* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Bipolar Junction Transistor
Variable-frequency drive wikipedia , lookup
History of electric power transmission wikipedia , lookup
Mercury-arc valve wikipedia , lookup
Electrical ballast wikipedia , lookup
Thermal runaway wikipedia , lookup
Electrical substation wikipedia , lookup
Stray voltage wikipedia , lookup
Voltage optimisation wikipedia , lookup
Voltage regulator wikipedia , lookup
Two-port network wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Mains electricity wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Schmitt trigger wikipedia , lookup
Buck converter wikipedia , lookup
Alternating current wikipedia , lookup
Current source wikipedia , lookup
Power MOSFET wikipedia , lookup
Rectiverter wikipedia , lookup
Opto-isolator wikipedia , lookup
Wilson current mirror wikipedia , lookup
History of the transistor wikipedia , lookup
unit-2 Bipolar Junction Transistor [BJT’s] S.Lakshmanan[ACED] Transistor is a solid state Device, whose operation is based on flow of electric charge carriers with in the solid. Used as Amplification and Switch It is analogous to a Vacuum triode. Transistor is a current controlling device. Vacuum triode is a Voltage controlled device. Transistor is only 5 decades old and replaces vacuum triodes in all applications. S.Lakshmanan[ACED] compact size, light weight, rugged construction, more resistive to shocks and vibration, instantaneous operation[ no heating required], low operating voltage, high operating efficiency[no heat loss] long life with efficiency. Draw back:- loud hum noise, restricted operating temp and freq. S.Lakshmanan[ACED] structure S.Lakshmanan[ACED] Types and symbol • NPN • PNP S.Lakshmanan[ACED] Transistor is like two PN-junction diodes connected back to back. Each Transistor has two PN-junction onejunction b/w Emitter and Base {Emitter junction} and other-junction b/w Base and Collector{Collector junction}. These two junction give rise to three terminals Emitter, Base , Collector. Transistor is single crystal in which there is twopn junction. Idea behind is one section to supply the charges to be collected by the third section through the middle section. Supply free charges is called Emitter, Collecting these charges Collector, the middle section formed b/w Emitter and Collector is Base. S.Lakshmanan[ACED] Terminals Emitter:- the E is always forward biased w.r.t base so that it can be supply majority charge carriers to the base. The E is heavily doped so that it may be able to inject a large number of charge carriers. Collector:- the main function is to collect the majority charge carriers. Collector is always reverse biased so as to remove the charge carriers away from its junction with the base. It is moderately doped. Base:- very lightly doped and very thin. It passes most of the injected charge carriers to the collector. Base forms two junctions i.e. emitter base and collector base. S.Lakshmanan[ACED] EB junction is F.B it offers low resistance to the emitter current. The CB junction is R.B offers high resistance to the collector current. The resistance of EB junction is very small as compared to the CB junction, therefore the F.B applied to the EB junction is very small whereas the R.B on the CB junction is much larger. Collector region is made physically larger than Emitter region Bcoz collector is to dissipate much power[heat]. No Arrow is marked for collector, Since its leakage current is always opposite to the direction of emitter current. S.Lakshmanan[ACED] Transistor Action • Transistor NPN PNP Behaves exactly in the same way except change in biasing and majority carriers. PNP the conduction is by holes NPN the conduction is by electrons NPN is more used than PNP bcoz it has better high frequency response & high mobility. S.Lakshmanan[ACED] Unbiased Transistor When no battery is connected b/w the terminals of a transistor, the transistor is said to be an unbiased transistor or open circuit state. Semiconductor is doped with donor impurity or Pentavalent it become Ptype. If its doped with acceptor or trivalent it becomes N-type semiconductor. S.Lakshmanan[ACED] When a N-region is formed next to P-region, a barrier potential is produced at the PN-junction. The free electrons in the N-region diffuse into the adjacent P-region to annihilate holes. Consequently, a layer of positive ions is formed on the N side, layer of Negative ions formed on the Pside. Creates the Barrier potential. In NPN transistor- the EB depletion region width is smaller than the CB depletion region width. S.Lakshmanan[ACED] S.Lakshmanan[ACED] Operation of Transistor The process of applying DC voltage across the different terminals of a transistor[to check whether the transistor is operating in the active, cut off or the saturation region] is called Biasing. Normal operation the EB junction is always F.B while the CB junction is R.B F.B at the EB junction reduces the barrier potential and narrows the depletion region. The lightly doped base and collector region produce a wide depletion region under R.B S.Lakshmanan[ACED] The electrons are injected into the E region by the EB supply VEB. These conduction band e have enough energy to overcome the EB barrier potential. The injected e enter into very thin, lightly doped base region bcoz the base is very lightly doped relative to the E region, only the few of the electrons recombine with the holes doped into the base. Injection of e makes the electron concentration on the emitter junction very large and collector junction the concentration of the e is extremely small and large in base. Injected