The transistor is a semiconducting element that has the property of being able to voluntarily govern the current intensity that circulates between two of its three terminals (emitting and collector), by means of the circulation of a small current applied in the third terminal (collector).
This effect is known with the name of current amplification, and it allows to apply to him in the emitter a very small current us with any form of variation in the time, and to obtain the same current, with the same variation in the time, but of greater amplitude.
They are used essentially in circuits that realize amplification functions, control, process of data, etc.
The internal operation can be described from already explained for the diodes, with the difference of which this last one owns two semiconducting unions, this is: the transistor respectively owns two semiconducting zones, that can be N or P, and between a both very thin one of the type P or N.
This set will form two unions: a N-P, between the emitter and the base, and the other P-N between the base and the collector (if the two outer zones are of the N type and the inner P type, that is to say a transistor NPN. If the outer regions are of the P type and the interior of the N type the transistor it will be of type PNP).
If we applied an external tension to him to union N-P, so that it is polarized in direct, a circulation of current between both regions will take place. Applying one second external tension to the other union, so that this one is in inverse (the positive terminal of the source connected to the collector and the negative to the base), the current generated in the other union, will be attracted by the positive difference of applied potential the collector, generating that practically all the originating current of the emitter arrives at the collector, except for a small amount of current month that will leave by the base. And it is exactly this smallest basic current the one that allows us to govern the circulating current from the emitter to the collector.
The sense of circulation of the adopted current until now is the one of circulation of electrons, and as the used convention takes the then opposite sense in a transistor from type NPN the current is incoming by the collector and the base, and projection by the emitter.
In figure c) we have a mnemotécnica rule to remember the relation between the currents that cross to the transistor.
Because the emitter current will be always a multiple of the basic ones we will obtain the wished results of amplification. We suppose that current happiness of collector (IC) is 100 times the emitter current (IE), then if
IV = 5 mA; IE = 500 mA. If now IV = 2 mA; IE = 200 mA. Where it is possible to be appreciated that a small variation in the basic current (3 mA), it produces a great variation in the one of emitter (300 mA). This amplification factor is denominated generally with Greek letter b (Beta).
We have already made notice that transistors of type NPN exist and PNP according to are the drugged ones of the three regions, but between both types does not exist any difference as far as the functional thing, unless all the senses of circulation of the currents are opposed in both, therefore, to polarize a transistor PNP, of equal way that one NPN, will be due to use tensions opposed in both.
The transistors have a very interesting characteristic that is the capacity that has these to give a constant current intensity to a resistance, independent of the value of this one, that is to say that the variations of current obtained by the action of the base, will produce in the resistance a variation of the tension, which will be, according to the law of Ohm: V = I x R. Then V will depend on the value of the basic current and the resistance in the collector, being greater greater V when is R, being determined the limit by the value of the external tension applied to the circuit.
This effect is in a “voltage amplification”, that is one of the characteristics but important of the transistors and reason by which they are of almost essential use in the electronic assemblies. This voltage amplification calculates like the relation between the voltage in the resistance of load and the tension applied between the junctions base-emitting.
The transistors, according to are the manufacture technology, are classified in great groups with different characteristics: Bipolar, Fet, MOSFET, UNI UNION. Until the moment we have talked about to the first group of them.
The study and analysis of the transistors are realized by means of the use of the “characteristic lines” of the same, with which the behavior or electrical operation of the transistor can be characterized completely, being this one expressed in graphical relations of currents IV, IC and IE, based on the external tensions and for the different configurations: Common emitter (EC), Bases Common (BC) and Common Collector (CC).
The curves describe the behavior of the transistors, but as these do not behave all of equal way, these vary according to the type of transistor, and, although they differ from a type to another one, they are very similar in the form. In addition they do not talk about one in particular, but they are an average of a great number of units. These graphs are provided by the manufacturer, and as the most common assembly is the one of common emitter, and in addition the manufacturers provide the curves to us based on this type of configuration, we will concentrate in the analysis of the curves referred to this type of assembly.
Also it is important to know the values máx, mín and typical of the most important characteristics, to be able to use, in the calculations, the value that will turn out more unfavorable in order to assure to us that the operation of any unit of the sample will be within the stipulated thing.
The characteristic lines but important are the characteristic of entrance and the one of exit. In those of entrance, the graphs of the relation between the basic current (IV) and the base-emitting tension (Vbe) for the collector-emitting tension are expressed (BCE) constant. From them we can calculate the current that circulates around the base when an external tension between this one is applied and the emitter.
As the transistor in assembly in common emitter has behavior similar to the one of a diode polarized in direct, the curves are similarly, that is to say, that a certain threshold voltage exists below which the current is practically null (You = 0.3 V for transistors of Germanium and 0.6 V for those of Silicon).
