is reduced . As a result, there is a tendency to decrease the collector current which compensates for the initial tendency.

    However, in order to make them sensitive to the variation of the junction temperature, the compensation circuits discussed are provided with a thermal coupling as tight as possible between the transistor and the diode, respectively the thermistor. Thus, in the case of the power transistors placed on the radiator, the non-linear device can be mounted in the body of a screw which is then screwed into the radiator as close to the transistor.


    12. Transistor types and families

    Various codes are used to identify different types of transistors. For example, in the European code the first letter signifies the nature of the semiconductor material: A = Germanium, B = Silicon. C = gallium-arsenic, D = indium-anti-monium. The second letter shows the scope: C = transistor of small and low frequency signals; D = low frequency power transistor; F = high frequency transistor; L - high power and high frequency transistor; S = switching transistor; U = switching power transistor. In the American code, the transistors are designated by the 2N code. The last figures indicate the respective type of transistor.


    At the current stage of electronics development the base material is silicon which is less affected by temperature. It allows transistors to be obtained in a wide range of powers and frequencies. It is also the main element of integrated circuits.

    In certain electronic circuits it is necessary to sort the bipolar transistors. In general, the sorting criteria refer to the deviation of the parameter ß (current amplification factor) from a value ß taken as a reference in the respective application. The deviation can be positive or negative and is expressed as a percentage. The more rigorous the selection criteria, the more likely there is to obtain a more expensive circuit, because it is necessary to purchase and sort more transistors. In more demanding applications, transistors are sorted not only by the factor ß, but also by other factors, such as: the noise factor. The noise factor is defined as a ratio between the signal / noise ratio at the output of the transistor and the same ratio but referred to the input.

    Following are the main types and families of Si transistors used in consumer electronics.

    a). Transistors with Si and AF switching, low power.  The family of NPN transistors in TQ-18 metal capsule, includes as representative types: BC 107, 108, 109. The complementary PNP types in the same capsule are: BC 177, 178, 179. By using waterproof plastic capsules, the technology has become more productive and cheaper by 20-40%. From the class of low power transistors in plastic capsule are: BC173, BC174, BC546, BC556, BC550, BC560, BC639, BC640 etc.

    b). Medium-power Si transistors
    The most common family is BD 135, 137, 139 (NPN) together with its complementation BD 136, 138-140 (PNP). They are generally used in final floors with powers of up to 3-4 W. The maximum dissipated power is about 12W at a capsule temperature of 25 ° C and the maximum collector current of IA. In practice, however, it is not used at currents greater than 0.5 A since (3 greatly decreases above this value. For amplifications at currents of order 0.5-1A other families are used: BD 233-235-237 (NPN) and BD 234-236-238 (PNP). The maximum dissipated power is 25W, and the maximum collector current of 2A. The capsule is plastic type SOT 32 or TO-126 with the collector removed to the surface to allow direct contact with the radiator. for a good cooling

    . Transistors with Si NPN for the final video floors
    These transistors have the task in the TVs to amplify the complex video signal from a level of 3-4 Vvv to an amplitude of 90-100 Vv as it is needed for a modern kinescope. As the spectrum of a luminance video signal ranges from 0 to 5 MHz, the gain in the EC connection must be uniform in the band (20-35 times). It follows that the cutoff frequency must be high (fT> 50 MHz). To achieve the desired output level, the supply voltage of the floor is high and therefore the UCE0 is between 100 and 300 V. The usual load resistances of 3-5 kOhms will determine an average current of 10-30 mA which implies, from reliability reasons, maximum collector currents between 50 and 100 mA. Also for stable operation at high frequencies in the connection ??,

    d). RF and FI-MA-MF Si
    transistors These transistors equip the high frequency blocks of the radio receivers, the FI-MA-MF amplifiers, as well as the AFI-sound from the TVs. Also some types (BF214) can be used as oscillator in FIF channel selectors. The limit parameters are of the order: Pmax = 120-300 mW; I C  = 15-30 mA; U CE0  = 20-25 V. However, to work in a frequency range from 0.15 to 20 MHz, these transistors have a high cut-off frequency (200-300 MHz) and a low collector-base capacity (0 5-LPF).

    e). Transistors with Si for AFI video-sound TV.
    The high frequency range (30-40 MHz) in which this transistor has to work involves, on the one hand, high cutoff frequencies of the order of 400-600 MHz, and on the other hand CCB reaction capacities reduced by half compared to the types. earlier. Are they usually used in connection? without neutrodyne and in adjustable amplification (RAA) or non-adjustable mode.

    The reduction of the collector-base reaction capacity is achieved through an integrated screen during the manufacture between the metal foil island which is the base contact and the collector area. The screen, being connected to the transmitter the effect of parasitic capacity is greatly diminished. Because the screen is a layer P made in the base material N of the collector, a PN junction appears that acts as a diode connected between the emitter and the collector. Therefore, in the current measurements with the ohmmeter, these transistors between the collector and the emitter have the character of a diode and not a blocking state in both directions.

    Representative families are BF 167-173 in metal capsule TO-72 and BF 198-199 in plastic capsule similar to BC transistors. Types BF 199 and BF 173 are used in fixed amplification mode, and types BF 167, BF 198 in adjustable amplification (RAA) mode. The adjustment can be done in voltage or current.

    f). Transistors with Si for FIF - FIU domains
    These transistors equip the TV channel selectors for receiving FIF (50-230 MHz) and UIF (470-860 MHz) bands. Due to the high working frequencies, the cutting frequency is high, the low reaction capacity and the low noise factor. NPN transistors were used in the first step of introducing Si transistors into the channel selectors. The representative family consists of types BF 180-181-200, which result from the same technology, the sorting being made by the noise factor F.

    g). Power Transistors with Si for AF
    These transistors are used in AF amplifiers with output powers of tens of W, regulators or voltage sources. The most common family is 2N3055, where the sorting is done taking as criteria the voltage U CE0 and ß at a specified collector current. The typical transistor of this family is characterized by: U CE0  = 60 V. I C  = 15A and P M  = 117 W. The figure of 117 W for the maximum dissipated power is valid for ideal cooling conditions, ie the transistor is considered mounted on a radiator. infinitely so that the temperature of the capsule does not exceed 25 ° C.

    h). Transistors with Si for horizontal sweep
    The operation of the final stage of BO from the pulse TVs requires for the used transistors, high working voltages and currents, high switching frequency and speed as well as low saturation voltages. Because the operation of a final stage of the BO implies the existence of a bipolar switch, in some types of transistors, a fast diode is installed internally in contrast to the collector current. This diode is called a parallel recovery diode.


    • [1] - Schett Z. et al. - "Semiconductors and applications" - Facla Publishing House, Timişoara, 1981
      [2] - Găzdaru C., Constantinescu C., - "Guidance for Electronists Vol.I" - Teh. Publishing House, Bucharest, 1986
      [3] - Vasilescu G., Lungu Ş. - "Electronica" - Didactic and Pedagogical Publishing House, Bucharest, 1981
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