electron diffuse into collector region due to extremely small thickness of base which is much less than the diffusion length. Most of the electron cross into collector junction. S.Lakshmanan[ACED] Collector is reverse biased and creates a strong electrostatic field b/w base and collector. The field immediately collects the diffused electrons which enter the collector junction. Flow of electrons into the base region when confronted with hole, a few electrons [1 to 5%] combine and neutralize, rest of the electrons[95 to 99%] of the injected electrons diffuse into the collector region and collected by the collector electrode. To maintain the base neutrality base electrode provides equal No.oF electrons which have combined with holes and results in a base current. The emitter current is equal to the sum of collector current and base current. IE = IC + IB S.Lakshmanan[ACED] The ratio of collector current to emitter current is called α [alpha] The ration of collector current to base current is called β [beta] In a transistor the α can not be greater than unity. The FB tends to align the V.B and C.B while the RB tends to increase their misalignment. Further, the electron lose energy in moving from base region into collector region. Only a few amount e is recombined with h in base region. S.Lakshmanan[ACED] Why the C region is more than E region? • The potential hill b/w the base and collector region is very steep. • Specially, electron much loos energy when they move from B to C. the electron give up their energy in the form of heat. • The C region must be able To dissipate this heat energy. S.Lakshmanan[ACED] S.Lakshmanan[ACED] Configurations • Common Emitter • Common Base • Common Collector S.Lakshmanan[ACED] Common Emitter Configuration S.Lakshmanan[ACED] Common Base Configuration S.Lakshmanan[ACED] CB Configuration In a common base configuration, the input current is the emitter current. and the output current is the collector current I. The ratio of change in collector current to the change in emitter current at constant collector-base voltage is called current amplification factor. A test circuit for determining the static characteristic of an NPN transistor is shown. In this circuit, base is common to both the input and the output circuits. To measure the emitter and the collector currents mull ammeters are connected in series with the emitter and the collector circuits. Voltmeters are connected across the input and the output circuits to measure VBE and VCB .There are two potentiometers R1 and R2 to vary the supply voltages VCC and VBE. S.Lakshmanan[ACED] CB - i/p Characteristics It is a curve, which shows the relationship between the emitter current, I and emitter-base voltage V at constant collector-base voltage V This method of determining the characteristic is as follows First by means of R1, a suitable voltage is applied to VCB from VCC. Next, voltages VBE is increased in a number of steps and corresponding values of IE are noted. The emitter current is taken on the Y-axis and the emitter base voltage is taken on the X-axis as shows the input characteristic for germanium and silicon transistors. The following points may be noted from the characteristics curves. This characteristic may be used to find the input resistance of a transistor. The input resistance (ri) value is given by the reciprocal of the input characteristic curve. The emitter current IE increases rapidly with small increase in emitter- base voltage VBE. It means that the input resistance is very high. The emitter current is dependent of collector voltage. S.Lakshmanan[ACED] o/p Characteristics It is a curve which shows the relationship between the collector current IC and the collector-base voltage VCB at constant emitter current IE. This method of determining the characteristic is as follows. First, by means of R2 a suitable voltage is applied to the base and the emitter. Next, VCB is increased from zero in a number of steps and corresponding values of IC are noted. The above whole procedure is repeated for different values of IE for obtaining family of curves. The collector-base voltage is taken the X-axis. shows the family of output characteristics at different emitter current values. The following points may be noted from the family of characteristic curves. S.Lakshmanan[ACED] The collector current IC varies with VCB only at very low voltages. This characteristic may be used to find the output resistance (ro) A very large change in collector-base voltage produces small change in collector current. It means that the output resistance is very high. The collector current is constant above certain values of collector-base voltage. It means that IC is independent of VCB and depends upon IE only. The output characteristics may be divided into three regions 1. The active region 2. Cut-off region 3. Saturation region S.Lakshmanan[ACED] ACTIVE REGION • In this region the collector junction is reverse biased and the emitter junction is forward biased. In this region when IE=0, IC =ICO. This reverse saturation current remains constant and is independent of collector voltage V as long as is below the break down potential. When emitter current flows in the emitter circuit then a fraction (- IE) of this current reaches the collector. Hence IC= -IE+ICO. Thus in the active region the collector current is independent of collector voltage and depends only upon the emitter current. But due to Early effect there is a small increase (0.5%) in IC with increase in S.Lakshmanan[ACED] VCB Saturation Region The region to the left of the ordinate VCB=0 is called the saturation