Also of the entrance characteristics we can deduce the resistance of entrance of the transistor, that is the variation of the base-emitting tension (Vbe) with respect to the basic current (IV).
In the curves of graphic exit the current of collector IC based on collector-emitting tension BCE when we maintain constant IV. A family of different curves for IV draws itself generally. In this graph she is observed that over a value of tension emitting collector Vce1 the current practically stays constant, independent of the value of BCE. Below this value quite the opposite happens, IV varies quickly with small variations of BCE. This value of Vce1 is approximately 0.5 V. To this zone of operation where IC is almost constant, denominates active region and is in which it is desired that the transistor works when is used it in amplifiers. In this case IC it only depends on IV.
In the graph we can observe a denominated straight line RS, that delimits one of the 3 possible regions of work of the transistors.
The transistor will work in some of the 3 regions following the polarizations that receive each one of the unions P-N compose that it. The three regions are:
Region of saturation: The transistor behaves like a switch between emitter and collector.
Region of cuts: The transistor behaves like a switch abierto between emitter and collector.
Linear region (or it activates): Current amplifier of entrance behaves like a device (current basic).
Some of the important parameters of the transistors and that they are provided generally by the manufacturer are:
BCE (the Sat) = maximum Tension between collector and emitter working in saturation.
Vceo= maximum Tension between collector and emitter.
Vcbo= maximum Tension between collector and bases.
Vebo= maximum Tension between emitter and bases.
Maximum Current Icmáx= of collector.
Icm máx= Current principle of collector (value tip)
Basic maximum Current Ibmáx= (value tip)
Ptot= total dispersible Power.
In the same way that in the entrance characteristics we can deduce the entrance resistance, in the exit characteristics we can deduce the resistance of exit of the form: Variation of tension BCE with respect to IC. Another factor that we can deduce is the gain of current of the transistor (b).
Of the curves it is deduced, to the almost horizontal being, who the exit resistance very will be lifted.
He is advisable to determine the point of work of the transistor, following the task that we want that this one realizes before in a circuit and using the seen curves.
For it is had to polarize to the transistor with some of the circuits of polarization that we will see next, but before it we will make reference to the straight line of load of a transistor. In order to obtain it we will have to return to the family of curves of exit already seen. The load straight line is useful since it shows to us, in graphical form, all the possible points of work of the transistor for a given polarization.
In the figure we can see the straight line of superposed load the family of exit curves, in which we see several points of interest, those that we happened to explain next:
For the calculation of the load straight line we will consider to the transistor in two of its states: it cuts and saturation.
In the state of it cuts IC is practically zero, then we can conclude that Vc = BCE, the one that in our example is of 12 V. Then with IC” 0 V and 12 BCE V we obtain the first point of the load straight line, to that we called P1 in the graph.
In the saturation state we have BCE” 0 V and so then we can calculate the value of IC that will be IC = Vc/Rc that in our example gives to 12 V/2000 W = 6 mA. To the point BCE = 0, IC = 6 mA we called P2 in the graph.
If we united P1 and P2 we will obtain the straight line of looked for load.
In order to obtain the point of work (q) of the transistor we needed to know how IV, of this form point Q is the intersection point of the straight line of load with the curve corresponding to the value of the current that operates the transistor at that moment (Ib).
The load straight line can be different with each transistor and each point from polarization.
Projecting to point Q on the Y-axes of the graph we will obtain the values of IC and BCE, denominated in the graph like Ic1 and Vce1.
We will begin now yes with the circuits to polarize to the transistors.
The task of these polarizers is not other than the one to cause that at the different legs from the transistor different tensions arrive to him, but from a unique power supply, trying, in addition, to cause that parameter b is most stable possible, that is to say, than does not vary with the diverse external factors that can get to alter the same.
In the figure we can see several of the configurations to polarize to the transistor:
The first diagram (a) shows to a denominated configuration polarization by tension division. The resistance R1 and R2 form a tension splitter, which gives the name him to the configuration. This type of polarization is one of the most suitable and best one to work in the active zone of the transistor.
In part B of the figure we see another form of polarizer, denominated “basic polarization”. Now the basic current is obtained through R1. This type of polarization is used in circuits that work in commutation, not being advisable its use in transistors to which it is desired work in the active zone.
The polarization that is in C is denominated “polarization by emitter refeeding” and by means of this one we obtained a greater stability of point Q.
To the configuration shown in D the flame “polarization by collector refeeding”.
More usual applications of the transistors:
Or we commented that to the transistor it is possible to be mounted in common emitter (EC), bases common (BC) or common collector (CC). Each of these configurations owns advantages and disadvantages one with respect to the other, being the one of common but resorted emitter at the same time as she is the one of better answer in most of the applications.
Each configuration obtains different coefficients from gain in tension (GV), as well as different impedances as much from entrance as from exit.
