TRANSISTORS

 TRANSISTORS 

A transistor can also be called as bipolar junction transistor has two junctions formed by sandwiching either p - type of n - type semiconductor between a pair of opposite types . There are two types of transistors namely

                                   1. pnp transistor                           2. npn transistors .

 In each type of transistor :

                         1. There are two pn junctions .

                        2. There are three terminals taken from each type of semiconductor .

                        3. The middle section is very thin layer . This is most important factor in the function                                 of  a transistor . 

shows the two types of transistors while Fig . 1.17 shows the circuit symbol of the two types . The three constituent portion of a transistor are called emitter , base and collector .

 1. Emitter : The section on one side supplies charge carriers is called the emitter . It is more heavily doped than of the other regions because its main function is to supply majority charge carriers to the base . The emitter may be distinguished from the collector by an arrow head represents the direction of the emitter current with the forward bias on the emitter .

 2. Base : The middle section which forms two pn junctions . between the emitter and collector is called the base . It is very thin as compared to either the emitter or collector and is very lightly doped .

 3. Collector : The section on the other side collects the charge carriers is called the collector . The collector region is physically larger than the emitter region because it has to dissipate much . power . Because of this difference , there is no possibility of inverting the transistor i.e. , making its collector the emitter and its emitter the collector .

 TRANSISTOR CONFIGURATIONS 

There are three leads in a transistor viz . , emitter , base and collector . However when a transistor is to be connected in a circuit , we require four terminals two for the input and two for the output . This difficult is overcome by making one terminal of the transistor common to both input and output terminals . The input is fed between this common terminal and one of the other two terminals . The output is obtained between the common terminal and the remaining terminal . Accordingly ; transistor can be connected in a circuit in the following ways . 

                                1. Common base configuration .

                                 2. Common emitter configuration .

                                 3. Common collector configuration . 

                Each circuit connection has specific advantages and disadvantages . It may be noted that regardless of circuit connection . The emitter is always biased in thje forward connection. the emitter is always biased in the forward direction, while the collector is always a reverse biased.

 COMMON BASE CONFIGURATION 

Fig  Shows a pnp transistor with bias voltage VEE and Vcc and connected in common base configuration . Here IE & Ic are emitter and -collector currents and VEB & VCB are emitter base voltage and collector base voltage . Thigh FIGURE 1.18 : Common Base . Configuration Graphs may be plotted choosing two out of four variables as dependent variables and other two as independent variables . In transistor it is most useful to select input current and output voltage as the independent variables . The output current and input voltage then form the dependent variable . Thus in CB configuration , we can write the following equations VEB = f ( V CB , IE ) ( 1 ) Ic = f₂ ( VCB IE ) ( 2 ) VEB VCB VecT

Input Characteristics : The curves of Fig . 1.19 constitute input characteristics of pnp transistor in CB configuration . These are obtained by plotting equation ( 1 ) i.e. , plotting the emitter base voltage VEB voltage against the emitter current IE , with collector to base voltage Vca as the parameter . 

The input characteristics of Fig for VCB opens but the forward characteristics of the emitter to base diode . Fig . 1.19 also shows that for a given level of input voltage , more input current flows when higher level of CB voltage are employed . This because the reverse CB voltage cause the depletion region to penetrate deeper into the base of the transistor , thus shortening the distance and the resistance between the EB and CB depletion regions .

Output Characteristics : The relation of equation ( 2 ) is depicted in Fig . 1.20 for a typical pnp transistor and plots the collector current Ic against collector to base voltage VCB using IE as the parameter . 

Active Region : The active region of the characteristics is one in which the collector junction is one in which the collector junction is reverse biased and emitter junction is forward biased. With zero emitter current

 the transistor behaves as a reverse biased base collector diode So that the collector current is small and equal to the reverse saturation collector current Ice lc 

 Suppose now a forward emitter current le flows in the emitter circuit . Then fraction of a le of this current reaches the collector . In the active region the collector current is independent of collector voltage and depends only on the emitter current IE

Saturation Region : The region of the output characteristics . to the left of the ordinate VCB = 0 and I = 0. In this region both emitter and collector junctions are forward biased .

 Cutoff Region : The output characteristics for IE = 0 is similar to all other characteristics except that it passes through the origin . The characteristics for IE = 0 is not coincident with the voltage axis , but the current le is only a few micro amperes . The region below and to right of the le = 0 characteristics for which region both emitter and collector junctions are reverse biased is called the cutoff region . Active Region

COMMON EMITTER CONFIGURATION

Fig  shows a npn transistor in common emitter or grounded emitter configuration . In the CE configuration also , the input current and the output voltage are taken as the independent variables where as the input voltage and the output current form the dependent variables . We may then expresses the output current Ic and the input voltage VBE as functions of input current Ig and output voltage VCE as below . ( 3 ) VBE 1 ( VCE . IB ) Ic = f2 ( VCE , IB ) ( 4 ) Le In Vcc VBB FIGURE 1.21 : Common Emitter Configuration

 Input Characteristics : Equation ( 3 ) gives the input characteristics . In Fig . 1.21 ( a ) the base to emitter voltage VBE is plotted against the base current Ig with collector to emitter voltage VCE as the parameter . With VCE = 0 the collector is shorted to emitter and the transistor acts as forward biased diode . If IVCE is increased , the base collector junction depletion layer increases and results the decreases of base width , finally decreases the recombination base current 

 Output Characteristics : Fig . 1.21 ( b ) shows the output characteristics curves of a npn transistor in CE configuration . Here collector current Ic is plotted against the collector to emitter voltage Vce using the base current la as the parameter . In CE configuration the slopes of the curves are larger than slopes in CB configuration . Further the output current le is much larger than input current 

 In the active region the collector junction is reverse based while the base emitter junction is forward biased . Thus in Fig . 1.22 ( b ) . the active region is the region to the right of the ordinate VCE = a few tenths of a volt and above lg = 0. In the active region , the transistor collector current le responds more readily to any input signal . The operation of CE transistor used as an amplifying device must be restricted to the active region .

 In Fig . 1.22 ( b ) , the collector current le varies with VCE for VCE between 0 and 1 V only . After this the collector current becomes almost constant and independent of VCE . Above this voltage a small increase in Ic with increasing VCE is caused by the collector depletion layer getting wider and capturing a few more majority carriers electron hole combinations occur in the base area .