region. In this region both junctions are forward biased. This is also called as bottomed region because the voltage has a fallen near the bottom of the characteristic where VCB=0. In this region IC increases rapidly with even small increase VCB in as shown in Fig. Cut-off Region The region below the IE=0 characteristic, for which the emitter and collector junction are both reverse biased, is called cut-off region. This portion of characteristic is not coincident with the voltage axis as shown in Fig. S.Lakshmanan[ACED] Region of Operation Conditions Emitter-Base Junction [EB] Collector-Base Junction [CB] Region Of Operation Forward-Reverse Forward-Biased Reverse-Biased Active Forward-Forward Forward-Biased Forward-Forward Saturation Reverse-Reverse Reverse-Biased Reverse-Biased Cut-Off Reverse-Forward Reverse-Biased Forward-Biased Inverted S.Lakshmanan[ACED] Forward-reverse biasing:- the transistor is in active region and the collector current depends upon the emitter current. The transistor is operated in this region for amplification. Forward-Forward biasing:-in this, both collector and emitter are forward biased, the transistor operates in the saturation region, the collector current becomes independent of the base current. Transistor acts as Closed switch. Reverse-Reverse biasing:- both the junction are reverse biased. In this the transistor has practically zero current because the emitter does not emit charge carriers into the base and no charge carriers are collected by the collector. Transistor acts as Open Switch. S.Lakshmanan[ACED] Bipolar Transistor Configurations Characteristic Common Base Common Emitter Common Collector Input Impedance Low Medium High Output Impedance Very High High Low Phase Angle 0o 180o 0o Voltage Gain High Medium Low Current Gain Low Medium High Power Gain Low Very High Medium S.Lakshmanan[ACED] Transistor biasing The most basic ways of Biasing are.. Base-Current Bias (Fixed Bias) Self-Bias Combination Bias. S.Lakshmanan[ACED] Base-Current Bias (Fixed Bias) • In this bias method, a biasing resistor is connected between the collector supply and the base. This is a very simple arrangement but carries a heavy withdraw. It is very unstable in means of temperature changes. • If the temperature of the transistor changes, either by changing of ambient temperature or from current flow within the transistor, the dc operating point (AKA quiescent or static point) will change. • This change is most undesirable because it affects the amplification gain and may also result into distortion on the output signal S.Lakshmanan[ACED] Self-Bias In this type of biasing, the biasing resistor is placed directly between the collector and the base of the resistor. With this biasing, feedback voltage can be fed from the collector to the base to develop forward bias. This is a better connection because if temperature increases and cause an increase in collector current, the collector voltage will fall because of the increase of voltage produced across the load resistor RL. This drop in VC will be fed back to the base and will result in a decrease in the base current. S.Lakshmanan[ACED] The decrease in base current will oppose the original increase in collector current and tend to stabilize it. The exact opposite effect is produced when the collector current decreases. The temperature change with this way will be managed as long as it is a moderate ambient temperature change. It cannot handle large temperature changes. Also, Bcoz the signal on the collector affects the base voltage and because they have 180 degrees phase difference, the amplification is slightly reduced. This is also known as "negative feedback", sometimes useful to prevent output signal distortion and self bias may be used for this purpose. S.Lakshmanan[ACED] Combination Bias. It’s a combination of fixed and self bias. the stability of the operation of the transistor is improved and also the disadvantages of the other methods are overcome. The fixed bias is implemented using the voltage divider R1-R2. The current flowing through the voltage divider network biases the base positive with respect to the emitter. the voltage divider tend to keep the base bias steady while the emitter bias changes with emitter conduction. S.Lakshmanan[ACED] The self bias is implemented using the resistor R3 connected in series with the emitter. If IE increases, the voltage drop across R3 will also increase and thus the Vc will be reduced. A drawback is that again the amplification is slightly reduced, but the stability is dramatically increased. To provide further more thermal stability and allow minimal signal degeneration, a bypass capacitor Cbp is places parallel to R3. Cbp is large so rapid signal variations will not change its charge materially and no degeneration of the signal will occur. S.Lakshmanan[ACED] AMPLIFIER IMPEDANCE’s S.Lakshmanan[ACED] Transistor as Amplifier S.Lakshmanan[ACED] the resistor R1, R2, and RE from the biasing stabilization circuit. The biasing circuit must establish a proper operating point otherwise a part of the –ve half cycle of the signal may be cut-off in the output. RL is the load, it represent the i/p resistance for the next stage. Cin is called i/p capacitor, used to couple the signal to the transistor base. CE is the bypass capacitor used in parallel with emitter resistance RE in order to provide a low reactance path to the amplifier ac signal. In the absence of this capacitor, the amp ac signal flowing through RE will cause a voltage drop across it, which in turn will feedback the i/p side and reduce the o/p voltage. S.Lakshmanan[ACED]