Next we see a summary of the main characteristics of each one of the three possible assemblies:
| Assembly | G.V. | Desfasaje (v) | Ze | Zs |
| E.C. | Discharge | 180º | average | average |
| B.C. | Discharge | 0º | low | discharge |
| C.C. | < 1 | 0º | discharge | low |
The assembly in Common Base owns a greater gain of tension against the other two. Also it has low impedance of entrance, which makes quite inadequate to operate in circuits of LF (B.F.).
With a assembly in Common Collector we obtained a very low distortion on the exit signal and, along with the assembly in Common Base, he is quite suitable at the time of designing impedance adapters.
It is the practical but important application for which the transistors are used. The diagram shows an amplifying stage in common emitter:
The transistor has been polarized by means of polarization by tension division.
As we know, a capacitor in high frequencies behaves like a short circuit whereas to LF the same increases until behaving like a circuit abierto for C.C.
Seeing it from this point of view it agrees to analyze to the amplifier in two stages, one from the point of view of a.c. and the other from the point of view of the C.C.
With this subdivision we will be able to analyze to the circuit by means of two circuits but simple, consequently, thanks to the theory of the superposition, which will happen will be that the total answer will be from the sum of the data collected in both circuits in which we disturbed the original one.
We will begin the analysis in the dominion of the C.C., for it we followed the following steps:
1º) cortocircuita the generator of entrance of alternating.
2º) the capacitores like open circuits are considered.
3º) analyzes east resulting circuit.
Abriendo C1, C2 and C3 and cortocircuitando to the generator of entrance in our circuit we obtain the resulting circuit that we see next:
Now, and with the references already explained, it is come to the resolution from the resulting circuit. With these data we obtain the point of polarization (q).
For the analysis in a.c. we resorted to the following rules:
1º) cortocircuita the source of C.C tension.
2º) is considered to the capacitores like closed circuits (short circuits).
3º) studies the resulting circuit.
In the figure we see how we have come to obtain the resulting circuit:
The capacitores have disappeared of the circuit having become short circuits, the R4 resistance disappears to be in parallel with a short circuit, the resistance R1 and R3 are now in parallel, consequently we obtain Ra. With the exit resistance it happens the same, and we obtain Rb.
In order to finish with our analysis we must suppose that now we applied a signal to the circuit and will see how it varies point Q
In the figure we see an example, where is point Q in the absence of signal and how it varies with the application of an entrance signal.
One sees that signal IE is not a direct correspondence of applied in the base of the given transistor the curvature of the graph of the characteristic of the transistor.
It is important to verify the station of point Q well, since if we want that the transistor operates in the active zone and polarized to this one in a point Q near the zone of saturation, we run the risk of which when we applied an entrance signal to him, Q moves towards the zone of saturation, leaving the active zone. In order to avoid this problem it agrees to always before analyze the variation of Q in our transistor and to verify that it does not leave the region where we want that it works.
Another very important family of transistors is the one of those of field effect, of which she is divides the FET. The same realize the function of flow control by means of a applied tension in one of their terminals.
They are constructed with a semiconducting zone P type or N that unites both terminal (Fuente and Drenador), to this region the flame channel and on this one exists another one with opposite sign that is connected to the door, between both forms a union PN or NP, according to is its topology. This set is mounted on a semiconductor with equal sign to the one of the door. When a tension between Drenador and Fuente is applied, there will be circulation of current by the channel.
The control of current happiness will take control of a variable tension that is applied to the door, since, when applying this tension, unions P-N are polarized in inverse form, causing that the channel becomes thinner and, therefore, increases the resistance of this one, generating therefore a variation of the circulating current by him.
As this current of Door is extremely weak because it is a polarized union in inverse, it will be possible to vary the current that circulates around the transistor without it is necessary to absorb current of him.
Also the family of transistors MOS or MOSFET (Metal, Oxide, Semiconductor) is part of the transistors of field effect.
This type of transistor is made starting off of a semiconducting P type in which they spread to two regions N type that form the source and the Drenador, and, upon the surface of these, is applied a silicon dioxide layer (SiO2), that the property of being has very insulating, on that is located the Door. Between Fuente and Drenador also a channel similar to the one of the type FET will exist, whose resistance and width will be controlled with the door tension.
In the characteristic lines of the transistors of field effect the current of Drenador (YOU GO) based on the tension applied between Drenador and Fuente imagines (YOU). As in the case of the transference of the bipolar transistors, a curve for each one of the values of wished VGS draws up. Also in these curves two zones are observed; from the origin the current grows with the tension, but reached certain Vp value, becomes constant and it forms there from the second zone, to these two zones the flame linear region to the first and region of saturation to the last one.
This type of transistors can be used in the circuits in a disposition similar to the one of the bipolar ones, that is to say: Common source, common Door and common Drenador, although the first and last one is used actually.
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